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1 THE LAKE RUDOLF CROCODILE ( Crocodylus niloticus Laurenti) POPULATION by A listair Graham Thesis submitted for the degree of Master of Science in the University of East Africa 1968 Author s address: P.O. Box 21199, Nairobi, Kenya (as at 201 2, PO Box 124, Cervantes 6511, Australia)
2 Preface This work originated as a result of a suggestion by Ian Parker of Wildlife Services Limited to the Kenya Game Department (now Kenya Wildlife Services) that a survey be conducted of the Lake Rudolf (now Lake Turkana) crocodile population to assess its present status and thereby provide the basis for planning its future. In the past it has been the practice to exterminate crocodiles whenever they begin to conflict with human interests. Nearly all the Eas t African populations have either undergone this treatment or been subjected to indiscriminate exploitation. Lake Rudolf supports the last relatively undisturbed population in Kenya, and to forestall a similar fate an understanding of their ecology has bec o m e a matter of urgency. The Game Department, while supporting the idea was unable to provide any funds and so the sample of 5 00 animals killed during the surv ey was used to finance the work, which was c a rri ed out by Wildlife Services Ltd This proved i nadequate as a source of revenue resulting in abandonment of many aspects of the work. Nevertheless, sufficient data w ere amassed to consider exploitation possibilities and ensure their continued survival.
3 S ummary 1. A 14 month st udy of the Lake Rudol f crocodile population is described during which 500 animals were killed and examined. 2 Lake Rudolf in northern Ke ny a is 186 miles long, 6 30 miles wide, with 576 miles of shore. Less than 250 m m of rain falls per ann um Shade temperatures over 24 hour s vary between 26 .5 30.5 o C with little annual variation. There is a s trong, persistent easterly wind. 3. The population is estimated by aerial and night ground counts at 12,439 animals in June 1966. Discontinuities in distribution are related to habitat, w ith high densities in sheltered and low densities in exposed situations, v arying from 1 90 per mile. 71.9 % of the population occupies the east shore. 4. The main food animals, fish, are caught in shallow water at night. 5. 48.4 % of stomachs are found emp ty, a much higher incidence than i s seen in other populations. 6. 90.9 % of stomach contents are fish. Of stomachs with fish 87 % contained tilapia (Cichlidae). 7 An annual breeding season is confi rm ed. In mal es a quiescent phase in January March culmina tes in a peak of sper matogenetic activity in October December. 8. The relationship between testis weight and cubic dimensions is established to provide a field technique for estimating testis weight in ani mals near maturity. 9. 50 % or more males are matu re at testis weights > 10g. The mean body length at a testis weight of 10 g is 270 cm. 10. The onset of female sexual maturity is well marked at 100 cm body length. 11. Seasonal breeding in the female is evident but some reproduction apparently takes plac e throughout the year. 12. Very old females appear to cease reproducing.
4 13. The mean clutch size in Rudolf crocodiles is about half that observed elsewhere. 14. Clutch size is s hown to increase with increasing body size the relationship being the same in Rudolf and Uganda animals. 15. Layers are observed in the dentary bone and evidence is presented that they are fo rm ed annually. The number of layers increases with length and is used in the male to give absolute values to the age length relationship. 16 Lens growth is seen to be independent of body growth and is used to show the relative difference in the age/length relationship of males and females. 17. More rapid growth in males res ults in a difference of size at age between sexes so that the olde st males of 470 cm are 3.5 times as heavy as the oldest females of 500 cm. 18. There is no weight difference between males and females of the same length. 19. Males seldom exceed 470 c m length and females 320 cm. 20. The growth of Alligator mississipien sis is compared with that of C. niloticus and seen to be similar w ith the latter species growing larger. 21. Growth in immature croc odiles is analyzed by plotting the frequency of occurrence of different growth rates on arithmetical probability paper. Two modes are obtained and assumed to represent mean annual length increments in ma les of 52 cm and in females of 2 0 cm. Growth evidently slows after maturity. 22. Based on immature growth rates and the maximum size obtained hypotheti c al growth c urves for ma les and females are drawn up using an age scale of 0 6 0 years. 23 3 recaptured crocodiles f rom a sample of 152 marked f or growth rate info rmation showed ann ual length incre me nts o f only 1.7 cm indicating that growth in wild crocodiles can be severel y in hibited. Captive animals from the same area yielded growth rates o f 30 cm per annum.
5 24. Age speci fi c fecundity is calculated f or each age class of breeding females. The age structure o f the populatio n being unknown a hypothetical f emale li f e Table is dra wn up u sing the observed f ecundity values and assuming a net reproductive rate o f 1.0. 25. Mortality is discussed. Owing to the high reproductive rate immature mortality can approach 100 % in a stable population. 26. The f actors a ff ecting the commercial va lue o f crocodile skin are discussed. The incidence o f "buttons", a hard deposit in the ventral scales, is seen to be the most important f actor reducing value 27 Skins currently increase in value with incre asing body size up to a length of 190 c m after which the value per unit area remains constant. 28. Evidence is presented that the most valuable age classes of a crocodile population are those near the point of maturity. 29. Ways o f exploiting the high potential productivity are discussed and suggesti ons f or reducing the immature mortality rate are made. 30. It is co ncluded that a management plan f or the Rudolf crocodiles could be drawn up ai med at cropping the younger age classes possibly supplemented by artificial rearing for a period of early lif e Exploi ting eggs is considered and the dangers of permitting unplanned exploitation emphasized
6 CONTENTS Introduction Material and Methods Taxono mic status Section 1. De ns ity and Distribution 1a Habitat 1b Population Counts 1c Factors influencing Crocodile Distribution Section 2. Food 2a Feeding Behaviour 2b Analysis of Stomach Contents 2 c Q uantity of Food Consumed Se c tion 3. Reprod uc tion 3 a Reproduction in the male 3 b Reproduction in the female Section 4. Age and growth 4a Age 4b Growth Se ction 5. Population Dynamics 5a Recruitment 5b Morta lity 5c Survivorship in cro c odiles Section 6. Exploitation 6a Value of crocodile skins 6b Productivity and yield
7 6c Crocodile Management on Lake Rudolf Section 7. Conclusions Ac kn owle dgeme nts Refere nces Appendi ces Water analysis and one off night counts.
8 Figure 1. in the text with the proportion of the total population occurring in each section and the density per mile o f shore.
9 Introduction This thesis comprises information coll ected during a 14 month survey of the Lake Rudolf (now Lake Turkana) crocodile population. Information was col lected on feeding biology, growth and reproduction with the aim of assessing the po pulation as a whole and considering its exploitation possibilities. The consulting firm of Wildlife Services Limited was engaged by the Ke ny a Game D epartment (now Kenya Wildlife Services) t o ca rr y out the survey with the East African Wild Life Society Sc ientific and Technical Committee acting in an adviso ry capacity. This activity was assumed latterly by the Zoology Department, University College, Nairobi. No other crocodile population has been s u bjected to comparable analysis before and info r m a tion oth er than of an anecdotal nature is scanty. Hippel (1946) recorded the stomach contents of 587 crocodiles shot on Lake Kioga in Uganda, finding a predominance of fish. Cott (1961) in his monograph on the Nile crocodile gives a good review of the available in formation up to that time and includes some previo usly unpublished data of Cott examined 651 crocodiles during the course of his work and records much useful data on feeding, growth and reproduction, and many other aspects of their biology. Peabo dy (1961) describes growth zones in the dentary of a crocodile and ascribed these to annual climatic variations causing discontinuities in bone growth. Poole (1961) describes the process of tooth replacement in crocodiles which appears to be continuous and notes that there is a constant size increment in each new tooth. Modha (1967) records observations on territorial and reproductive behaviour in crocodiles on Central Island, Lake Rudolf. Reference to other work is made, where appropriate, in the text.
10 Material and Methods Detailed methods are set out at the beginning of each section, while a general outline of the work follows. Fieldwork covered the 14 months from J uly 1965 to September 1966. Preparation of the results occupied the 6 months from F eb ruary 1967 to July 1967. A 5 day preli minary survey in February 1965 s howed most of the population to be on the north east shore of the lake a nd consequently this was select ed as the main study area. The Game Department agreed to 500 ani m als being killed f or investigation, and as no other funds were available the project was financed entirely from the sale of skin from these animals. Collection of specimens was made from two sites, Allia Bay and Moite (see Figure 1) with the original intention of collect ing approximately 40 per month from each site. Towards the end, crocodiles were difficult to obtain at Moite and ultimately 305 were collected at Allia Bay, 187 from Moite and 8 from North Island. The first month was devoted to the capture, marking and r elease of animals at Moite for growth rate studies. The first half of September 1965 was spent on North Island eliminating the crocodiles there as part of a long term experiment to observe the rate of recolonization. Regular collection of specimens began i n September 1965 and continued so as to cover a complete twelve month cycle. A Cessna 182 aircraft was used for most travelling and transport, with a small boat for local movements. The work was carried out entirely by the author although 5 Turkana were engaged as skinners and M. Watson and R. Bell acted as observers during the count in June 1966.
11 Taxonomic status and Distr ibution of the Order Crocod y lia The order Crocodylia contains 13 families grouped under 5 suborders. Only the family Crocodylid ae remains extant with 8 genera and 25 species. There are 2 genera of crocodiles, Crocodylus with 12 species and Osteolae mus with 2 species, the latter confined to West Africa and the Congo, the forme r occurring throughout the Old W orld tropics an d the nor thern part of the New W orld tropics. There is one genus of alligators Alligato r with 2 species, one in the South Eastern United States ( A. mississipiensis ) and the other in the l ow er Yangtze Valley of China ( A. sinensis .) The related c aimans, with 3 gene ra and 9 species, occur in Central and South America. 2 monotypic genera of gavials, Gavialis and Tomisto ma occur respectively in India and the Malay region. (Sc h midt 1944, Wermuth 1953, Darlington 1957.) Crocod y lus niloti cus Laurenti the common A frican species and known as the Nile crocodile, occurred in recent times throughout Africa, Madagascar, the Seychelles and Comoros. Hunted for its skin, or merely exterminated, it has vanished from much of its former range and significant populations are now conf ined to parts of Ethiopia, Sudan, Tchad, Botswana, East Africa, Zambia and Mozambique. The first 4 areas, with the largest remaining populations, are being extensively exploited at present. Isolated populations survive throughout most of the former range i n rivers and lakes but most of the larger populations are being hunted for s kin. Lake Rudolf and Murchison F alls Nation al Park support the only large E ast African populations that have not been extensively Deraniyagala (1939) considered the Lakes Rudolf and Baringo crocodiles to differ from those of the Nile system. He accordingly named the Rudolf animal C. niloticus pauciscutatus and the Baringo animal C. niloticus Worthingtoni However, until the differen ces are shown to be other than environmentally caused there seems little value in these sub species.
12 Section 1 Density and Distribution 1a Habitat Ge ological History The most widely accepted opinion is that Lake Rudolf has been separated from the Ni le system since the beginning of the Upper Pleistocene and Fuchs (19 39 ) prese nt s a d etailed account of its geology General The centre of the lake lies approximately 3 o 30 North and 36 o East. It is 186 miles long a nd 6 30 miles wide. The shoreline inclu ding the 3 main islands is approxima te ly 57 6 miles and the surface area of water (variable) about 6,870 square miles. The water is saline (Appendix 1) but potable, and the author detected no ill effects after drinking it f or 15 months. Th e taste of the w ater varies from place to place and is generally saltier near the shore There is only one permanent river, the O m o, which drains into the lake f rom the Ethiopian highlands to the North. N umerous seasonal rivers (that do not always flow every year) drain i nto the lake f rom which there is no surface outlet. The depth between Central Island and Gulf averages 174 feet, while Worthington (1932) records a depth of 256 feet near Central Island. No other soundings of the lake appear to have been made. S tron g currents are evident in ma ny places but these have not been studied. From Fuchs (1935) description it appears that the lake today is similar in size to what it was in 1934. However Fuchs, w ithout giving his sources states that "during the last 45 years the lake has sunk at least 30 40 feet." There is reason to doubt this state m ent since the description given by Von H hnel (1894) of Allia B ay fits very well its present day appearance. He describes 2 small islands in the bay which are similar in position and size to the islands present today which are less than 10
13 feet above lake level. The littoral is generally very s h allow and changes in depth of 3 0 40 feet would produce very extensive alterations to the shore line. H owever there has been a steady retreat by the lake since the upper Pleistocene which may, in the long term sense, be continuing. The lake is continuously rising or falling over a small range according to rainfall. In 1965 66 the change was about 3 feet, highest in November and lowest in April, but considerable variation exists year to year. Climate Occasional recordings of shade tempera ture were made wi th a whirling h ygrometer. The highest temperature obtained was 56.5 o C at 1430 hours and the lowest was 26.5 o C at 0600 hours. Similar temperatures ha v e be en recorded on Central Island (Mo dha 1967, p.78 ). Water temperatures were measured at various times thr ough the 24 hour cycle 15cm below the surface in 50 100cm of water, and ranged from 21 27 o C. These m a y be regarded as typical far the shallows on the east shore, with little variation through the year, or the 24 hour cycle. The area has a probability of le ss than 25 cm rainfall per annum (Atlas of Kenya 1959), and Lodwar, 40 miles west, has averaged 12 17 cm for the last 30 years. Most rain falls in March, April and May and lake levels are highest after this time. Insolation measurements are not available but cloud cover for most of the year is slight, with only occasional c l oudy days. Wind measurements are not available but generally the wind prevails easterly to south easterly throughout the year. The daily pattern on the east shore I 10 a steady east erly wind throughout the morning, of 10 20 miles per hour, gusting frequently 30 40 miles per hour. After mi dday there is a tendency for this easterly to be replaced by a slight westerly "sea breeze" until dusk. (Values derived from variations in ground sp eed observed while flying.) This pattern is however, erratic, wi th frequent periods of continuous wind lasting up to 3 d a ys. Calm periods occur on most days at any time in the 24 hour cycle, variable in duration, but not usually lasting more than 12 hours During periods of rain, this pattern is further disrupted, and it may blow from any d irection, usually the
14 n orth. The importance of this wind on crocodile biology is discussed again in the section on "distribution." Habitat Lake Rudolf is surrounded b y arid c ou ntry, with a gradient north to south of increasing desolation. The west shore is dry and sandy with sparse vegetation for its whole length. There is contin uous human activity along this s hore with Tur kana settlements at several points. No signifi cant numbers of wild mammals occur except in the extreme north west, where herds of topi and o ryx border on the lake. The east shore is gener ally less arid than the west except in the south east, where old lava flows predominate, with little vegetation or wildlife. The north east section is open bush count ry with an evidently higher rainfall than any o ther part, and with numbers of topi, oryx, both species of zebra, Grants gazelle, giraffe, lion, leopard, cheetah, h yaena, wild dog and many small mammals E lephant, rhinoceros and b uffalo, common 80 years ago (von H hnel 1894) have vanished. Whether the disappearance of these species, and the resident human population s described by von H hnel, has been associated with any change in the climate and vegetation, is unknown. The north end of the lake appears to have a higher rainfall than any other part and is extensively cultivated. Description of the lake shore ( Figure 1) For the purpose of comparing crocodile distribution and possible future exploitation th e l ake shore has been divided into 13 sections based on habitat. These are somewhat arbitrary but serve to make the discontinuities in crocodile density clearer. Section 1 (Plate 1) Omo River delta. 88 miles of shore, but very variable according to l ake level. Marsh made up of extensive reed beds with weed filled open water. Similar swamp is not found elsewhere on the lake. Moderate
15 crocodile density of 14 per mile. T his section is m ostly in Ethiopia, with considerable human activity on the shore. Pl ate 1. Omo River delta. Extensive marsh of reed beds with weed filled open water. This comprises the whole of Section 1; similar habitat is not found elsewhere on the lake except for small marshes at the Turkwell and Kerio deltas. Moderate crocodile densit y of 14/mile. Section 2 (Plates 2 & 3 ) Illeret to Allia Bay. 7 miles of shore. Mostly swampy consisting of small scattered reed clumps, grass and inundated bush, with extensive weed beds (species not identified) Occasional stretches of sandy beach. Cro codile density of 90 per mile is m uc h higher than an ywhere else on
16 Plate 2A (S2). Illeret to Allia Bay. Extensive weed beds and inundated bush typical of this section. This photograph is of the northern end of Allia Bay, which supports the highest local density of crocodile on the lake Plate 2B (S2). A recently flooded area caused by a small rise in lake level. Such sheltered water is favoured by small crocodiles that rapidly accumulate in such transient areas. The section has a density of 90/mile.
17 Pla te 3A (S2). The passage of the aircraft has disturbed at least 10 crocodiles while at least 1 remains ashore. Plate 3B (S2) An isolated sandbank in Section 2 that illustrates the tendency for crocodiles to clump together. The hippo (upper right) and skimme rs and Egyptian geese give some idea of scale.
