Front Cover
 Front Matter
 Summary and conclusions
 Literature cited
 Back Matter
 Back Cover

Group Title: Behavioral ecology of basking in the yellow-bellied turtle, Chrysemys scripta scripta (Schoepff) (FLMNH Bulletin v.20, no.1)
Title: Behavioral ecology of basking in the yellow-bellied turtle, Chrysemys scripta scripta (Schoepff)
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00027854/00001
 Material Information
Title: Behavioral ecology of basking in the yellow-bellied turtle, Chrysemys scripta scripta (Schoepff)
Alternate Title: Bulletin of the Florida State Museum, Biological Sciences ( Vol. 20 ; No. 1 )
Physical Description: Book
Language: English
Creator: Auth, David L.
Publisher: University of Florida
Publication Date: 1975
 Record Information
Bibliographic ID: UF00027854
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.

Table of Contents
    Front Cover
        Front Cover
    Front Matter
        Front Matter
        Page 1
        Page 2
        Page 3 (MULTIPLE)
        Page 4
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        Page 20
        Page 21
        Page 22
        Page 23
        Page 24
        Page 25
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        Page 29
        Page 30
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        Page 34
        Page 35
        Page 36
        Page 37
        Page 38
        Page 39
        Page 40
        Page 41
    Summary and conclusions
        Page 42
        Page 43
    Literature cited
        Page 44
        Page 45
    Back Matter
        Back Matter
    Back Cover
        Back Cover
Full Text

of the

Biological Sciences

Volume 20


Number 1





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SYNOPSIs: The purpose of this study is to determine if basking in turtles is thermoregulatory and
to describe the attendant behavior. Observations were made from a blind on a marked popu-
lation of Chrysemys script script (Schoepff) and an unmarked population of Chrysemys flori-
dana peninsularis Carr at a north Florida sinkhole pond. The study was conducted between
August and December, 1968, thus providing a wide range of environmental temperatures and
sunshine in which to observe any variations in basking behavior.
The number of atmospherically-basking C. s. script and C. f. peninsularis per day, as well
as the number of times individual C. s. script basked per day, reached a peak at a daily mean
water temperature of 28.50C. Basking decreased during overcast periods. Aquatic basking in
C. s. script and C. f. peninsularis increased as water temperature approached 31.50C, the max-
imum mean water temperature recorded during the experimental period. Temperature-
telemetry revealed that an adult basking script could elevate its body temperature at least 100C
above near-surface water temperatures.
Because of decreased sunshine and air and water temperatures from August to December,
and the resultant slower rate of body temperature rise, mean basking duration in C. s. script
increased during those months. Basking duration is also directly related to body size.
Basking does not appear, to be a daily requirement, even under optimum weather condi-
tions. Some C. s. script basked up to five times a day in favorable weather; however, a consid-
erable percentage of the population did not bask every day.
Basking activity started about 0800 and reached a daily maximum between 1000 and 1100
each clear day in August and September. The time at which activity started and the time it
reached a daily maximum shifted to a later part of the day during October and November.
Thermoregulation appears to be the primary function of the basking act in C. s. script and
C. f. peninsularis. This conclusion is based on the following basking behavioral patterns:
1) body orientation, head and leg extension and retraction, leg kicking, and rear-foot digit
2) remaining on a site for considerable periods of time after it becomes shaded and the turtle's
body temperature has increased, but rarely emerging at an already shaded site;
3) varying frequency and inversely varying duration with decreasing air and water tempera-
tures and sunshine (shown for C. s. script only); and
4) sitting on submerged basking sites or floating in warm surface water.

'The author is currently a graduate student in the Department of Zoology, University of Florida. This paper
in an altered form was submitted as partial fulfillment of the requirements for the degree of Master of Sci-
ence at the University of Florida.

AUTH, DAVID L. 1975. Behavioral Ecology of Basking in the Yellow-Bellied Turtle, Chrysemys
script script (Schoepff). Bull. Florida State Mus., Biol. Sci., 20(1):1-45.

S'1 o. 8,

F> 3 ( ,k

Vol. 20, No. 1

,lc /


IN T R O D U C T IO N .. ............................................... ..... ............................................... .................

. ......... 3


MATERIALS AND METHODS............................

R ESULTS ......................... .. ... ........................................ 5
Basking and the Physical Environment ........................................................ .................. 5
B askin g D u ration ......................... ...... .............. .............................................. ............. 15
B asking g F req u en cy ................................................ ................................. .......................... 19
Daily and Monthly Variation in Basking Number, Duration, and Frequency .................. 26
T he Basking A ct .................. ................... .............................. ................. 30
T em p eratu re-T elem etry ....................... ................................................................................ 3 7

D ISCUSSION ................... ........... ... ................ ...