18 the lake and accounts for 52. 6 % of the whole population. No resident humans but subject to sporadic hunting by Merrille from Ethiopia. Plate 4A (Sections 3, 6 & 7). Extensive sand and shingle beaches inte rspersed with rocky (lava) stretches. Low densities of 3 9 crocodiles/mile. Se c tion 3 (Plate 4) Allia Bay to Moite. 24 miles of shore, all rocky or shingle beaches and backed by a range of hills which provide shelter from the wind. Low cr ocodile density of 9 per mile. Section 4 (Plate 5) Moite. 24 miles of shore. Mostly sheltered swampy water, similar to Section 2, and with a high density of 26 per mile. No resident humans, but subject to r egular, small scale hunting by the resident El Molo. Considerable numb ers of Rendille and EI Molo domestic animals are watered along this section during the dry season but the nomadic herders do not disturb the crocodiles.
19 Se ction 5. (Plate 5) Moite to Loi a ngalani 68 miles of shore consi sting of i solated stretches o f sheltered, swampy water, similar to Moite, separated by extensi v e sandy beaches. A moderate crocodile density of 19 per mile. The southern half has resident EI Molo who hunt c o ntinuously along all this section Plate 4B, Sections 11,12 and 13. Volcanic islands with occasional sandy beaches. The high densities of 24 39/mile P late 5 Sections 4,5,8 and 10. Open shore, but with numerous weedy patches and sheltered water. Moderate densities of 11 26/mile is obscure, but possibly associated with nesting.
20 Plate 6A. S9. Uninterrupted sand and shingle beaches backed in places by doum palms, extending almost the entire length of the west shore Very low density of 3/mile. Section 6 (Plate 4) South East Shore. 24 miles of shore consisting of c ontinuous s hingle and rocky beaches. With Section 9, this section has the second lowest density on the lake of 3 per mile. Continuous human activity along the shore. Section 7 (Plate 4) South West Shore. 64 miles of shore, backed by a range of hills and very simila r to Section 3. Roc k and shingle beaches with no swamps, exposed to the almost continuous easterly wind. The low density of 7 per mile is made up almost entirely of anim a ls living in sheltered inlets at the extreme south end of the lake. Continuous human a ctivi ty. Section 8. (Plate 5) Turkwell and Kerio deltas. 36 miles of shore, very similar to Sections 4 and 5, but less sheltered Due to the prevailing easterly wind, the swampy areas are confined to irregularities in the shoreline w hic h provide
21 s helter. A moderate density of crocodiles of 11 per mile. Considerable hunting of crocodiles is carri ed out by the resident Turkana. Plate 6 B S9. Uninterrupted sand and shingle beaches extending almost the entire length of the west shore mostly devoid of eith er aquatic or terrestrial vegetation Very low density of 3/mile. Section 2. (Plate 6) Turkwell delta to Omo delta (excluding Gulf (Kalokol) ) 121 miles of shore consisting of sand and shingle beaches mostly exposed to the full effect of the su rf generated by the easterly wind. A very low density of 3 crocodiles per mile. Resident Turkana e xist all along this s ection who hunt and collect eggs.
22 Section 10 Gulf (Kalokol) 16 miles of shore, enclosing a shallow bay about 2 miles acro ss. Completely sheltered water with small swampy patches. A high density of 22 crocodiles per mile. Resident Turkana all round the bay. Section 11 (Plate 4) North Island. 5 miles of sho re. Mostly recent lava flows with two sandy beaches. Very low density of 1 crocodile per mile. No resident humans. Section 12 (Plate 4) Central Island. 15 miles of shore, including 2 completely sheltered small lakes. Rocky shore with one beach. The relative ly high density of 39 crocodiles per mile is associated with 2 smal l lakes on the island and along with Gulf (Kalokol) this Section must be regarded as a special case, as similar habitats are not found elsewhere on the lake. No resident humans. Section 13 (Plate 4) South Island. 18 miles of share. Mostly rock y b ut several sandy beaches and sam e sheltered water formed by partly submerged lava. A high density of 24 crocodiles per mile, possibly associated with the isolation of the Island from the mainland. No resident humans.
23 Population Counts Methods Distr ibution and density was assessed by aerial counting. As the study area was not large at the resolution of aerial counting total count s were made rather than samples. Two total counts were carried out, one in February 1965, and a second in June 1966. Duri ng the 1966 count a considerable amount of aerial photograp hy was experimented with. Using a milit ary F.24 5 inch format camera, about 10 % of the shoreline was photographed from about 800 feet above lake level. The purpose was two fold: to establish wheth er photograp hy would confer any advant a ges over direct vis ual counting; and to sample stretches of shore to obtain a breakdown of animals into size groups. The only advantage indicated by the use of photogra p h y in counting is to eliminate variations in ab ility among observers. The photographs do not reveal more animals than are seen by an experienced observer and in fact are less efficient in turbulent weather when the resolution on the negatives is impaired by scattered light, despite the use of Polaroid filters. Time and expense prevented a usable sample for breakdo wn into size groups being obtained as this could only be done in calm periods which did not occur on suitable occasions. Day Counts Counting in Februa ry 1965 was by one observer (the autho r) from a Piper Colt aircraft at 50 100 feet above water level. This gave good results on narrow shorelines, but where the animals were spread out over a wide area counting was impaired due to the relatively small visibilit y profile of the observer. The J u ne 1966 count used two experienced observers (M. Watson and R. Bell) both counting f rom the right hand side of a Cessna 182 from which the passenger
24 door had been removed to improve visibility. Airspeeds of 70 1 00 miles per hour were maintained a nd height at 700 800 feet above lake level T he technique adopted during the June 1966 c ount was designed with the follo wing points in mind. A total count was made rather th an samples to provide an index for the whole lake. An estimate of the actual numbers present was then made by correcting the day count by a factor derived from night counts (Table 2.) Rudolf crocodiles are particularly susceptible to total counting since the area involved is not large and the animals are confined to a relatively narrow belt of op en habitat. Water more than 0.25 miles from shore was not examined as in more than 500 crossings of the lake the author saw only one crocodile more than 0.25 miles from land (except in bays such as Gulf (Kalokol) .) The height of the aircraft abo ve ground was chosen to provide the observers with a visibility profile wide enough to include the entire habitat. This height meant that virtually no animals less than 100 cm long were seen. This actually impro ves the consistency of the count as animals t his size are difficult to see from a ny height, and at best will only be recorded erratically when they occur in exposed situations. In counting by this tec hnique there are 2 main sources of error: those animals simply missed by the observer ; and those an imals underwater at the time of observation. The proportion of animals missed by the observer varies slightly according to the time of day and counting conditions. Crocodiles are naturally cryptic and w hen the water is rough, or flying conditions turbulent somewhat more pass unnoticed than when con d itions are calm. One of the major pro blems in counting crocodiles is a psychological one of having to concentrate on searching for individuals of greatly varying size: there is a tendency to pick on a given size range only. The second source of error, those animals submerge d at the tim e of the aircrafts passage has not been assessed quantitatively. However, general observations suggest that only a small proportion is underwater at any given moment. In assessing t his, account must be taken of the pattern of movement in and out of the water during the 24 hour cycle (Cott 1961, p. 217.) Basically this pattern of activity consists of movement
25 into and out of the water at intervals throughout the day. (Mos t of the nig ht is spent in the water.) Two peak periods of lying ashore are evident, one in the morning and one in the evening. Crocodiles are somewhat easier to see when lying ashore but this behaviour does not greatly affect the proportion potentially visible as mos t animals in the water are on the surface, and therefore still visible. Night Counts In order to determine and correct the error present in day counts a technique of night counting has been developed. Hunting and capture experience has shown that an ac curate estimate of numbers can be obtained by counting eye reflections from a torch. The technique involves moving along a stretch of shore, usually wading to ensure that the entire occupied habitat is in the range of the observer. Distances more than 50 m from the torch (6 volt unter" lantern) cannot be considered accurately counted. Movement of the observer must be continuous, or the animals become di s turbed, and the norma l precautions of remaining down wind, silent and out of the light must be maintaine d. Most important is the necessity for an absolutely dark night, as the slightest moon provides enough light for a significant proportion of animals to see the observer. Approximately 3 mi les of relatively high density s hore gives a sufficiently large samp le. Results The results of counting are Shown in Figure 1, and Tables 1 & 2. Figure 1 shows Lake Rudolf with distribution expressed by the proportion of the whole population that occurs in each s ection. Also s hown is the actual density per mile of s hor e. Table 1 shows the results of both visual counts with estimates of actual numbers calculated with correction factors derived from night counts (Table 2). The two night counts in Februa ry 1965 gave an average correction factor for day counts of 3 1, and t he two night counts in J une 1966 an average
26 factor of 2.2. The technique on both occasions was the same and the lower difference between day and night counting in 1966 is attributed to the improved daytime counting on this occasion. February 1965 June 1 966 Section Miles shore Number counted estimate Number counted estimate Proportion of total (%) Density per mile 1 88 202 546 1,201 9.6 14 2 73 1,946 2,974 6,543 52.6 90 3 24 162 97 213 1.8 9 4 24 130 288 634 5.0 26 5 68 300 574 1,263 10.2 19 6 24 45 34 73 0.6 3 7 64 114 195 429 3.4 7 8 36 138 173 381 3.0 11 9 121 170 147 323 2.6 3 10 16 41 165 363 3.0 22 11 5 10 2 4 1.0 1 12 15 218 267 587 4.7 39 13 18 97 192 422 3.4 24 TOTALS 576 3,573 11,763 5,654 12,439 Table 1 Shows numbers of animals seen and estimated in February 1965, and June 1966. The estimates were obtained by multiplying day counts by a factor of 3.1 in 1965 and 2.2 in 1966 (Table 2.)
27 Locality & size Date Numbers (day) Numbers (night) Difference Corr ection factor used in Table 1 South end Moite Pool 1 X 0.25 miles Feb1965 67 259 2.86 3.1 Allia Bay 2 miles of shore Feb 1965 61 205 3.36 Allia Bay 3 miles of shore June 1966 344 672 1.96 2.2 North end Allia bay 2.5 miles June 1966 193 458 2.37 Tab le 2 Shows results of 4 night counts and the difference between the corresponding day counts.
28 Two night counts were made for each day count, and the sections of shore involved were recounted by day from the air to provide the comparative Fig ures for the correction factors (Table 2.) The total number of animals estimated to be in the lake in June 1966 was 12,439. This Fig ure was obtained by multiplying the 5 654 animals actually seen by a factor of 2.2, being the average of the two factors obtained from n ight counts. This total ignores ve ry young crocodiles less than 6 months old, as during the peak hatching period there may be large numbers of newly hatched animals. It can be seen that 79.9 % of the whole population occurs on the east shore of the lake, and 52.6 % on the north east section alone (Section 2.) The whole west shore only s up ports 12 % of the population, and the three major islands 8.1 %. Section 9 o n the west shore, of 121 miles, only supports 2.6 % of the population. Density has been expressed a s corrected act ua l numbers of crocodiles per mile of shore, although the animals are really clumped along the shore, as would be expected. The variation in densities is s hown in Figure 1. The highest density occurs in Section 1, with 90 animals per mile, a nd the lowest density on North Island, of one animal per mile. The west s hore (except for the Tu rkwell delta and Gulf (Kalokol) supports very low densities of 3 7 animals per mile. The high density of 39 per mile on Central Island is due to the accumulation of crocodile s in the small lakes there Density expressed as numbers per mi le of shore is useful o nly for comparative purposes as the animals actually live in the narrow belt made up of the immediate shore and the littoral stretching out 100 5 00 yards fr om land, depending on the degree of shelter. Thus the animals actually occ up y approximately 144 square miles of habitat (576 x 0.25 miles) and in fact the majority spends mo s t of the time within 100 yards of shore, and thus occupies only about 34 .6 square miles (576 x 0.06 miles). This represents a density of approximately 360 crocodiles per square mile of habitat. The highest density is found in Section 2 where 52.6 % (6 543 animals) occupy 7 3 mile s of shore. This represent s a habitat o f 4.38 1 8.25 square miles (100 500 yards from land) and
29 a density of 358 1 494 crocodile8 per square mile. Based on the age structure indicated in Figure 1 3 the total biomass o f crocodiles on Rudol f is e s timated as 3 25 ,000 kg. This repre s ents, in Section 2, 9000 59 000 kg per square mile an d compares interestingly with so me of the higher densities in terrestrial situations (Bouliere 1963, Lamprey 1964) Their pre f erence for shallow water thus confines them to a relatively small total area of habitat compared to t errestrial animals, a fact not generally appreciated, and one that imposes low limits on the relative number s of crocodiles in East Africa. Cott (1961) p. 278, has commented o n the apparent scarcity of small crocodiles 60 120 cm long even going so far as to say that these animals" go into retreat," and notes that many other observers make similar comments. This effect is almost certainly a result of confining observations to the day. In order to gain a subjective impression of the numbers of crocodile in a ny given area it is essential to observe them at night with a torch when a very different impression will be gained to that obtained by day. Croc odiles are exceptionally cryptic wary animals and even o n Rudolf where cover is sc ant, casual daytime observat ion reveals only a few larger animals. At night on Rudol f the impression is gained of gradual ly decreasing numbers with size: certainly no particular size gro up is absent. Cott, p. 316, also comments o n the segregation o f age gro ups. During t he day there is a tendency for the larger an imals to be associated with exposed basking sites but at night when actively feeding, crocodiles of all sizes may be found together and no pronounced segregation into size groups was observed on Rudolf. As large crocodiles will eat smaller ones there may be same avoidance of bigger animals by small ones at close quarters, but if this occurs it does not result in segregation into size groups.
30 1c Factors influencing crocodile distribution At the beginning of the survey 14 days were spent on North Island to eliminate the crocodiles there. It was intended to make regular aerial counts of the Island from then on to observe changes in numbers, but expense prevented this, so the experiment was abandoned. No other experimental work on movement and distribution was attempted. The main factors influencing dist ri bution on Rudolf are thought to be shelter and possibly food availability. Figure 1 shows that most of the population (79.9 % ) occurs on the east shore and locally distri bution is clumped with concentrations occurring always in relatively sheltered places. The long sandy beaches of the west, exposed to the surf and with no vegetation, are very sparsely populated. Gulf (Kalokol), the only large body of sheltere d water on the west shore, is noteworthy in supporting a relatively high density of crocodile despite continuous shooting of crocodiles there by the Game Department, and year round human activity along the s hore. When hunting crocodiles it is very obvious that they avoid rough water. C ott (1961) p. 218 notes that crocodiles show a marked tendency to come ashore during rough weather. It was often observed on Rudolf that during spells of rough weather, particularly when winds blew from unusual directions caus ing water to surge into normally sheltered places, that crocodiles rapidly moved from such areas. These animals would then accumulate in sheltered spots, often not normally frequented. Rough water generally is avoided and almost certainly is the major fact or governing crocodile distribution on Rudolf. The persistent easterly wind generates an almost continuous swell and the resultant surf along the west shore prevents the growth of aquatic plants. The long sandy beaches are only occasionally interrupted by sheltered inlets and the entire habitat is avoided by crocodiles. The reason for this avoidance of turbulent water is not immediately apparent but may be partly due to difficulty in breathing. Crocodiles swimming in rough water tend to keep the mouth open so that the nostrils are clear of the surface. Although when not actively swimming they can stay submerged for long periods
31 and rise to breathe for a few seconds only, it is likely that when engaged in continuous activity such as swimming, their oxygen requirements are much greater, necessitating frequent aspirations. Thus merely submerging to avoid waves may not be possible if continuous activity is required as well. While hunting or observing crocodile the impression is gained that long distances are c overed by swimming on the surface, not under water. Crocodiles on beaches with surf tend to come ashore again rapidly after being disturbed, while crocodiles in sheltered water may remain off shore for protracted periods possibly many days at a time, as was observed by the author on North Island. Crocodiles in Gulf (Kalokol) spend extended periods, probably many days, off shore, where the continuous human activity discourages them from coming out. It is highly like ly that this is in part made p ossible by th e shallowness of the water (10 20 feet,) permitting them to 27 rest on the bottom with little effort getting to and from the surface. It is of interest to note here that it is clearly unnecessary for a croc odile to adhere to the normal r hy thm of movement in and out of the water in which much of the day is spent lying ashore. The function of this behaviour is not clear but may be nothing more than for relaxation after the feeding activity of the night. It may also be necessary for the anima l to absorb solar energy but this is possibly still done when floating on the surface. Associated with the tendency of crocodiles to avoid rough water, and therefore accumulate in sheltered water, is the obvious advantage of calm water in feeding (see s ection on feeding behaviour.) Further, sheltered water tends to have much more vegetation in and near it, providing the shelter favoured by small crocodiles (Cott 1961, p. 277) and probably necessary for their survival. To what extent fish densities are a ffected by the wind is unknown but casual observation suggests that fish on Lake Rudolf are more numerous in sheltered areas. The availability of food fish may well va ry according to the degree of turbulence of the lake and this may be influencing crocodi le distribution. 28
32 There is evidence that considerable distances are covered by cr o codiles on Rudolf. Modha (pers. comm.) observed crocodiles with pieces of fish net caught on their heads at Central Island : these could only have come fro m G ulf (Kalokol) 9 miles away. Much of the lake is only 6 15 miles wide and it is possible that crocodiles regularly cross the lake. A crocodile shot on North Island contained the remnants of a large mammal it must have eaten on the mainland 9 miles away, as none occur on the island. They certainly move along the shore. In the 4 years since 1962 over 500 crocodiles have been shot in Gulf (Kalokol) by the Game Department and many others wounded and lost. The present population of 363 a nimals is not noticeably different to what it was in 1962, according to R. McConnell (pers. comm.) the fisheries o fficer stationed there. As it is unlikely that any eggs laid near Gulf (Kalokol) escape discovery by the human residents, steady recruitment of t he Gulf animals must be occurring from s o me other source. As it is located on the extensive sandy beaches of the west shore with a density of only 3 animals per mile, there is obviously considerable movement of animals along the shore resulting in an accum ulation in the Gulf.
33 Section 2. Food choice Methods All stomachs were examined in the field and their contents recorded. U nidentified remains were preserved for identification in the laborato ry Three categories were recognized; stomachs containing fr esh food, those containing remnants only, and empty stomachs. If fresh enough, the approximate size of the food animals was recorded. Observations of feeding behaviour were made in the field and on captive specimens. Results 2a Feeding Behaviour To fa cilitate comprehension of the observed food choice and the differences between various populations a superficial account of feeding behaviour is presented first. The Nile crocodile is basically an aquatic opportunistic predator. Animals at the e dge may be taken and there is some evidence that large crocodiles may lie ashore and ambush animals c om ing down to drink although this may only occur fortuitously. Cott (1961) p. 302, records that ve ry small crocodiles may hunt terrestrial insects on land but most insects are probably caught near the edge, or after having fallen into the water. As carrion feeders, they will travel considerable distances overland to carcasses and the author has observed a crocodile following a dragged lion kill, pre sumably by the scent trail. Feeding is done mostly at night. A high proportion of crocodiles are in the water at any given time at night and considerable feeding activity is commonly o bserved.
34 Fish, the main food source of Nile crocodiles generally, are caught mostly in shallow water. Observations of animals at night with a torch (most individuals if unaware of the observers are undisturbed by a moderate torch light) s how most feeding activity to occ ur in very shallow water. They float in shal low water w ith their feet on or near the bottom, mouth open, and eyes half above and half below t he surface. Movement is slow, half swimming, half walking, with frequent periods of inactivity. Sho als of fish moving through the shallows are snapped at when within a fe w inches of the crocodile. Frequently no attempt at catching is made until the fish actually touches the crocodiles head and fi s h have been observed swimming right into the open mouth. Although no quantitative data exists it is evident that the majority of feeding "bites" fail. Bites are made with a rapid sideways movement of the head, the closing of the mouth being accompanied by a rolling over of the head about the longitudinal axis, so that the jaws meet with the lateral face of the head parallel to the ground. The function of th e turning over of the head is not clear, but in catching fish like tilapia it may be to overcome the tendency of these fish to escape by leaping out of the water. Rolling the head over the fish will pin it down, reducing the opp ortunities for escape. In picking up objects from the ground, such as a dead fish, turning the head over allows the teeth to be brought right down over the object concerned. The tendency to feed in shallow water has the obvious advantage of keeping the p rey anim a l and crocodiles head on the same level, thus minimizing possible avenues of escape. In captive a ni mals a tendency to utilize obstru c tions such as rocks or the corner of the ta nk to co nfine the prey has been observed. Two other, more specialized methods of catching fish have been observed. The first, seen only in captive animals, involves the crocodile approaching a shoal of fish near the shore from deeper water. The crocodile slowly runs
35 aground so tha t its head becomes half awash. It then gr adually curves its body a round, so that the tail tip eventually co m es ashore It then bites at the trapped fish, the whole pattern being perfo rm ed smoothly, in one movement. The second technique is associated with their observed tendency to approach spla shes made by shoals of fish jumping. Any disturbance caused by fish, or by other crocodiles feeding, will often stimulate a nearby crocodile to approach. A crocodile thus alerted, and if within a suitable distance, sometimes leaps right out of the water to wards the fish. If the animal lands among the fish, it may bite at them immediately. Often the startled fish become temporarily disabled by swimm i ng or jumping into excessively shallow w ater. Crocodiles, like most predators, are quick to notice animals i n difficulties and to take advantage. Pitman (1961) gives a description of the attempts by 8 crocodiles to catch a flightless (moulting) spur w ing goose that took to the water at the approach of a boat. The bird was evidently recognized as being i ncapacitat ed and immediately became an object of a ttention to the nearby crocodiles. It may be that the primary function of the spectacular leaps that crocodiles frequently make is to frighten fish into situations where they are easier to catch. Of interest is the fact that identical leaps are sometimes made by crocodiles disturbed by the observer. In such instances, it serves as an escape mechanism to get the crocodile into deeper water rapidly. Crocodiles frequently catch fish too big and strong to be easily su bdued. Several observers have watched such catches being taken ashore and battered on the ground until they cease struggling. In battering a fish thus, the head movements are identical to the feeding bites, except that the jaws are kept closed. The rolling over of the head here serves to ensure that the fish, held sideways in the mouth, is brought into contact with the ground.
36 The normal fish catching bites are in response to two basic stimulation sources : vis u al and tactile contact. They will frequently bite at inanimate objects such as stones which are accidentally touched while the animal is moving about During the survey many nights were spent wading in water up to 5 feet deep while hunting. Crocodiles were frequently seen feeding, but always in ver y shallow water. Crocodiles have been observed catching fish in deep water (Cott p. 30 3) but the technique adopted in such instances is unk nown. Kinloch (in Cott, page 3 03 ) mentions the taking of angler s artificial lures in deep water by crocodiles. These instances may, h ow ever, stem from the attraction of an unusual object to the predator, a phenomenon possibly associated with the abno rmal movements of distressed animals. The s ignificance of the extensive weed beds on Rudolf as a high density crocodile h abitat is probably associated with the conditions necessary for catching fish. The weed provides support for the crocodiles, and restricts the movement of the fish. Crocodiles have often been observed feeding in the weed beds, where conditions often resemb le those of shallow water, the tops of the weed here simulatin g the ground in the shallows. In feeding on animals other than fish, two basic techniques may be recognised. Firstly, there is the captur e of animals without the use of elaborate stalking or a mbush, in which approach is made openly. In this catego ry is the fortuitous catching of passing insects Secondly, animals which would recognize a crocodile as a potential source of danger, such as most mammals and birds, have to be take n by surprise. In t hese instanc es, the crocodile m ay stalk a watering m a mmal by approaching underwater, or it m ay take advantage of ambush. Like snakes, small crocodiles have to swallow their food whole and thus the size of the food animal taken will be determined partly b y the relative size of predator and prey. But with large crocodiles the situation is different, as these
37 can overpower animals too large to swallow, whose inertia as a carcass is sufficient to permit the crocodile to tear off pieces to swallow. Crocodiles feed readily on carrion and descriptions of the techniques for tearing off pieces are given by Attwell (1954). The animal may tear off fragments simply by shaking the head from side to side, like a dog does, or by holding the flesh and then spinning the bo dy about the longitudinal axis, thus tearing off a piece. 2b Analysis of stomach contents The stomach contents of 493 animals examined during the survey are s ummarised in Table 3. Category Number Proportion (%) Total number of stomachs with food 25 4 51.6 Total number of stomachs without food 239 48.4 Number of stomachs containing fish only 223 45.4 Number of stomachs containing fish and other animals 8 106 Number of stomachs containing animals other than fish 23 4.6 Table 3. S tomach contents of 493 Rudolf crocodiles. Of those stomachs containing food, 90.9 % contained fish, and 87.8 % fish only. 9.1 % contained other animals only. 48.4 % of the stomachs were empty 22 % contained hard food remnants only such as bones o r scales and 29.6 % contain ed soft, relatively fresh food. No difference in food animals or incidence
38 of e m pty stomachs was observed between males and females, between croc odiles from Moite or Allia, or from month to month, although slight differences would pass undetected, as the s amples generally were small. In addition to food, the stomachs of all 493 animals examined contained stones. Cott (1961) discusses so called stomach stones at some length and suggests a possible hydrostatic function No other data on stomach stones were c o llected during this survey.
40 Table 4 sho w s the proportion of various food animal types in the Rudolf sample and compares this with 9 other localities. Individual s a mples are too small for critical comparison, but the results are sufficient to ill ustrate a number of points. First ly in the majority of localities, insects are the most frequently recorded food animal. However, as pointed out by Cott, p. 279 they are taken exclusively by small crocodile s. Insects are not recorded from crocodiles >250 cm long, presumably owing to the size difference between predator and prey. After insects, fish are the most frequently taken food animal. Cott found crocodiles of all sizes feeding on fish and this is the case on Lakes Rudolf, Kwania and Kioga. Molluscs are frequently taken and in a wide range of localities. Reptil es, mammals and birds form a si gnificant part of crocodi le diet in most localities and c rustaceans (mainly crabs) are eaten where they occur. Fish are clearly the most important food source for Nile crocodiles generally as they are taken throughout life and in higher proportions than other animals. Cott, p. 278, concluded that the very wide variety of food animals that have been recorded from crocodile stomachs reflected a progressive change in d iet with age. His data do not however support this conclusion. Of the 9 animal types recorded by Cott, 6 are eaten by crocodiles of all sizes from 50 cm onwards. Three groups only eaten by very small crocodiles were insects, spiders and amphibians, almost certainly a reflection of the discrepancy in size between larger crocodiles and arthropods and amphibians. It seems more likely that crocodiles will eat any available animal and the stomach contents of any particular crocodile will reflect the local availa bility of potential food animals and the relative sizes of predator and prey. Crocodiles from Rudolf, compared to most other habitats, show two striking differences: a high incidence of empty stomachs, discussed later, and an almost exclusive feeding on fish. This latter is readily explained by the very obvious
41 lack of other food animals in and around the lake, such as large insects, amphibia ns and small mammals. Also scarce are small birds (except waders), lizards and snakes. Molluscs and crustaceans ava ilable to crocodiles are non existent. The lake and its surrounds constitute, from the biological point of view, a very barren environment compared to other East A frican crocodile habitats. The incidence of reptiles (other crocodiles and chelonia) taken is similar to elsewhere, but birds evidently fewer. Here again, the difference is probably due to the lower incidence of darters and cormorants which are the birds most commonly recorded by other observers. The only birds common on Rudolf (except in isolated suitable habitats such as Gulf (Kalokol) and Central Island) are various waders and Egyptian Geese, which are probably infrequently taken as their occupation of s hallow water could be expected to make it easy for them to detect and avoid an att acking crocodile. Table 5 compares the incidence of various fish types in stomachs of crocodiles from 11 localities in Zambia, Uganda and Kenya. Again samples are mostly small, but trends are detectable. Firstly, c ichlids, mostly tilapia species, are every where except lakes Mweru and Kalunguishi taken in relatively high numbers and must be considered the most widely eaten fish type. Other types are taken in variable proportions, but m ochokids ( S ynodontis spp.) c lariads ( Clari a s s pp .) and c haracins, ( Hydrocy on and Alestes spp.) are taken in the widest range of habitats. All these fish frequent the shallo w s. But the fish most frequently taken varies considerably. In Uganda above Murchison Falls (including Lakes Kioga and Kwania) it is a Protopterus ; in Mweru a nd Kalunguishi almost exclusively Clarias mossambicus Peters, and on Rudolf almost exclusively Tilapia nilotic a vulcani Trewavas. In other areas more than one variety may be taken in equal proportions.
43 The preponderance of tilapia in Rudolf stomachs is obscure. To superficial observation Clarlas mossambicus and Alestes spp are common fish of the shallows and could be expected to form a more important part of their diet. Of interest are Cott s observations from Mweru wa Ntipa where 49 % of all stomach contents were fish. Of these 97 % were Clarias mossambicus despite the abundance of tilapia There are obviously many factors influencing food c hoice Table 6 compares the incidence of empty stomachs among crocodiles from Rudolf and other habitats. s data, which he does not present separately, are from a variety of Uganda and Zambian localities. Locality and source Sample Percentage of stomachs with food Percentage of stomachs empty Zambia Cott (1961) 591 85.9 14.1 Lakes Kioga and Kwa nia (Uganda) (from Hippel (1946) 587 76 24 Rudolf 493 51.6 48.4 Table 6. Compares the proportion of empty to food containi ng stomachs in crocodiles from 3 localities. The very much higher incidence of empty stomachs on Rudolf (48.4 % ) compared t o elsewhere (Hippel 24 % ; Cott 14 1 % ) is outstanding and unexplained. The highest local incidence recorded by Cott was 27.5 % (Mweru and Kalunguishi), still much lower than for Rudolf. In both these localities fish
44 form a high proportion o f all stomach conte nts and in both instances there is alm o st exclusive feeding on only one fish species. Cott, p. 282 found a higher incidence of empty stomachs in crocodiles over 180 cm than in immatures (29.5 % and 9.1 % respectively) but on Rudolf the proportions were si milar (51 % and 41 % respectively.) The relatively high incidence of empty stomachs on Rudolf is not accompanied by any obvious deterioration in the p h ysical conditi on of the animals. Less than 1 % of the animals examined were described as being in "poor co ndition". Nevertheless, they obviously feed less frequently than crocodiles elsewhere and while this may only mean that fish are a more easily convertible food source t han other animals, it is suggested her e that it is a consequence of so me unexplained fun ction of availability. Young crocodiles in captivity if supplied with an excess of fish will feed every day. This suggests that the hi gh incidence of empty stomachs o n Rudolf is a consequence of low food availability and that the animals would feed more fr equently if they could. If, as postulated, the animals are not obtaining a maximal amo unt of food then a likely co m pensation would be a l ow ering of growth rate. This is discussed further in Section 4. 2c Quantity of Food Cons umed There are no quantitati ve data on the amo unt of food consumed by wild croco d iles and the results just discussed show that this varies considerably. However, it is possible to calculate approximately the amount of food that could be consumed by a wild population from a considerat ion of captive animals. Pooley (pers. co mm ) has extensive, a s yet unpublished, data on the amount of food cons um ed by captive animals. He observed hatchling crocodiles to consume approximate ly 3.8 % of their weight in food per day under conditions where e xcess food was available. Further he found that they consumed similar quantities per meal whether they were fed daily, or at 3 day intervals, s uggesting that a stomach fill is digested in 24 hours. Essentially similar results
45 were observed by the author in 3 captive hatchlings fr om Rudolf that were kept in Nairobi in a tank with the water temperature maintained at approximately 260 o C and fed on live fish. Cott (1961) describes observations on 2 crocodiles of 230 cm which consumed an average of 0 8 % of the ir weight in food per day, although the details of the conditions under which they w ere kept are not given. Nevertheless, a falloff in the rate of food intake would be expected to accompany a falloff of growth with age Based on the age structure in Figur e 1 3 the biomass of crocodiles on Rudolf has been estimated as 525,000 kg. Ingesting 3.8 % of their weight in food per day would represent 4,000,000 kg fish per annum (90.9 % of food is fish). Wild crocodiles on Rudolf at their current density, are obviou s ly feeding at a much lower rate and will consume only a small fraction of this maximal amount. Nevertheless the Fig ure is of interest to the prospective crocodile farmer who will aim at maximising the rate of growth in his animals.
46 Section 3 Reproducti on The purpose of this section is to describe, superficially, the reproductive process in both sexes, emphasizing those as pects relevan t to crocodile population dynami cs at the expense of a more detailed treatment of the me chanics of reproduction. 3a Rep roduction in the male M ethods All an ima ls killed were examined in the field and their re productive state assessed. Testes were weighed on an Ohaus triple beam balance measured (lengt h, breadth and d epth) and the vas deferens examined for the presence o r a bsence of semen. Several entire reproductive tracts were collected S emen samples were examined microscopically on the spot for the presence of sperm. Testis tissue from 52 animals of all sizes and at all times of the year was collected and fixed in a 10& formaldehyde solution. T issue from 24 anima l ranging in length from 150 470 cm was ultimately sectioned for histological examination. Sections were cut at 5 and stained with a standard ha ematoxylin/eos in technique. Tissue stored in f ixative for up t o 18 months was found to be satisfactory f or the determination of spermatogenesis and the measurement o f tubule diameters. R esults A well established breeding season occurs in most reptiles and is described for the Nile crocodile C ot t (1961) and Modha ( 1967) Basically, in the male there is a n annual build up of spermatogeneti c activity c ulminating in a period of sexual ac tivity. This is followed by a q uiesc ent phase during w hich no spermatogenesis o cc urs and the testis de c reases in size. In the male cro codile the events ac c ompanying the onset of maturity and the cycle of reproductive activity are
47 briefly as follows. In the immature animal the testis is small, weighing less than 10 g. With the onset of spermatogenesis the testis increases in size weighin g, in an animal 470 c m long, over 700 g. The a c tive testis is pink in c olour while the m ature but inactive testis darkens to a mottled purple; this is not however an accurate indication of the state of sexual activity of the testis. With the production of sperm, se minal fluid begins to accumulate i n the vas deferens. Microscopic examination up to 4 hours post mortem of semen from both the testis and the vas deferens shows a dense mass of motile sperm A tendency for the spe rm to occur in clumps of 15 30 arr anged symmetrically with the sperm bodies together and tails radiating outwards was frequently observed. Histologically the immature testis is characterized by extensive interstitial tissue and small tubules with a relatively thin germinal epithelium. As spermatogenesis begins the epithelium starts proliferating and the tubules enlarge until their diameter is three times that of the immature testis. Very little interstitial tissue is present and the testis is composed almost entirely of seminiferous tubule s. In the mature but quiescent testis the tubule diameter is reduced by almost half and no spermatogenetic activity is seen. Figure 2.A. Z.67. Immature testis with extensive interstitial tissue, small tubules with only a thin germinal epithelium. 63 di am.
48 Figure 2.B. Z 68. Mature testis at the height of spermatogenesis. The tubules make up most of the testis volume and interstitial tissue is reduced to a thin layer between tubules. A thick germinal epithelium is present with a columnar appearance. C lumps of sperm tails are seen projecting into the lumen of the tubule. 63 diam. Fi gure 2. C B.10 Mature but quiescent testis. Tubule diameter is reduced, there has been an increase in interstitial tissue and the germinal epithelium is reduced and som ewhat disorganized. 63 diam.
49 Figure 2.D. X. 3. Mature testis considered to have become non functional. Tubules are similar in size to an immature animal and the interstitial tissue is extensive. The germinal epithelium is almost completely brok en down 63 diam. The 3 basic conditions of the male testis, namely immature, mature and, active and mature but quiescent are shown in Figs. 2A, 2B, 2C and 2D. The product of length, breadth and depth in 35 a n imals with testis weights in the range 1 40 g are l isted in T able 7 with the corresponding testis weights. Th e s e data have been plotted in Fig ure 3 as the cubic factor against w eight and the regres s ion of cubic factor (y) on weight (x) c alculated. The regression coefficient was 1.5492. This w as done to pro vide a ready means in the field, where a n accurate balance may not be available, of determining the t estis weight in animals that are near the point of sexual m aturity.
50 Testis wt (g) Testis cubic factor (cm) Testis wt (g) Testis cubic factor (cm) 1.0 0 .6 16.5 22.5 2.0 1.4 16.5 24.0 2.7 2.7 17.0 27.6 3.5 4.3 18.0 36.0 4.5 5.5 18.5 22.6 5.0 5.5 19.0 44.8 6.0 10.5 20.5 30.2 7.0 22.1 20.0 37.8 9.4 12.4 21.0 35.0 10.0 11.2 20.0 40.0 11.0 19.1 23.0 50.4 11.0 18.5 24.0 28.0 13.0 33.6 25.0 30.4 14. 0 35.8 25.0 38.5 14.0 22.5 29.0 50.0 15.0 35.0 35.0 45.0 15.0 24.4 38.0 61.0 16.0 32.5 Table 7. Lists testis weights and the corresponding product of testis length, breadth and depth, in 35 animals with testis weights in the range 1 40 g.
51 Figure 3. The product of length, breadth and depth plotted against weight in 35 testes in the range 1 40g. The regression coefficient was 1.5492 D etermination of Sexual Matur ity It is important to determine the point of sexual m aturity, but in an animal where the mature but quiescent t estis superficially resembles that of an immature animal this i s not always easy. Cott (1961) p. 253, recognized "breeding" m ales as those with testes "enlarged and distended." During t his survey the criterion of sexual matu rity adopted was t estis weight, and the criterion of sexual activity wa s the p resence or absence of semen in the vas deferens. There is no other structure in the male reproductive tra c t capable of s toring significant quantities of semen and it is assumed t herefore that any male in this condition is mature and p otentially capable of fertile copulations. W hether recently m atured males do in fact breed is unknown. Observations by M odha (1967) p. 83, suggest that it is the older males that are chiefly involved in reproduction. This however,
52 probably depends on the age structure of the population concerned with the younger males reproducing where no older males are present. In order to determine the weight at which the testis begins to mature the presence or a bsence of semen in the vas deferens has been related to testis weight in 70 males examined between July and December, the period of greatest reproductive activity (Table 9 and Modha 1967.) Animals examined during the first half of this year were eliminated to minimize confusion with matur e but quiescent individuals. Fro m Table 8 it is evident that no animals with testis weights of < 5 g are mature. In the range 6 10 g 35.7% were producing semen while from a t estis weight of 11 g onwards 50% or more are matu re. Those testes in the weight range 11 35 g that were not producing semen may have been quiescent rather than immature, but short of histological examination there is no way of deciding. For the purposes of this survey 50% or more males are considered to be mature at a testis weight of 11 g. Testis weight (g) Sample Number with semen Percentage with semen 0 5 17 0 0 6 10 14 5 35.7 11 15 5 3 60 16 20 5 3 60 21 25 6 3 50 26 30 1 1 100 31 35 4 2 50 >36 18 18 100 Table 8. Shows the incidence of se men in the vas deferens of 70 male crocodiles examined between July and December, expressed as the percentage of animals with semen in the sample for each of 8 testis weight ranges.
53 The mean length of 21 animals with testes weighing 5 15 g was 270 cm, v arying from 212 336 cm. Cott found in a sample of 65 animals from Zambia that none were breeding until 280 cm length suggesting that crocodiles on Rudolf mature at shorter lengths than animals from other localities. The Breedin g Season The incidence of semen in the v as deferens of 144 crocodiles recorded over a 12 month period is shown in Table 9. Testes <10g Testes >10g Sample No with semen Percentage with semen Sample No with semen Percentage with semen Jan Mar 20 0 0 13 0 0 Apr Jun 7 0 0 34 19 56 Jul Sep 12 5 41.6 18 12 66.7 Oct Dec 19 0 0 21 20 95.3 T able 9. Shows the incidence of semen in the vas deferens of 58 crocodiles with testes weighing less than 10 g and 86 crocodiles with testes weighing more than 10 g collected over a 12 month p eriod. The results are expressed as percentages of animals with semen in the samples for each 3 month period. In the sample of animals with testis weights of more than 10 g it c an be seen that there is a well m arked cycle of sexual activity through the y ear, with no animals active in Jan uary March and 95.3 % active by October December. It is interesting to note that by April June 56 % were potentially active. How much reproductive activity actually accompanies the appearance of these potentially active mal es is not known. Modha (1967) did not observe copulation on Central Island until the second week in October, las ting until December; but this is
54 certainly not a true reflection of the events on the mainland where hatching nests were found in late August an d September at Allia bay. Fertilization of these eggs would have occurred in May June which agrees with the condition observed in the female. These points are discussed again below. 3b Reproduction in the female Metho ds All animals killed were examin ed in the field and their reproductive state recorded. Ovaries and oviducts were weighed on an Ohaus triple beam balance whenever possible. The number and size of developing follicles or ova was recorded. Ovary and oviduct tissue from a representative samp le of animals was fixed in a 10% f ormaldehyde solution for histological examination. Whenever possible the whole ovary was collected and preserved. Four entire reproductive tracts were collected. Results Reproduction has not previously been described in any crocodilian, but a number of workers have investigated various lizards, snakes and turtles. C agel (1944) working on the turtle Pseudemys scripta found that the mature ovary has 2 3 sets of enlarging follicles, development of which result in one set a season maturing. In Lacerta agilis (Marshall 1956) the various stages of oogenesis may be seen at any time of the year but mature follicles are present only in April and May. In many reptiles the ovarian follicle after disc h arging the ovum develops into a str uc ture that has been considered analogous to the mammalian corpus luteum (M arshall 1956 p. 468.) The function of these "corpora lutea" is u nknown. Accompanying development of the follicles the oviducts increase in size and vascularization
55 In the N ile crocodile t he immature ovary is small, weighing up to approximately 26g, and consist s of a clear cortical tissue with numerous clusters of oocytes distributed evenly throughout the organ. The c lusters are close together giving the whole ovary an even, granular appearance. As the animal matures, follicles begin developing. These grow and begin to protrude from the ovary, eventually separating almost entirely from it. Development continues until the ova are 4 5 cm in diameter and weigh 30 40 g. At this stage they are shed through an aperture on the surface of the follicle and pass into the proximal oviduct. After ovulation a "corpus lute um develops from the ruptured follicle. This consists of a thick walled brownish sac about 1 cm in diameter) which, i n recently ovulated ovaries, is found everted and connected to the ovary by a short stalk. More usually it is inverted with the body of the structure visible through the clear cortical tissue of the ovary. After ovulation the corpus luteum regresses, event ually vanishing altogether. The time taken to do so is unknown, but it appears that by the time the next set of follicles is well developed no signs of the previous corpora lutea remain. Immediately after the ova are shed the ovary is flabby, enlarged an d heavy, weighing, in animals of 275 320 cm 400 2 000 g. It then undergoes resorption eventually be corning small and firm, 30 300 g in weight, with corpora lutea in various stages indicating recent activity. The ovary then appears to go through a stage o f quiescence as regards oogenesis (Table 12). Of 121 sexually mature (but not senescent) animals examined 15 (12.4 %) were evidently quiescent with no large follicles developing. 106 of these 121 animals were either ovulating or had just laid, and of thes e 16 (15.1 %) had another, smaller set of follicles developing concurrently, producing a conditi on of the ovary similar to that observed by Cagel (1944) in P. scripta Thus some animals may only have one batch of ova developing at a time, while others have 2 batches. The inconsistency on this point and its significance is unclear, but it appears that : either an animal may produce one batch of eggs with no other folli cles developing until regression of the corpora lutea is well advanced or c o mplete,
56 which cy cle of events probably occupies a complete season (one year) ; or maturation of a batch of ova m ay be accompanied by development of another, youn ger batch so that soon after egg deposition a second set of ova may be near maturity. The obvious consequence of this latter condition is that 2 batc hes of eggs might be produced in one season. This point is discussed again in the section on breeding season. Of the total of 127 mature females examined 6 (4.7 %) appeared to be totally inactive rather than quiescent. No c o rpora lutea were present and the o vary presented a s mooth mottled appearance caused by confinement of the oocytes to small, widely spaced clusters so that the cortical tissue comprised the major part of the ovary. Such ovaries had probably ceased pro ducing ova altogether, particularly as t hey were all from large (and presumably old) females of > 290 cm. Altogether 35 animals > 290 cm long were examined of which 6 (18%) were inactive. It thus appears that in the older f ema les a cess ation of reproductio n eventually occurs, suggesting that senescent processes affect crocodiles, which will have some consequence on their population dynamics The oviduct consists of 2 readily distinguishable sections The first part, here called the pro ximal oviduct, is th in walled, much convoluted and slightly longer than the second part. It is more extensively vascularized, blood vessels being especially prominent in the mature, active animal. The second part, or distal oviduct, is shorter, less convoluted and with much t hicker and whiter walls. The whole length is deeply folded longitudinally presumably to allow f or expansion when containing eggs. After ovulation th e ova pass into the proximal ovi duct. No animals were found during the survey with ova in the proximal ovid uct suggesting that they do not remain t here for long. F ertilisation p ossibly occurs here and it may participate in the initial formation of the egg membranes and shell. The distal oviduct may be found containing eggs, either still soft with very little of the hard shell formed, or with fully formed and shelled eggs Thus the
57 distal oviduct is where at least part, if not all, of shell formation occurs. Prior to deposition the eggs are seen fully formed and covered in a thick, gelatinous substance. The ovid ucts lead straight int o the cloaca and the only other macroscopic organs which may be accessory to the reproductive tract are the cloacal glands. These are small and lie lateral to the cloaca with a short duct opening into it and secrete a viscous orange s ubstance with a strong, distinctive smell. Their function is unknown. At this point it is worth mentioning the gular glands which are almost identical in size and appearance to the cloacal glands and secrete what appears to be the same substance. Their function is also unknown. The final stages in the reproductive process have been described by Cott (1961) and Modha (1967). When ready to deposit the eggs the female comes ashore and digs a hole in the ground, usually s omewhat above high water level and not far from the shoreline The environment of the nest site does not appear to be critical as nests may be made on exposed sandbanks with the eggs only 10 cm beneath the surface, or in sheltered positions in vegetation. The eggs after deposition are cov ered up and throughout the ensuing 3 month incubation period the female tends to spend considerable time ashore on or n ear the nest. If disturbed by activities such as hunting the females may not actually lie ashore, but evidently continue to frequent the vicinity as such animals unearth the ready t o hatch eggs in the no rmal manner. The hatching embryos e mit a variously described "squeak" which probably stimulates t h e female to unearth the eggs. Most nests seen on the east shore were constructed in a substr ate which, during incubation, becomes so hard that the young are unable to dig themselves out. Unearthing by the parent is essential in these cases for survival. In the authors experience it has been the rule rather than the exception for the young to requ ire digging up by the female and the significant feature of
58 the female s presence by the nest during incubation is probably the necessity for unearthing it rather than any factor concerned with incubation. The consequence o f this behaviour on crocodile co n servation is o f great importance as severe di sturbance of females before the nests have been dug up will result in high egg mortality. The f emale also acts in a guarding" capacity as evidenced by the aggressi ve behaviour observed by M odha (1967) p. 90, al thou gh her continued presence near t he eggs may attract rather than repel predators such as monitor lizards ( Varanus niloticus ), h yaenas ( Crocuta species) an d baboons ( Papio species). Length (cm) Sample Number mature Percentage mature < 160 30 0 0 161 170 19 1 5 171 180 23 1 4 181 190 23 10 43 191 200 23 11 47 201 210 35 18 51 211 220 42 33 79 221 230 18 10 55 231 240 2 1 50 241 250 9 3 33 251 260 3 2 67 261 270 4 3 75 271 280 5 4 80 281 290 8 7 88 291 300 13 11 85 301 + 23 23 100 Table 10. Lists the numbers of mature animals in each of 16 length groups in 280 female crocodiles from Lake Rudolf T he onset o f sexual maturity This was determined simply by considering an animal potentially mature when de veloping follicles were visible in the ovary. Cott (1961) used a similar criterion
59 when he considered females with ova more than 0.2 cm in diameter to be mature. The same criterion was adopted during the present work Figure 4. The proportion of matur e females in each of 16 size groups plotted against body length for 280 female crocodiles from Rudolf. The regression coefficient was 0.4065 Table 10 shows the proportion of mature to immature animals in each o f 16 length groups in 280 f emales from Rudolf I t can be seen that although some animals are maturing at lengths of 161 180 cm, the onset o f maturity in Rudolf crocodiles may be considered well marked from 180 cm onwards. An increasing proportion of mature animals is seen with increasing length and i n order to
60 d erive a value for age specific fecundity for use in the female life Table the regression of maturity (y) on length (x) has been calculated. The regression coefficient was 0.4065 and the result is shown in Fig ure 4. Comparable data for crocod iles elsewhere does not exist but Parker (unpublished data) has records of 8 mature and 15 immature animals from the Galana river, shown in Table 11. Length (cm) Sample Number mature Number breeding <190 14 0 0 191 200 2 1 0 201 210 2 2 1 21 1 220 0 0 0 221 230 3 3 2 231 240 0 0 0 241 250 1 1 0 251 260 1 1 0 Table 11. Shows proportion of mature animals relati ve to length in 23 female crocodiles from the Galana river (Kenya). (Parker: unpublished data). Although too small a sample for critical comparison the abs ence of mature animals < 190 cm length and the onset of maturity at about this length is consistent with the Rudolf data. Cott (1961) while giving no details, states that for crocodiles from several Zambian localities about hal f the females are breeding at 300 cm length. The smallest breeding female he found was 238 cm. These comments suggest that maturity is attained at considerably longer body lengths than on Rudolf. Co tt (1961) p. 254, also quotes previously unpublished data o f collected during a Uganda Game Department campaign to exterminate crocodile s on Lake Victoria. He records a sample of 855 females shot and measured on the "breeding grounds" and therefore considered to be representative of the female
61 b reeding p opulation. The mean length of these 855 females was 318 cm varying from 219 462 c m. Less than 2 % were breeding at lengths below 244 cm suggesting that maturity is reached, as with Zambian animals, at much longer body lengths than on Rudolf. The Breeding Season Seasonal breeding in crocodiles is well known and is described by Cott (1961) : Modha (1967) has confirmed its existence on Central Island. C ott observed that in 14 Uganda n and Zambian localities, egg deposition occurs from 1 3 months before the ti me of lowest water level so that hatching after the 3 month incubation period takes place during, or shortly after the time when the water level begins to rise. The onset of the rains and resultant rise in w ater level could be expected t o favour survival in young croc odiles. The habitat is enlarging and there is an increase in emergence of insects, important in small crocodile diets. It is obvious too, that egg deposition should take place sometime during the year when water levels are subsiding as the da nger of inundation of nests by rising water is great in African waters characterized by seasonal flooding. Modha (1967) describes a clearly defined breeding season on Central Island during 1965 66. Copulation was observed beginning in October and lasting until December with a peak in November. Nesting started at the end of November, lasting until Jan uary with a peak in the second half of December. Hatching l asted from Febr uary to April with a peak at the end of March. The lake level was lowest in April, r ising again in that month and the pattern of the breeding season was thus seen to be the same as that described by Cott for crocodiles in Uganda and Zambia. Modha s observations suggest that ova are maturing in October and November and that the interval between fertilization and the completion of egg formation is approximately one month, the time between the observed peaks of copulation a n d egg laying. There is then a 3 month incubation period.
62 Table 12 shows the condition of the ovaries of 121 mature females exa mi ned on the east shore over a 12 month period in 1965 66. It is evident from T able 12 that no clear sequence of events emerges from an examination of the ovaries. While this is in part due to the small samples it is probably also a reflection of a much more diffuse breeding season than that recorded by Modha on Central Island. Animals with follicles in all stages of development are found throughout the year and while a peak of egg production occurs at the end of the year there is evidence of so me reproductive activity throughout the year. This could mean that either some animals reproduce more than once a year, or that there is considerable variation in the time of reproduction. The time sequence of events would permit an animal to breed twice. From fertilization to hatching occupies about 4 mo nt hs. An ani mal with two batches of ova developing concurrently co u ld, after maturatio n of the first batch, be ready to produce the second in a few months time, say shortly after the first lot of eggs have hatched. On Rudolf where fluctuations in water level are relatively slight there may not be so close an association of nesting and time of year, since this
63 is not as critical as it may be on most rivers where changes in water level are more pronounced. N umber o f eggs produced Tab le 13 lists mean clutch sizes from 7 sources. + While the various Uganda n and Zambian records are similar much smaller clutches are found on Rudolf. Cott (1961) p. 275 observed that larger clutches were from larger anim al s : thi s Locality & source Mean clutch size Range Sample Uganda, L. Victoria. Carpenter (1920) 59.9 38 76 13 Uganda, L. Victoria. Pitman (in Cott 1961) 60.4 25 95 775 Uganda, Murchison. Cott (1961) 54.9 17 Uganda, Murchison. Parker (1967) unpublished. 52 32 67 22 Zambia. Cott (1961) 56.2 23 Rudolf, Central Island. Modha (1967) 33.5 14 46 65 Rudolf, Sections 2,3 & 4. This study 33 16 55 18 Table 13. Nile crocodile clutch sizes from 7 localities. Table 14 lists clutch sizes and corresponding body leng ths of 19 crocodiles recorded by Cott (1961) from Uganda and Zambia, and 10 crocodiles from Rudolf.
64 Uganda & Zambia Rudolf Clutch size Body length Clutch size Body length 40 275 16 207 57 290 28 250 45 292 37 275 55 292 40 283 50 293 44 294 49 302 49 302 50 306 41 305 55 307 55 307 45 308 35 316 46 309 52 316 60 309 66 311 50 313 69 313 65 316 70 319 87 330 69 333 70 345 Table 14. Lists clutch sizes and body lengths of 19 crocodiles from Uganda and Zambia, and 10 cro codiles from Rudolf means that the difference in mean clutch size between Rudolf and other localities might be due either to a difference in the number of eggs produced relati ve to crocodile size, or a difference in the mean size of breedi ng females. Th ese two samples are compared in Fig ure 5 which sho w s the cube root of clutch size plotted against length. The regression coefficients of clutch size (y) on length (x) have been calculated and were 0.0150 (Uganda and Zambia) and 0.00978 (Rudolf). These were not significantly different at the 5 % level. It is evident that clutch size bears a direct relationship to length and that this is the same for the 2 samples considered. The diff erences in mean clutch size betw een Rudolf and Uganda and Zambia therefore r ef lect differences in the
65 mean si ze of breeding females. Of interest are observations of (in Cott 1961, p. 274) in which the mean size of 775 clutches from Lake Victoria was 60.4, varying from 25 95. Recorded concurrently with these observations we re the lengths of 855 females from the same localities in which the mean length was 318 cm varying fr o m 220 470 cm. Inspection o f the regression line for data shows that a clutch size of 60 corresponds to a length of 315 cm. Figure 5. the cub e root of clutch size plotted against body length in 19 Uganda crocodiles (Cott 1961) and 10 Rudolf crocodiles. The regression coefficients were respectively 0.015 and 0.00978. Closed circles: Uganda. Open circles: Rudolf
66 Section 4. Age an d growth Attem pts to establish age criteria for Rudolf crocodiles were hindered by the absence of sequential, morpholog i cally different structures, such as occur in mammalian molar teeth, so that even a crude division of animals into relative age groups was not possible Generally speaking an adult crocodile is simply a larger replica of a juvenile with the most obvious and easily measured expression of age being size. Evidence is presented later that for most, if not all, of the animals life growth is continuous of wh ich the most convenient measure is body length. Throughout this work attempts to relate length to age have b een made. Nothing on ageing crocodiles has been published and info rma tion on rate of growth is scant. Cott (1961) p. 240 records data on one wild and 6 captive, known age animals) and Pooley (1962) records growth in 10 captive hatchlings. Poole (1961) describes tooth replacement in crocodiles and concludes that as each successive tooth is larger by a constan t increment the total number of replaceme nts in any individual can therefore be determined. In the absence of information on the rate of replacement it is, however, impossible at this stage to relate tooth size to age, although this could be a fruitful line of research. Laws (1965 B) and Watson (1967) have used growth layers in the teeth of various mammals as age indicators. These layers are formed in response to environmental changes at known intervals that were, in the case of wildebeeste (Watson ibid.) the alternating wet/dry season and its a ccompanying changes in food supply and metabolism. Although layers are present in crocodile teeth, investigation was abandoned in favour of a similar investigation of the layering that occurs in the dentary bone. Attempts to relate tooth layers to age are hampered by ignorance of the replacement rate, and the high incidence of damaged teeth seen in Rudolf animals would prevent the same tooth being available from each jaw.
67 Peabody (1961) described zones in the dorso ling u al region of the denta ry in a speci men of C. niloticus from Ngamiland (Botswana) ascribing these zones to variations in growth caused by the well marked summer and winter climate of the region. He assumed that differences in growth rate were reflected in differences in the formation of bon e, seen as symmetrical layers of alternating darker and lighter bone and that each layer represented a growth. He describes 4 layers in the specimen he examined concluding that it was thus in its fifth year. The animal in question was 81 cm long an d Pooley (1962) found that captive crocodiles in Zululand grew to similar lengths in 2 years with an optimal food supply. It is not unlikely that a wild crocodile living under more rigorous conditions and with a less favourable food supply might take 4 yea rs to attain the same size. As similar layering was present i n Rudolf animals a series of jaws was sectioned and examined. Methods Cro s s sections were cut through the ramus of the mandible at 4 sites along i ts length. The most suitable was found to be that between tooth number 8 and 9 (counted from the front) Th is section goes through the dentary and splenial, layering being especially obvious in the dorso lingual regi on of the denta ry Peabody does not state where his section was cut but in appearance it is similar to those of the present study Of 68 sections cut, 23 male and 24 female sections were finally compared The remaining 21 were rejected owing to damage during preparation or because they were too obscure for interpretation. The technique u sed for preparing the sections was similar to Peabo dy and is essentially the same as the method of preparing rook sections. Sections were ground d own with carborundum c om pounds until thin enough to moun t on a mi c ros c ope slide in a mounting me dium, thickness being judged by eye. Examination was made under the low power of a binocular microscope with oblique reflected lighting.
68 Results T he 47 sections considered are listed in Table 15. Males Females Body length (cm) Dentary layers Body len gth (cm) Dentary layers 146 14 157 15 150 8 164 19 159 10 167 19 164 21 171 18 168 23 176 20 175 17 181 14 190 21 183 23 202 22 190 14 203 15 190 21 206 18 191 29 216 18 197 22 235 30 198 16 237 30 198 18 244 33 198 30 270 20 203 29 283 33 205 24 287 35 209 26 291 28 215 29 297 35 215 31 308 40 219 26 309 24 223 18 309 44 226 17 453 51 227 28 272 25 Table 15. Lists body length and the corresponding number of dentary layers in 23 male and 24 female crocodiles from Lake Rudolf. The data have been plotted in Fig ure 6 as the number of layers on length. The regression of layers (y) on length (x) was calculated in the male sample and the regression coefficient found to be 0 1295. In the female s ample there was n o regression of layers on length which is probably due to the restriction of the sample to a small body length range of 160 270 cm. It was expected that in the female a greater number of layers at a given body length would be found and the limited data in Fig ure 6 do suggest thi s as the female scatter tends to lie above the male regression line. However in the absence of a better distributed sample it is impossible to determine the relative difference, if any, between male and female denta ry layers relative to length.
69 Figure 6. The number of growth layers in the dentaries of 23 male and 24 female crocodiles from Rudolf, plotted against body length. The male regression coefficient was 0.1295. Closed circles: males. Open circles: females. The results for the male sample in Fi gure 6 suggest a relationship between layers and age and in order to determine this in absolute terms it is necessary to consider what might be governing their formation. Seasonal changes on Rudolf are so slight that it is unlikely that they could be dir ectly influencing growth to the extent observed by Watson (ib id.) in wildebeeste. But a well marked cycle of reproductive activity, particularly in the male, is pre s ent which appears to have evolved in response to the seasonal fluc tuations in water level that oc cur i n tropical Africa as a result of the alternating wet and dry seasons. The continuation of this cycle in Rudolf despite its separation from the Nile system since the middle Pleistocene indicates the fundamental nature of this reproductive cycle The build to a peak of reproductive activity, followed by a sudden cessation and quiescent phase, is likely to be accompanied by a gross
70 cycle of metabolism that could result in discontinuities in bone growth such as occur in other animals. The considera ble variation in the male scatter could be due to several factors. Variation in individ ual growth rate, variation in the age at sexual maturity and the possibility that some animals do not breed eve ry year. From the foregoing it seems possible that growt h layers in the dentary are formed on an ann ual cycle in response to a rhythm of reproductive activity with intervals of a year. A ssuming the number of dentary layers to represent an equal number of years the data in Figure 6 for males has been replotted in Fig ure 7 as body length on age. Ignorance of the form of the growth curve in crocodiles precludes any transformation of the data at this stage; consequently, the regres s ion line has been drawn in as for Figure 6. The corresponding curve for absolute ag e in the female has been derived from the relative age difference in males and females indicated by the relationship between lens weight and body l ength This is discussed further in the next section. Eye lens weight and age Since in some mammals (Lord 1959) it has been shown that lens weight increases with age after gr ow th of the organism as a whole has ceased, so that lens weight can be used as an age indicator, a collection of lenses was made to determine whether a similar phenomenon occurred in croc odiles. Since absolute ageing of any lenses has not been possible the series proved useful only in indicating relative ages. Methods Lenses were collected by dissecting the eye, removing the lens and ciliary body together and preserving this in a 10 % f o rmaldehyde solution. In animals killed by day lenses were discarded if not collected within 4 hours post mortem: in animals killed at night lenses were collected up to 18 hours post mortem, provided the eyes had not been exposed to the sun. Since al l anima ls were shot through the brain many lenses were accidentally damaged but one or both
71 lenses (273 in all) were obtained from 180 animals. In the laborato ry the fragment of ciliary body and the s uspensory ligament were dissected off leaving the lens capsule free of extraneous tissue. Minor damage was detected at this stage and resulted in 42 of the 273 lenses being discarded, as it was found that even small injuries to the capsule resulted in significant weight losses. Lenses were dried in an oven at 78 80 o C to constant weight. Weights in milligrams were measured on a sensitive balance. A test series s howed that drying was slow, so that after 10 da ys, a loss of 2 mg a day was still occurring. By 20 days weight loss was insignificant and these weights were tak en as constant. The full series of 231 was dried under the same c onditions for 25 days. Results Compar ison between ri g ht and left lenses Right and left lenses of the same individual were collected undamaged from 51 anima ls and their weights compared The mean difference between pairs was 1.5 % and did not exceed 4 % of the weight of the heaviest lens.
72 Compari son of male and female lenses
73 D ry lens weights were obtained for 55 male and 83 female crocodile s and are shown in Table 16. The s e data ha ve been plotted in Figure 8 as the cube root of lens weight against body length. The regressions of lens weight (y) on length ( x ) Figure 8. Shows the cube root of dry lens weight plotted against body length in 55 male and 83 female crocodiles from Rudolf. The regression coefficients were respectively 0.01097 and 0.0161. Closed circles: males. Open circles: females. have been calculated and the regression coefficients were 0.01097 (males) and 0.01610 (females). For males and females up to a length of 180 c m no difference is detectable, but from this point on lenses of females are heavier than males of the same body size. This is what would be expected if lens growth continued indepe ndently of body growth since ma le crocodiles after maturity grow faster tha n females. Assuming lens growth in the two sexes to be similar the discrepancy between the male and female regressions is taken to
74 represent the relative difference in ages. This relationship has been utilized in Fig ure 7 so that a curve for the absolute a geing of females may be obtained. The male curve in Figure 7 has been taken directly from Figure 6. The female curve is the lens weight curve relative to the male lens weight curve after the latter has been adjusted to the same scale as that for male den tary layers. Figure 7. Sh ows the male data from Figure 6 repotted as body length on age in years (where years equals dentary layers). The female curve is female lens weight (Figure 8) relative to male lens weight after the latter has been adjusted t o the same scale as male layers. The broken line represents immature growth. In the absence of other evidence the curves in Figure 7 have been used as the means of ageing Rudolf crocodiles in the section on population dynamics. Since it is apparent (see s ection 4b) that growth in crocodiles elsewhere is more rapid, the age/size relationships in Figure 7 should only be applied to Rudolf
75 crocodiles and until confirmatory evidence from other sources is available the conclusion drawn from dentary layering sho uld be treated with caution. 4 b Growth Size and shape The more rapid growth of male crocodiles results in a size differential between the sexes so that on Rudolf the longest males (470 cm) are 3.5 times as heavy as the longest females (320 cm). This condition also exists in al ligators and other crocodilians (Schmidt and Inger 1957) During th is survey 27 standard body measurements were taken from all animals examined to establish : whether any change in proportions t hat occurred with growth could assis t in ageing ; and to determine whether anim als could be sexed by external examination. Although a full analysis of th ese data is not complete, preliminary results suggest that any differences are too slight to use as age or sex indicators. In Table 17 are listed the mean weights in kg for males a nd females in 437 Rudolf crocodiles, and 42 Ugandan and Zambian crocodiles (Cott 1961) for each of 15 size groups rising in 30 cm cm intervals. Also shown is the comb ined mean weight for males and f emales. In Tabl e 18 are listed male and female weights for 49 crocodiles from the Galana river (Parker: unpublished data). No difference is apparent between the mean male and female weight for each length group in the 3 samples. The conclusion of Cott (1961) p. 255 that females are heavier than males of the same length is not borne out by his data. Further, no difference among the 3 samples is apparent. The data in Table 17 have been plotted in Figure .9 as the cube root of the mean combined weight against length and the r egression of weight (y) on length (x) calculated for the Rudolf sample.
77 Length group as for Table 16 Males mean weight (kg) Females mean weight (kg) Males and Females mean combined weight (kg) 1 2 2 2.2 2.1 3 4 4.1 4.0 4 8.5 7. 7 8.1 5 16.3 15 15.7 6 25.7 24.6 25.1 7 46.4 42.8 44.6 8 62.7 62.7 Table 18. Shows the mean weight in kg for males and females in each of 8 length groups in 49 crocodiles from the Galana river, Ke nya (Parker: unpublished data) The regression coe fficient was 0.01684. The regression s for the Uganda and Galana samples have not been calculated but the points for both these samples are found to lie close to the regression line for the Rudolf sample. Thus no difference in the body weight to body leng th relationship between animals of the 3 samples is apparent. Because of its importance in fundamental considerations of the size/time relationship it is useful to determine how large crocodiles may grow. Casual observation suggests that few individuals in a population exceed certain sizes and this is borne out by examination of killed samples. Size of Males In the Rudolf population 50 % or more males are sexually mature at about 270 cm long. There is however, considerable variation, with some animals mature at
78 189 cm a nd others not mature until 338 c m. Growth evidently still continues after sexual maturity, so that the largest males of 470 cm are nearly twice as long and weigh ten times as much as those of 270 cm. The largest of 202 males examined duri ng the survey was 472 cm long, with 11 animals in the range 430 470 cm. Of several hundred animals shot by the Kenya Game Department in Gulf, none were larger than this. The largest of 324 males examined by Cott (1961) p. 251 in Uganda and Zambi a were 7 animals 430 475 cm long. Cott does however quote several, apparently authentic, records of animals up to 600 cm long. The author has two records (Turner and Bell, pers. co m m.) of animals more than 500 cm long fr o m the Grumeti River, Tanzania. Ther e thus appears to be a size that very few individuals exceed. There is no suggestion that growth ceases altogether at a particular size and in fact is probably continuous, but a limit on the averag e size of the oldest individuals in any population wi ll be imposed by the complex of mortality factors that govern the maximum age attained. Occasional very large animals would be the result of unusually rapid growth and long life. Size of females Sexual maturity in females of the Rudolf population is reached at about 190 cm, when 43 % or more are mature. But, as with males, considerable variation exists, with some mature at 170 cm and others not mature until 300 cm. This variation in the size at maturity could result from differing growth rates, so that individ uals maturing at the same age vary in size; or it could reflect differences in the age at which various individuals mature, assuming relatively constant gr owth rates. In practice it is probably a combination of both effects. Of 278 females examined durin g the survey the largest were 9 animals in the range 310 320 cm. The largest of 327 females examined by Cott in Uganda and Zambia were 11 in the range 325 350 cm. Cott also quotes unpublished data of in which the 16 largest of 855 females from Lak e Victoria were 380 470 cm long, and another 74 of 350 380 cm. This sample was shot while
79 presumed to be lying near nests and it is possible that the very large animals were erroneously assumed to be females because of their presence near nests. The lar gest females on Rudolf are about 320 cm long while in Uganda they may grow larger. Preliminary results of a survey of the Murchison Falls population suggest that the size limits for both sexes there may be considerably greater than on Rudolf. Lifespan N either the potential nor actual lifespan for any crocodilian is known. Information is confined to records of captive animals and such data do not Age (years) 20 30 31 40 41 50 51 60 Alligator mississipiensis 10 8 4 2 Crocodylus niloticus 3 3 1 Alli gator sinensis 3 1 Caiman niger 1 Caiman crocodilus 2 Crooodylus oataphractus 3 2 Crooodylus intermedius 1 Crocodylus palustris 1 Crocodylus porosus 1 Crooodylus siamensis 1 Gavialis gangeticus 4 Osteolaemus spp. 2 Tom istoma tetraspis 1 Table 19. Recorded longevities in 30 individuals of 13 species of crocodylians. From Flower (1937), International Zoo Year Book (1966) and present study. indicate average longevities. To obtain some idea of lifespan, 110 zoos compr ising all the larger ones in the world were consulted for information on growth and age. The records obtained are shown in Table 19, to which have been added similar records from Flower (1937) and the Inte rnational Zoo Year Book (1966). Reptiles are widel y considered to be ve ry long lived and Flower discusses several claimed instances of tortoises living more than 100 years, but none of
80 these are recorded in a ve ry convincing manner. Only in the case of the chelonian Emys orbicularis has an age of over 70 years been authentical ly recorded. For crocodilians only two ages of more than 50 years have been recorded, both in A. mississipiensis One of 53 years is alive at the time of writing (South Australia Zoological Gardens) and the other of 56 years was aliv e when recorded by Flower (1937). 20 years have been recorded for at least 13 species of croco di lian (Table 19): but ages of more than 30 years are unc ommon and more than 60 years unknown. It appears that 60 years can be considered an exceptional age in ca ptive animals. To what extent these ages reflect ages in wild animals is unknown, but there is no reason to suppose that a healthy captive animal should differ greatly from a healt h y wild animal. The age scale in Figure 7 suggests that crocodiles on Rudol f do not survive for more than 70 years. That various species of crocodilians may live for many years without growing is well known to zoo keepers Three alligators in the Be rlin Zoo (in litt.) all about 33 5 cm long ( 30 cm shorter than the usual size lim it for males) lived 25 years without growing. A caiman in the Munster Zoo (in litt.) lived 28 years without growing. This phenomenon occurs in the wild (see section on "growth in known age crocodiles") and while the causes are unknown it is likely to be in response to conditions such as overcrowding or insufficient food. Th e causes of mo r ta l ity in older animals are discusse d in the section on Population d ynamics. Gr owth in Allig ator missis s ipiensis Because of the general similarity between A. mississipi ensis and C niloticus it is of interest to consider growth in the former species since more is known about it, and see how far it compares with growth in crocodiles. Some information on growth in wild alligators comes from observations by McIhlenny (1934) who released 38 marked hatchlings on Ave ry Island, Louisiana, recovering a number of individuals at intervals during the next 11 years. By 5
81 Figure 19. The age/length relationship in Alligator mississipiensis. Suggested curves for adult growth have bee n sketched in by hand to become asymptotic at their respective size limits. Closed circles: males. Open circles: females. + are known age, unsexed animals. years of age a well marked discrepancy in growth between the sexes was apparent, with males growing faster. This discrepancy continues thr oughout life, so that the effective size limit for most males is 370 cm and females 250 cm. (Chabreck, in litt.) The result s of these observations are shown in Figure 10. Up to age 11 the curves are for McIhlenny s d ata and from then on have been sketched in by hand to become asymptotic by their respectiv e size limits. C habreck (in litt.) has growth data from a large sample of wild alligators, as yet unpublished, which confirm that normal adult size is reached at abou t age 20 25 years. The known age animals plotted in Figure 10 are McIhlenny (up to age 11) and 2 animals in the New York Zoological Park (Downing and Brazaitis, 1966). Also
82 shown are 5 unsexed animals from the Berlin Zoo and the South Australian Zoolo gical Society (in litt.) Sexual maturity in the f emale alligator is reached at abo u t 5 years and 190 cm length (Chabreck 1967, p. 8) and inspection of the curves in Figure 10 show that growth begins to f all o ff gradually after maturity, becoming neglig ible by the t ime males are 370 cm long and f emales 250 cm. As with crocodiles captive alligators may live f or many years without apparent growth. Growth in known age crocodiles In an attempt to collect information on growth in cap tive animals all t he m ajor zoos of the world, 110 in all, were con s ulted for records. Data covering various stages of the first 7 years of life (juvenile lif e) were obtained f or 14 unsexed immatures from a variety of zoos. The growth of 3 Rudolf hatchlings kept in artificial co nditions in Nairobi was recorded during the survey. Pooley (1962) has data on 10 hatchlings for the first 2 years of life. Cott (1961) quotes records of 5 immatures. Record s of 8 known age adults were obtained but none were accompanied by gr ow th rates for any part of adult life. Davison (in litt.) has records for a single, unsexed wild animal covering 31 years of life. Since growth in juveniles and adults is different the two stages have been considered separately These 32 known age immatures are shown in Table 20, where their growth rates, expressed as annual length increments, are grouped into categories at 3
83 Length increment (cm/y) 10 12 13 15 16 18 19 21 22 24 25 27 28 30 31 33 34 36 37 39 40 42 Increment group 1 2 3 4 5 6 7 8 9 10 11 Sample 0 1 3 3 5 2 4 4 8 2 0 Percentage 0 3.1 9.4 9.4 15.5 6.3 12.5 12.5 25 6.3 0 Accumulating percentage 0 3.1 12.5 21.9 37.5 43.8 56.3 68.8 93.8 100.1 0 Table 20. Annual length increments in 32 immature crocodiles grouped at 11 categories at 3 cm intervals. cm intervals. These growth rates have been analysed as follows, using the method of Cassie (1954). The frequency of occurrence of each growth rate was plotted on arithmetical probability paper ( Figure 11). A normal distribution of growth rates would give a straight line but the plot showed 2 modes of growth rate. Furthermore, the point of inflexion occurs at the 50 % level which is what would be predicted if the sample comprised equal numbers of males and females with different mean growth rates. The two co mponent no rmal distributions have been plotted and the best straight lines fitted by eye. The mean growth rates and standard deviations are given by the ordinate value when the straight lines cut the 50 % and the 15.8 % and 84.13 % verti c als respectively. The 2 means were 31 33 c m and 19 21 cm per year and compare favourably with Pooley male and female growth rates of 34 and 26 cm per year respectively. Although other interpretations of these growth rate frequencies can be made this constitutes the simplest reasonable explanation and the values so obtained have been used to draw the curves for immature growth s hown in Figure 12. Male and female growth curves up to 300 cm and 200 cm respectively have been drawn from the growth rates derived in the preceding paragraph (Table 20 and Figure 11). These assume a nearly constant rate of growth up to sexual maturity, as in alligators. What is known of
84 Figure 11. Analysis of growth rate in 32 immature crocodiles. Accumulating frequencies of 9 incremental rates are plotted on arithmetical probability paper indicating 2 modes. The 2 component normal distributions are derived and the means and standard deviations are given by the ordinate value where the straight lines cut the 50%, 15.8% and 84.13% verticals. growth in crocodiles suggests that maturity is attained in approximately 7 8 years, assuming a mean length at this point of 300 cm in the faster growing males and 200 cm in females. Thereafter, growth in length falls off with age and purely speculative curves hav e b een sketched in to follow the sort of slope observed for alligators ( Figure 10) becoming asymptotic at an arbitra ry 500 cm (males) and 350 cm (females). These represent the maximum sizes reached by the majority of older animals in a population; obviou sly some individuals will grow larger and some never reach these sizes. There is no information to say whether growth ever ceases or whether it becomes as slow as indicated in Figure 12; nor are there any data on mean life spans in any population and the se curves must be regarded as hypothetical curves for "normal wild crocodiles. As
85 indicated in Figure 7 this consideration of growth does not apply to Rudolf animals where growth is very much slower and in which animals appear to be stunted. Figure 1 2. Growth curves for male and female immature crocodiles (values from Figure 11) up to 300 cm and 200 cm respectively. Suggested curves for adult growth have been sketched in by hand, becoming asymptotic at 500 cm (males) and 330 cm (females). Also shown age animals are shown. Closed circles: males. Open circles: females. + are unsexed animals. Also shown in Figure 12 is a wild crocodile in the Wankie National Park Zimbabwe, for which da ta are available covering the first 31 years of life (Davison in Cott 1961, and in litt). Growth in this animal was continuous until age 7 and a length of 213 cm, thereafter falling sharply so that by age 31 growth was only 1 cm per annum, or may even have ceased. The curve for this animal
86 fits fairly well the female curve and sugg ests that growth in older animals slows possibly after sexual maturity. At the start of th is survey attempts were made to capture and mark animals for release and s u bsequent rec apture to obtain growth data on wild animals. Initially efforts were concentrated on adults, as they were of more interest, using a 10" nylon shark net and snares. Neither technique was successful enough to catch sufficient animals in the limited time ava ilable, so activity was confined to capturing small an im als by hand at night while dazzling them with a torch. 152 animals were caught and marked by clipping off a combination o f the vertical caudal scutes and f astening self locking, plastic tags through a similar scute. The recovered animals showed that both techniques were satisfactory and could be expected to last for years. Recapture effort was combined with the routine collection or specimens and totaled 61 nights plus various times during a greater number of days. Only 3 animals (of 78 88 cm total length) were subsequently recovered, after an interval of 8 9 months, during which they grew respectively 0.4; 0.75 and 2.6 cm, or an average of 1.7 cm per annum. It is u nlikely that such low growth rates r eflect normal growth in R udo lf. animals as the slower growing animal would, at this rate, take 900 years to reach 470 cm. The importance o f these observations lies in demonstrating the extent to which growth in crocodiles may vary although the causes of th is variation remain largely speculative. Growth in Rudolf crocodiles It can be seen ( Figure 7) that up to a length of 190 cm there is a mean immature growth rate of 10 cm a year (body length). This is less than half the growth rate Shown in Figure 12 and is continued through adult life. That considerable depre ss ion s in crocodile growth can and do occur is demonstrated by the 3 marked animals mentioned in the preceding paragraph. These animals, although appearing to be in "good condition" grew, in 9 mo nths, an average of
87 only 1.7 cm per annum. 3 hatchlings captured from the same locality and kept in Nairobi in a tank where the water temperatu re was maintained at approximate ly the same level as in Rudolf ( 26 o C ), but with an optimal f ood supply of live f ish, averaged 30 cm per annum. The slow growth rate observed on Rudolf is clearly not intrinsic but determined by some environmental factor or factors. The most obvious one for w hich there is any evidence is a poor food supply. An exceptionally hig h incid ence of empty stomachs (48.4 % ) is observed in Rudolf animals and this could result from an inadequate food supply. It would be expected that such an inadequacy would be compen s ated for by a depres s ion in growth rate w hich, other thing s being equal, is pro portional to the rate of food intake. Another factor known to influence croc odile growth is the temperature of Figure 9. Shows the cube root of body weight plotted against length in 437 Rudolf crocodiles. Male and female weights have been lumped and the mean weight of each size group calculated. The regression coefficient was 0.01684. Also shown (open circles) are the weights of 42 Ugandan and Zambian crocodiles.
88 their environment. Pooley 1962 found that g rowth in captive animals bore a close relationsh ip to enviro nm ental temperature. Growth slowed with falling temperature and ceased altogether for a period during winter. Since the high air and water temperatures on Rudolf vary lit tle over a 12 month period, or during the 24 hour cycle, this cannot be th e inhibiting factor there There is also the possibility that some direct, densit y dependent effect is exerting an inhibito ry influence on growth Although in the overall sense crocodile numbers on Rudolf are surprisingly low compared to localities lik e Murchison Falls N ational Park, this is not a valid observation on real density. Using the age st ruc ture indicated in Figure 13 (assuming a similar curve for males) and the weight/length relationship in Figure 9, the total biomass of crocodiles on Rudol f is estimated as 325,000 kg. This represents, in Section 2 on the east shore, densities of 9 000 39,000 kg per square mile. While our knowledge of crocodile biology is insuffic i ent to judge whether such densities are "high" or "low", it seems possible tha t they are in fact high when compared to terrestrial situations such as those summarize d in Lamprey (1964). Occupance, in the sense used by Watson ( 1967), of favoured areas in continuous and local densities may be at a maximum with no opportunities for dis persal. Emigration is also impossible in Rudolf crocodiles since the only possible route, the Omo river, already contains large numbers of crocodile.
89 Section 5 Population dynamics The object of this section has been to determine s o me population paramete rs of the Rudolf population. Recruitment, in this instance the addition of fertile off spring to the breeding population, is considered first followed by a discussion on mortality. The age structure of the population is speculated on and some theoretical aspects of the female life Table and the net reproductive rate considered. 5 a. Recruitment Reproduction This section considers those aspects of reproductio n that are influencing the age specific fecundity, or the number of hatching eggs produced per year by each age class of breeding female. Several factors, considered below, contribut e to this value. The ill defined reproduction cycle prevented accurate assessment of the proportion of mature fe m ales that reproduce eve ry season (1 year). In the absen ce of this data all mature animals have been assumed to breed each year and examination of Table 12 would suggest that the pr op ortion is high. The proportion of females in each age class that are mature (and assumed to breed) has been determined (Table 10, Figure 4) and these values incorporated. The pr op ortion of older females that have ceased reproducing has also been taken into account (Table 12.). Val ues for age specific clutch size were read off Figure 5 and have been included in the calculation. M odha (1967) gives some data on hatching success on Central Island in 1965/66 and fr o m this an estimate of the hatching rate has been calculated. Modha lists a total of 152 nests made on the island in 1965/66. 126 (82.9 % ) of these hatched. He does not state what his criteria for assuming 152 to be the
90 total were, but since the nests are easy to locate once they have hatched we can accept these data as fairly accurate, since the number of unkn o wn nests that failed to hatch is likely to have been negligible For the nests that hatched Modha estimated the hatching rate by counting egg shells and unhatched eggs from each nest, and assuming a mean clutch size of 33 (where the clutch size was unknown) computed the hatching success. From an examination of this da ta (given in Tables A 10 and A 11) a hatching rate of 82.8 % is apparent. Thus for the total of 152 nests, representing 5 016 eggs, an overall hatching rate of 68.6 % (3 441 eggs) is calculated.
91 T he calculation of age specific fecundity, expressed as an m x value where this is the mean number of female offspring produced per female in a period of time x, is summarized in Table 21, for each age class of mature females. Since a pivotal age of 3 years has been adopted the m x values have been multiplied by 3, since the length of each generation is one year. Mortality up to sexual maturity Where it occurs the Nile m onitor lizard ( Varanus niloticus ) may be an important agent in egg mortality as suggested by the numerous observations of predation by this anim al (Cott 1961, p. 304.) However, during th is survey no evidence was found for the existence of monitors on the mainland of Rudolf (although they occur on North and Central Islands) and it is not, therefore a significant agent of egg mortality on the lake as a whole Cott (ib id .) records a variety of animals that have been observed to prey on crocodile eggs, but of these the only ones present on Rudolf in any numbers are hy aenas ( Crocuta cro c uta Erxleben and Hyaena hy aena Meyer) Both species are common alo ng the shores of the east side but during the 14 months of the survey only one nest was found destroyed by hyaena s in an area where an estimated 100 or more nests were made. The human populations of the south east and west shores destroy a high proporti on of nests made there and although these are low dens ity areas (for crocodile) man is almost certainly the most important egg predator on Rudolf. A form of catastrophic mortality which must take place occasionally is inundation by early, exceptionally hi gh floods. In East Africa where deterioration of the catchment areas of many rivers is resulting in increased run off the c hances of this sort of mortality are increasing. Modha (1967 ) records 3 nests on Central Island destroyed by waves during a sto rm.
92 M any other factors will contribute towards egg mo rtality. Disease, adverse environmental conditions, infertility and failure of the parent animal to unearth the hatching eggs have all been known to occur (Mo dha 1967), but unfortunately no quantitative asses sment of any of these exists. Of 26 nests that Modha recorded as failing to hatch the fates of 9 are described while for the remaining 17 no information is given. M o rtalit y in immature c rocodiles The small size of hatchlings could be expected to make t hem available to a wide spectrum of predators and a variety of animals have in fact been recorded feeding on young crocodiles (Cott 1961, p. 304). While predation on hatchlings at Rudolf may be high no actual instances are on record and the extent to which it occurs remains speculative. The finding of a hatchling in the stomach of a catfish ( Clarias lazera Val) on Central Island (Modha 1967) is of interest. While it may have been scavenged it could represent active predation by a fish which, in the extensiv e unsheltered water of Rudolf, might be significant. Modha (1967) p. 92 concl u ded that most hatching took place at night which m ay favour s urvival at a particularly vulnerable stage, since a nocturnal predator would have to be near a hatching nest to detec t it while a diurnal avian predator such as a marabou ( Leptoptilos crumeniferus Lesson) could see a nest hatching from some distance. The appearance in a given habitat of a large number of h atchlings in a relatively short space of time, a concomitant of a breeding season tied to climatic cycles, may or may not favour survival. As has been suggested by Darling (1938), this will prevent predators from building up to densities that could exploit hatchling crocodiles maximally. It would also preclude the evolut ion of predators exclusively dependent on very small crocodiles. Conversely, "specific search images" might develop, resulting in a high level of exploitation by some predators. The nature of the habitat could also be expected to influence the level of pre dation since very shallow water and dense c over would make crocodil es less vulnerable to predators than in habitats with scant cover like much of Rudolf.
93 Foo d supply would be important in that low availability might, apart from direct starvation, depress growth rates, seems to be the case on Rudolf, leaving immature animals susceptible to hazard for a longer time. There is evidence that in localities without shelter mortality may be very high. 15 nests were found on various occasions up to 3 days after hatching, on 2 sm al l islands 1 2 miles offshore in Allia Bay. There is no shelter of any sort on either island and frequent close examin a tion, both by day and night, failed to reveal any of the hatched animals. An appreciable current flowing away from land in a north westerly direction combined with the nearly continuous easterly wind makes it ve ry unlikely that these anim al s swam to s hore and it seems that nesting here resulted in, perhaps, 100 % mortality to the hatchlings. There is circumstantial evidenc e that a similar state of affairs exists on the larger North and Central Islands. (Only one visit was made to South Island during the survey and no relevant information was recorded.) 14 days were spent on North Island in August 1965 during which consider able effort was spent on locating crocodiles. Despite the fact that at least 18 nests, representing about 400 hatchlings, had hatched the previous season, only 1 animal < 90 cm long was observed on the island. Modha (pers. comm.) has remarked on t he scarci ty of small crocodiles < 100 cm on Central Island prior to the onset of the 1965/66 breeding season. In 1966 he observed 126 nests to hatch on the island which represents approximately 3 500 hatchlings. Assuming a similar number the previous season his observation would suggest a massive mortality among hatchlings. (This ignores the unlikely possibility that the young animals dispersed by swimming the 6 miles to the mainland). North and Central Islands are noteworthy for the complete absence of any swam py habitat which is where, on the mainland, young crocodiles tend to accumulate. What sheltered water that does exist on Central Island is occupied, at times, by high densities of large crocodiles. It is therefore possible that these islands are unsuitabl e as habitats for young crocodiles and mortality on them may approach 100 % Nesting there may be a response to some disturbing influence on the mainland
94 such as harassment by man at the expense of survival in the young. The implications of this are discuss ed further in Section 6 b on "Exploitation". Some idea of the order of magnitude of mortality in young crocodiles is given in Section 5 c. 5b Mortality in older animals As crocodiles grow larger they presumably become susceptible to fewer predator s pecies so that in the case of a full grown male the only one remaining is man. Two lions (the most powerful potential predators) were observed to kill a crocodile of 320 cm at Allia bay during the survey but individ u als larger than this have not been reco rded killed by lion (Cott 1961). The entire shoreline of Rudolf is subject to illegal hunting by man, but it is unlikely that the numbers of animals taken is large since, with the exception of Mer rille hunting sorties from Ethiopia and a few in Kenya, fir earms are not used, and traditional weapons are relatively inefficient. Also, until recently crocodile hunting was for food only and animals were only killed occasionally. Predators apart from man, are therefore unlikely to be causing much mortality in ol der crocodiles. Catastrophic mortality may occur from such agencies as lightening strikes, which, in the shallow, saline littoral of Rudolf could eliminate considerable numbers of individ u als. A volcanic area like Rudolf might suffer occasional underwater eruptions causing a local and rapid rise in water temperature to lethal levels. Although riverine crocodiles are known to aestivate in response to drying rivers, (Cott 1961, p.231) drying up of entire lakes, such as happened to Lake Rukwa (Tanzania) in rec ent years, can result in heavy mortality. Hippopotamus have been known to kill full grown male crocodiles ( Cott ibid.) but while the frequency of such conflicts is unknown it is unlikely to be high. Apart from disease, about which nothing is known, the other possible cause of mortality is the complex of conditions associated with "senescence." Whether senescent processes occur is not clear but the finding of evidently non f un ctiona l ovaries in large females (see section on female reproduction), and one male with possibly non functional
95 testes, does suggest senescence. This latter animal measured 445 cm with testis weights of 52 g and 59 g ( about 300 g in an active male this length ) No spermatogenesis was present, although it was at the height of the bree ding season, and the testes were enclosed in a thick, fibrous sheath. Histologically the testis was superficially similar to a mature, quiescent a nimal with large tubules and relatively little interstitial tissue. But the epithelium of the tubules ( Figure 2) was seen to be very thin and showed a tendency to break away from the underlying tissue. The interstitial tissue contained fewer stained nuclei than in an active animal. Although possibly a pathological condition, it may have been a manifestation of senescence. Pitman, in Cott (1961) comments that several exceptionally large females examined by him on Lake Victoria had ceased breeding, although he does not state what the criteria for determining this were. Old animals with inactive gonads have been de scribed by Legler (1960) in the turtle Terrapene ornata ornata and in Pseudemys scripta e l egans by Cagle (1944). There is then some evidence for supposing that senescent processes occur in crocodiles and this may account for ultimate mortality in ve ry o ld animals 5c Survivorship in Crocodiles N o assessment of the age structure of the Rudolf crocodile p op ulation was made during the survey and this section is confined to theoretical considerations only. (But see Table 23 .) In a crocodile population with a stable age distribution mortality from hatching to the onset of reproduction must be ve ry high since relatively large numbers of offspring are produced per breeding female. The order of magnitude of this mortality is indic ated in the following argum ent.
96 Figure 13. Hypothetical female survivorship curve for Rudolf crocodiles. The adult portion from age 16 was fixed by trial and error to give a value for R 0 of 1.0 (see text and Table 22). The curve from hatching to age 16 was sketched in by hand to represent total immature mortality (see text). Although no data exist there is no reason to suspect that any ma jor change has taken place in the Rudolf crocodil e population over the last 20 years. Assu m ing for the purposes of this arg ument that there h as been a stable age distribution over that time the following speculation on survi v orship has been made. First, a purely arbitrary survivorship curve, for females, was drawn, incorporating the following features Beginning at age 16 (onset of maturity) a decrease in m ortality rate until age 35 was followed by an increasing mortality rate to the
9 7 maximum age of 53. This curve, drawn as symmetrically as possible, is similar to type IV of Slobodkin (1964) in which a high rate of mortality in immatures is follo wed by a relatively good expectancy of further life in adults. The increase in mortali ty rate of very old animals is i mplied by the presence of senescent processes which will decrease the expectancy of further life in such individuals. The actual shape of the curve at this stage h ad no meaning and was drawn purely to provide a relative scale of survivors in each age class from 16 53 years. Then, by trial and error, using m x values from T able 21 and l x values from the curve just described, a net reproductiv e rate of 1.0 was calculated (actually 0.999894). In any population which is neither increasing nor decreasing the net reproductive rate will be 1.0. Thus, if the age distribution of adult female crocodiles on Rudolf is at all similar to that assumed above the scale of m ortality in immature crocodiles can be gauged. The absolute l x values so obtained (from a cohort of 1000) were then used to fix the position of the adult portion of the survivorship curve (shown in Figure 13) and a smooth curve from hatc hing to age 16 sketched in to represent immature mortality. In such a hypothetical population only 0.8 % of 1000 hatching females need survive to enter the breeding population, for that pop ula tion to be maintained. Thus a perfectly healthy crocodile populat ion could experien ce nearly 100 % mortality of its imma ture animals. Conversely the potential for recruitment is enormous. To what extent this immature mortality is density dependent is impossible to decide but it is highly likely that it is sens itive to p opulation density since a certain range of survivorship must be intrinsic to permit the animal to recover from catastrophe or exploit new situations. A certain level of mortality may be inevitable regardless of the suitability of an environment for young c rocodile survival. Lack (1954) discusses the question of survival in young birds and concludes that a high reproductive rate is a result of selection and an inevitable consequence will be a high mortality in young animals. It is variations to their mortali ty that provide the animal with the flexibility in recr ui tment it needs to survive as a species. This range in recruitment rate is the phenomenon of most concern to this survey since it is
98 from this that the productivity and hence the degree of exploitatio n by man can be judged. A life Table drawn up on the basis of the l x values in Figure 13 is shown in T able 22. Of interest is the age structure of the 500 animals shot during the survey, since it reveals the sort of result that can be expected from "ran dom These animals were collected in such a way that the larger individuals constituted a quasi random sample of those age gro ups at Moite and Allia B ay. Animals judged to be less than 190 cm long were avoided because most of them would be immatu re and consequent ly of little scientific value to this survey. Altogether 152 animals less than 190 cm long were collected, being most ly the result of misjudged size or to make up numbers when occasion demanded. But over this size no conscious selection wa s exercised since the time required for one p erson to hunt, recover and process the an imals was so great that selection was impossible and any availa ble animal was taken. Nevertheless, some bias towards larger animals was probably present since these were of much more interest than smaller ones. On at least 4 occasions smaller animals were passed up in favour of exceptionally large males, selected because of their relative scarcity. No difference in the ease with which animals of different sizes or sexes co uld be hunted was noticed. The 348 animals concerned were all collected from Moite and Allia Bay (excepting 5 from North Island) and although this involved hunting 30 40 miles of shore they are not necessarily representative of other parts of the lake. Th e sex ratio in the sample was 54 .3 % in favour of females. 651 crocodiles from a variety of localities examined by Cott (1961 ) showed a ratio of 50.2 % in favour of females. It thus appears that the sex ratio in crocodiles can be taken as 1:1. The 348 animal s are listed in T able 2 3 Although the sample may represent the age structure of the population as a whole, the erratic distribution of relative numbers remains unexplained and attempts to use the sample to construct survivorship curves were discarded in f avour of the theoretical treatment discussed in the previous section.
100 Section 6. E xploitati on To date crocodile exploitation h as consisted of sport hunting, and collection o f eggs for f ood by man (probably involving only small numbers on Rudolf) and hunting for s kins. It is important to consider the value of crocodile skin and the factors influencing it sinc e any management program must relate directly to this, particularly in a species where fairly precise selection of individuals is both feasible a nd desirable. 6a Value of crocodile skins The skins of all 500 anima ls killed during the surve y were collected and sold. Records of size and "grade" were kept, but the accidental loss of some records resulted in data being available for only 393 skins ( 78.6 % ). Gra din g The co mm ercial grading of crocodile skins is governed by several factors, the most important of which is the incidence of structures known as "buttons". These are hard, flat discs of a bone like substance that form inside the com me rc i all y valuable ventral scales. They begin in the centre and grow out radially, eventually occupying the entire scale and their effect is to render the normally pliable scute hard and brittle and unsuitable for leather work. A tendency for buttons to appear fir st in the pectoral region is observed. The nature and cause of buttons is unknown and although they occur in most if not all populations they are usually not found in animals less than 300 cm long. But lakes Rudolf and Baringo are peculiar in producing an ex c eptionally high incidence of buttoned skins even in small animals which is unexplained. The other factors governing skin grade are natural blemishes or injuries caused during skinning and dec omp osition. Three basic grades are recognised with occasiona lly a fourth for "reject" skins. No "fourth" grades were produced during the survey. "First" grades are those skins with no buttons damage or
101 decomposition. "Second" grades are those with buttons confined to the pectoral region or with minor injuries or d a ma ge. "Third" grades include s kins with a high incidence of buttons or wi th relatively severe damage or patches of decomposed skin. Although this system of grading is arbitrary it was found to be consistent among different c omme rcial fi rms. The value of a skin also varies according to size; the larger the skin the more valuable it is within each grade range. The size of a skin is commonly determined by measuring across the widest point on the ventral surface, starting and ending at the first li ne of rigid scales (which are useless as leather). Grade and value of Rudolf skins The 595 skins are listed in Table 24 as the number of first, second and third grade male and female Skins, occurring in each of 15 size groups (groups as in Table 17). As can be seen, the incidence of first grades is highe st in the smallest animals, decreasing to zero in animals over ~1 am. The incidence of second grades is similar to first: being higher in small animals and nil in ani m als over 541 c m The incidence of thirds is co nversely lowest in the smaller animals, be c oming 100 % in animals more than 341 c m long Firsts comprised 17 % of the total, seconds 17 % and thirds 66 % : these proportions will obvio usly vary according to the size distribution of anim als taken. Although th e data s ug gests that females produce more high grade skins than males the samples are too small for critical comparison and the sexes will therefore be considered together. The grade/length relationship is illustrated in Figure 14 where the percentage occ urrence of each grade is plotted against length and s uggested curves sketched in. Although comparable data for skins elsewhere does not e xist ( c ommercial records give only grades with no accompanying data) it is well known that the
103 Figure each of 12 length groups of Rudolf crocodiles. Data from Table 24 with males and females lumped and the mean for wach successive pair of size groups plotted. Closed circles : firsts. Open circles: seconds. + thirds. high incidence of buttons on Rudolf is not found in other localities except L ake Baringo. Buttons reduce the value of these skins by approximately half, compared to skins f rom other sources in East Africa. Table 25 shows the relationship between size, grade and value of Rudolf skins, the prices shown being those prevailing in Nairobi in 1966. The price of each grade rises as the animal grows longer, up to about 190 cm, after which it remains unchanged. The size is assessed by measuring the skin across the ventral surface at the
104 widest point. Figure 15 shows the change in value of a skin with crocodile length and age, the values being the average for each length class considered Figure 15. the increase in avera ge value of Rudolf crocodile skin (2) with length and age, and the value of the same skin if all were first grade (1). Also shown for co mpar ison are the values of the same skins assuming 100% first grades, which would more nearly approximate the position with most other East African skins, a t l east in the younger animals. It must be borne in mind that these value s depend entirely on the prices paid by com merc ial buyers which are a matter of negotiation between the parties concerned. The prices i ncorporat ed into the above analyses were those prevailing in Nairobi during 1966 and while probably higher than any previously offered will fluctuate according to demand.
105 Shown in T able 26 is the relative value of each age class of female crocodiles, assuming an age structure as in Figure 13 and values as for Rudolf animals. Assuming a similar age structure for males it is interesting to find that the most valuable age classes are the young animals just before and just after the onset of sexual maturity, of 7 19 years age. This is due to increasing mortality cancelling out the rise in value with size and age. This has very important implications in crocodile management since it suggests that the most valuable crop would consist largely of ve ry young animals. There are many other reasons why cropping young crocodiles may be desirable, discussed in the next section.
108 Productivity and Yield To compute the crop, or possible yield to man, the "productivity" must be known, which is the biomass of crocodile produced by the population in a period of time, for convenience, one year. The crop is that part of the gross production which man may remove, per year, without depleting the stock or production. Calculation of productivity requires knowledge o f population size and age structure, the potential recruitment rate, the actual recruitment rate and age specific mortality rates. The information presently available does not permit accurate prediction of cropping levels but s o me hy pothetic al consideratio ns are possible from which, at least, a management plan for Rudol f crocodiles could be drawn up. There is a widespread tendency to relate the possible yield or crop from a population of animals to the size of the stock, or total population, at any given m oment. But as Macfadyen (1963) points out there is little relation between stock and productivity, particularly in species with high reproductive rates The potential production of a crocodile population is obviously very high. From the data i n Table 22 th e total annual egg production on Rudolf has been estimated as 18, 000 30,000 eggs, and may be much greater, depending on the actual size of the breeding population. Taking, for the sake of argument a mean of 24,000 eggs this represents 14,000 hatchlings (6 8.6 % hatching rate). At the growth rate indicated in Figure 7 this would represent a gr oss production of 323,000 kg of crocodile at 15 years of age. The value, at this age, of Rudolf crocodiles is Kenya shillings 3 .24 per kg ( Figure 14) giving a total v alue of Shs. 1,046,52 0 (£5 0,000). If the growth rate were raised to that shown in Figure 12 then this same amount would be produced in 6 years. It is interesting to compare this with the value of elephant, the only other East African wild animal apart from hippopotamus and s ome fish for which a realistic val ue has been created, although this value is not always realisable due to
109 marketing problems. At 15 years an elephant is worth up to Shs. 2 70 per kg (Wildlife Services Ltd ., unpubli shed data), but th e overall value of th is species is greater due to its more rapid growth (Laws 1966). The actual crocodile crop realisable by man is the pr o portion of gross productivity remaining after recruitment the breeding population has been satisfied (which may be as little as 0.8 % of those hatching), and natural mort a lity up to the age of cropping. Maximising the crop thus consists of reducing the high immature mortality. Le Cre n (1962) bas shown that in some freshwater fishes recrui tment tends to be the same regar dless of the numbers of eggs produced, and that the efficiency of the reproductive and recruitment process in fish is inversely related to population density so that cropping at the correct level results in an increased yield, by relieving the pressure of density. A certain amount of immature mortality in the wild state is probably inevitable and has to be taken into account. There is also the possibility that survival from year to y ear is very variable as has been demonstrated in a variety of other animal s. On Rudolf there may be increased survival in very young animals in years when there is an unusual rise in lake level, resulting in large areas of inundated bush and grass providing s helter and good feeding conditions for small crocodiles (see plates at end). There are several ways in which immature mortality might be reduced. In Figure 13 a high rate of mortality from hatching right up to maturity is shown, but it is likely that in reality mortality soon after hatching is much higher and conversely muc h lower thereafter as has been ob s erved in birds (Lack 1954) and in many fish and invertebrates with high reproductive rates. A possible way of reducing this sort of mortality is to collect and hatch eggs artificially and only release the young animals af ter a period of growth that could be expected to leave them less available to predators and more experienced in movement and feeding. Simply ensuring that the young animals are released into a favourable e nvironment would improve survival in tho s e animals that woul d under natural conditions, have hat c hed into unsuitable situa tions such as most of the west
110 s hore of Rudolf, or the large, isolated islands. There i s also the alternative of m aintaining the animals in captivity right up to the point at which the y are ha rvested, altho ugh there are indications that s uch crocodile "fa rming may be economically unfeasible (Parker 1967, unpub. report to the Kenya Game Dep artment ). As already mentione d, cropping, will in i t i t s elf, relieve pressure on density dependent mortality, particularly if young animals are c ropped. 6c Cr ocodile management on Rudolf Since the continued survival of cro c odiles on Rudolf may depend upon their competing with other forms of utilization of the lake, such as the growing fishing indus t ry it is necessary to consider how th eir exploitation might best be c arried out. Cropping Some of the reasons for cropping those age classes approaching maturity have been considered. There are other reasons w hy this may be desirable. If adult animals are cropped then an essential requirement is that cropping operations be confined to the quiescent phase of the reproductive cycle, or serious interference with the reproductive rate is likely t o result. This means that the 8 month period October May, wh ich includes most laying, inc u bation and hatching, must be a closed season Cropping of imm ature animals on the other hand would interfere little with reproduction, although the 4 months Feb ruary M ay when the females are unearthing their nests is probably best kept a closed season at all times, particularly since some of the animals cropped will include recently matured females, even if the maxim um length is kept below 200 cm. Cropping young animals takes maximum advantage of the high rate of immature gro wth, and the still rising skin values which stab ilize after a length of 190 cm.
111 The imposition of an upper size limit in the vicinity of 200 cm wi ll introduce the danger of over c ropping females since the faster growth rate of males will leave them underr epresented in the se size (as distinct from age ) groups. This would have to be carefully monitored. There are important practical advantages in cropping young animals related to the fact that the conduct of a cropping operation is facilitated by both a cons tant s ize of product, and small unit size. In the field it is easier to train hunters to pick a consistent size gro up of animals than to select precise numbers of certain different sizes. Large animals are disproportionately difficult to handle and skin an d are awkward to pack and transport. Although the basis of the crop would consist of young animals analysis of the adult age structure might indicate some initial a d justment to maxim i se production. Some cropping of adults will be inevitable. The "control of crocodiles by the Game Department in response to the danger to human life and damage to fishing gear that crocodiles cause, presently involves the killing of several hundred animals a year, many of them adults. Illegal hunting removes an unknown but possibly significant number of adults per year. A certain quota of large males should be set aside for sport hunting since this is an important element in the tourist use of the lake. Removal of these animals probab ly has little effect on the stock since t heir expectanc y of further life is short. All these activities may combine to remove all the permissible adults of the crop, leaving the specifically commerc ial operation confined to young animals. A practic al consi deration which must be included is that 70.9 % of the lake s crocodiles occur on the east shore where the present political un certainties may inhibit operations. The extreme north end of the lake is in Ethiopia with about 8 % of the population. The rest of the lake falls under two different admin istrative areas, Marsabit and Turkana.
112 Egg production The other obvious form of e xp loitation is to consider the lake as a potential source of e ggs, either to support a crocodile farming indust ry, or for export to other areas to augment depleted stocks The estimated annual production of 26,000 eggs makes this a potentially sizeable prospect. The ve ry strong possibility that crocodile breeding on North, Central and South Islands c ontrib u tes nothing to recruitment makes them an ideal source of easily fou nd eggs. Section 7. Con c lusions Although further information is required the results of thi s work strongly suggest that Rudolf crocodiles presently exist at maximal densities, evidenced by a depressed growth rate, stunted size, small size of breeding females and small size at maturity. Within Lake Rudolf distribution is seen to relate closely to environmental conditions. Shelter m of shallow water, undisturbed by wind and with some cover provided by vegetation is essential. Such localit ies invariably contain relatively high densities whereas exposed shorelines, or those s u bject to the effects of wind, inv aria bly support low densities. This results in an accumulation of animals in the more favourable situations, producing the situation on Rudolf where 70.9 % of the population inhabits the east shore, the whole west side being exposed to the persistent easterly wind. Although considerable movement of individuals is thought to take place, occupance of favoured areas is at continuous high leve ls. The dependence of crocodiles on the shore and the shallow water near it effectively limits them to a relatively small area of habitat, on Rudolf approximately 2 % of the lake s surface area. Thus density in terms of biomass per unit area may be high wit h apparently low numbers of individuals.
113 The observed low incidence of food containing stomachs suggest that a primary agent limiting density is food availability, although it may be that when overcrowded crocodiles feed less frequently, thus depressing individual growth Whatever the causes it seems that Rudolf cro c odile n umbe rs have exceeded t hat which can exploit the food source opti m ally which has been compensated for by depressed gr ow th that in turn has led to a lowered reproductive rate. In plan ning the future of the population an important consideration is that in a rapidly developing environment management priorities cannot always wait for the completion of ideal long te rm research programs. The solution may be a compromise in which carefully planned exploitation, based on the indication s of this initial survey, is acc ompanied by research aimed at monitoring the effects of exploitation and finding ways of maximising it. It is certain that unplanned, uncontrolled exploitation will produce the sa me result as it has in other East African waters where something close to total extinction was achieved in a ve ry short space of time. Examples are Lake Rukwa in Tanzania, Lake s Victoria, Kioga and Albert in Uganda, the Semiliki R iver and much of the Nile: the Tana and Athi rivers in Ke ny a and more recent ly Lake Baringo. Thus sustained c ropping of adult animals offers little economic prospect due to slow growth and relatively small population size. But cropping youn g animals, supplemented by artificial red uction of i mm ature mortality, shows promise. Enough information is available to draw up a management program based on these indications, and which could be, at least in part, self financing. Proper cropping in a wider sense may be a highly desirable a d ju n c t to any conservation policy aimed at maintaining viable animal co mmuni ties, by a d justing the age structure and density so that reproductive and growth rates are maximised. This results in a more vigorous population better able to withstand catastrophe o r unfavourable conditions and to respond to favourable condition s
114 Acknowledgements The assistance of the professor and staff of the Z oology Department, University College Nairobi, in preparing the results of this work is gratefully acknowledged. Throu ghout the survey penetrating critic ism by Dr. R.M. Watson coupled with his clear perception of the problems of population research were of immense value. He and R.H.V. Bell, Serengeti Research Unit, contributed their skill and experience as observers durin g the counts. During the field work the several sear ch and rescue op erations and myriad other services perfo rm ed by R. Mc Connell, Fisheries Officer at Gulf were frequently the only means whereby the continuity of the s urvey was maintained. Ade quate re c ogni tion of suc h contributions is almost impossible. I wish to thank R. Bradley, University College, for help with the statistics. Peter Beard assisted in many aspe c ts of the field work. The trouble taken by E. D avison to measure the wild croc od ile in the Wankie National Park is much appreciated. Professor Loupekine, Geology Department, University College, kindly permitted his technician to prepare the denta ry sections. Miss Jabbal prepared the testis sections and Dr. A.H. Harthoorn provided f acilities for processing histological material. I am grateful to Bernd Fleischer of Achelis (Ke ny a) Ltd, and Dr. M. H y der, University College, for the photomicrograp h y. Space precludes listing all the zoos who replied to requests for growth data. Their w illingness to cooperate is gratefully acknowledge d. Interesting discussions were had with M ulji Modha, Kenya Game Department, and Dr. D.R.M. Stewart, Fauna Research Unit, lent equipment and assisted in a variety of ways.
115 Charles Little devoted consider able time and indu stry to maintaining my captive crocodiles. I wish to thank Miss Valerie Williams, Miss J. Reeves, Miss C. Graham and Mis s A. Jervis for h elp in preparing the manuscript.
116 References Atlas of Kenya (1959). F irst edition. The Survey of Keny a, Nairobi. Attwell R.I.G. (1959). Crocodiles at c arrion A fr. wildl 13 : 13 22. Birch, L.C. (1948). The intrinsic rate of natural increase of an insect population. J. anim. ecol. 17 15 26. Bouliere, F. (1963). Observations on the ecology of some la rge African mammals. African Ecology and Human Evolution 36: 43 54. Cagle, F.R. (1944). Sexual maturity in the f emale of the turtle, Pseudemys s cripta elegans Copeia 149 152 Carpenter, H.D. (1920). A naturalist on Lake Victoria. London. Cassie, R.M. (19 54). Some uses of probability paper in the analyses of size frequency distribution. Aust. J. mar. Freshw. Res. 5 (3) 513 522. Chabreck, R.H. (1967). Alligator farming hints. Typewritten report of the Louisiana Wildlife and Fisheries Commission, Grand Chen ier, La. Cott, H.B. (1961). Scientific results of an enquiry into the ecology and economic status of the Nile crocodile ( Crocodilus niloticus ) in Uganda and No rthern Rhodesia. Trans. zoo: Soc. Lond. 29 (4) 211 356. Darling, F.F. (1958). Bird flocks and t he Breeding Cycle. Cambridge. Darlington, P.J. (1957). Zoogeograp hy : the geographical distribution of animals. Mus. Of Co mparativ e Zoo. Harvard Univ. Deraniyagala, P.E.P. (1948). Same scientific results of two visits to Africa. S polia Zey lanica 25 (2) 31 32 D owning, H and Braizaitis, P. (1966). Size and growth in captive crocodilians. Int. Zoo Year Bo o k 6 265 269. F lower, S. (1957) Further notes on the duration of life in animals. Pro c Zoo. Soc. Lond. 1 07 A 1 59. Fuchs V.E. (1955). The Lake Rudolf Rif t Valley Expedition 19 3 4 Geog. J. 86(2 ) 114 42. (1959). The geological hi story of the Lak e Rudolf basin, Kenya. Philos. T rans. Roy. Soc. L ond. B. 229: 219 274. H ippel, E. V. (1946). Stomach contents of crocodiles. U g anda J. l2 : 148 9.
117 Internat ional Zoo Year Bo o k (19 66). A recent survey of longevi ty records for reptiles and amphibians in Zoos. 6: 487. Lac k D. (1954). The natural regulation of animal numbers. Clarendon Press, Oxford. Lamprey, H.F. (1964). Estimation of the large mammal densiti es, biomass and energy exchange in the Tarangire Game Reserve and the Masai steppe in Tanganyika. E. Afr. Wildl. J. 2 1 46. Laws, R. M. (195 3B ). A new method of age determination for mammals with special reference to the elephant seal. ( Mirou ga leonina Linn ) Falkland Is. D ep. Surv. Sci. Rep 2: 1 11 Laws, R.M. (1966). Age criteria for the African Elephant Loxod on ta a. africana E. Afr. Wildl. J. 4 : 1 370 Legler, J .M. (1960). Natural history of the ornate box turtle, Terrapene ornat a ornat a Aqassiz. Univ. K ans. Pubs. Mus. Nat. H is tor y 11: 527 669. Lord, R.D. (1959). The lens as an indicator of age in cottont ail rabbits J. Wildl. Mgmt. 23: ( 3 ) 558 360. Lord, R.D. (1959). The lens as an indicator of age in the grey fox. J. Mammal. 42: (1) 109 111. Macfadyen, A. (196 3 ) Animal Ecology: Aims and Methods. Sir Isaac Pitman. London. Marshall, R.H.A. (1956) Phy siology of Reproduction 3rd ed. 1 (1). Mc Ihl enny E.A. ( 1934) Notes on the incubation and growth of alligators. Copeia 80 88. Modha, M. I. (1967A). The eco logy of the Nile crocodile ( Crocodylus niloticus Lau renti) on Central Island, Lake Rudolf. E. Afr. Wildl. J. 5 74 95. Modha, M.I. (1967B). The ecology of the Nile crocodile ( Crocodyl us niloticu s Laurenti ) on Centra l Island, Lake Rudolf. M.Sc. thesis, Univers ity of East Afric a, 1 1 3 1. Peabody, F.C. (1961). Annual growth zones in living and fossil vertebrates. J. Morph. 108: (1) 11 62 Pitman, C.R.S. In Cott (1961). Nile crocodiles Crocodyl us niloticus versus Spurwing goose Plectropterus gambensis Bull. Brit. Ornith. Cl u b. 81 112.
118 Poole, D.F.G. (1961). Notes on tooth replacement in the Nile crocodile Proc. Zool. Soc. Lond. 136: 131 140. Pooley, A.C. (1962). The Nile crocodile. T he L ammergeyer 2: 1. Schmidt, K.P. and Inger, R.F. (1957). Living reptiles of t he world. Hamish Hamilton. Slobodkin, L.B. (1964). Growth and regulation of animal populations. Holt, Rinehart & Winston. Von H L. (1894). Discovery of Lakes Rudolf and Stefanie. Longmans. Watson, R. M. (1967). The population ecology of the wildebe este ( Connochaetes taurinus albo j ubatus Thomas) in the Serengeti. Ph.D. Thesis. Cambridge. 1 363. Worthington, B. (1932) The lakes of Kenya and Uganda. Geog. J. 79: 275 292
119 Appendix 1 Water analysis
120 Appendix 2. Two counts made to illustrate the high density of small crocodiles on Lake Rudolf. Count 1. Allia Bay, mid March 1966 at 2020 hours. Swamp east of sandbank. A single 180 o sweep of ght, no wind i.e. calm water & good conditions for seeing crocodiles. 126 crocodiles, mostly small, but no hatchlings. Count 2. A quarter of a mile further along the shore same evening a 90 o sweep returned 94 crocodiles. The maximum distance a Hunter la ntern with new battery can get a reflection off Effective distance (not all animals will be perfectly positioned to reflect) is less.