SUMMARY AND CONCLUSIONS .....................................
L ITERATURE C ITED .....................................................

.............. 39
................. 42
..................... 44


Basking is a characteristic habit of aquatic turtles. Moll and Legler (1971)
listed two forms of this behavior in Panamanian Chrysemys scripta: "aquatic
basking," or floating in warm surface water, and "atmospheric basking," or
basking in air. Chelydra serpentina and Sternotherus odoratus (Cagle 1944)
are two species that characteristically practice the former. Unless otherwise
stated, the term "basking," as used in the present paper, refers to atmospheric
McDowell (1964) and Weaver and Rose (1967) will be followed in this
paper, placing floridana, nelsoni, and scripta in the genus Chrysemys rather
than Pseudemys.
The yellow-bellied turtle (Chrysemys scripta scripta) periodically leaves
the relative thermal stability of water to bask atmospherically. Several people
have reviewed the possible functions of this activity in turtles (Boyer 1965,
Moll and Legler 1971, Spray and May 1972) and have offered several possible
explanations: (1) the increased temperature differential between the turtle's
body and the surrounding medium may expedite some discontinuous process
such as digestion (Cagle 1950); (2) drying of the integument; (3) removal of
algae and parasites; (4) synthesis of vitamin D in the skin; (5) increased ease
of respiration; and (6) allowing the turtle to rest, especially in flowing water
habitats. As Spray and May (1972) suggested, these explanations are probably
not mutually exclusive.
Field observation is one approach to answering the question of why turtles
bask. Boyer (1965) watched a population of Chrysemys scripta elegans under


a variety of environmental conditions, but did not trap and mark individuals.
During the present study individual turtles were marked so that I could de-
termine where a turtle basked, the number of times it basked, and the duration
of each appearance. I could then ask the question whether the turtles were
behaviorally thermoregulating by varying their basking movements, fre-
quency, and duration in correlation with daily and seasonal changes in solar
and thermal radiation fluxes and air and water temperatures.
Chrysemys scripta scripta was selected because intensive study of its bask-
ing behavior had not been attempted, and because it is relatively easy to
trap, mark, and observe in one of its preferred habitats, sinkhole ponds. A
group of Chrysemys floridana peninsularis was also present at the study site
but was not marked. Some information on the basking behavior of this species
is included.

I thank Walter Auffenberg for making the pond on his property available for the study, for
use of equipment, and for aid in preparing the manuscript. Some funds and supplies were pro-
vided by the Florida State Museum.

The study was conducted between 4 August and 4 December, 1968, at a single dystrophic
sinkhole pond, located seven miles northwest of Gainesville, Alachua County, Florida (Fig. 1).
The pond surface area was estimated to be 14,000 sq. ft., but varied seasonally by fluctuating
water levels. The pond, surrounded by a mesophytic hardwood forest, was covered with duck-
weed (Lemnaceae): Lemna minor L., Wolffia columbiana Karst, and Wolfiella floridana (J. D.
Smith) C. H. Thompson. The two major woody plants in the water were buttonbush (Cephalan-
thus occidentalis L.) and tupelo-gum (Nyssa aquatica L.).
The natural turtle population consisted of Chrysemys script script (Schoepff) (yellow-
bellied turtle), Chrysemys floridana peninsularis Carr (peninsular turtle), and Sternotherus
odoratus (Latreille) (common musk turtle). The latter species was never seen basking atmos-
I caught Chrysemys s. script in standard hoop net traps (Lagler 1943), using smelt as bait.
Chrysemysf. peninsularis could not be trapped in this way. A basking trap, consisting of a 10-foot
cypress plank with a chicken wire basket submerged on one side and a ramp on the other side,
was installed in the pond to catch C. f. peninsularis. Because of the time required to catch a sig-
nificant sample and the amount of the disturbance required to scare turtles into the basket, I
soon discontinued using this kind of trap.
Six dimensions for each turtle (Jackson 1964) were measured with a metal caliper. Carapace
area was approximated using aluminum foil. The foil was pressed onto the surface of the cara-
pace and the form cut along the outside edge of the marginals. Excess foil folded on itself was
then removed from the form. The weight of the foil form divided by the weight per unit area gave
the approximate carapace area.
Chrysemys script script individuals were marked in two ways: (1) by drilling holes in the
carapace marginals; and (2) by painting Arabic numerals on the carapace (Cagle 1944), with a
marine epoxy paint. Four numbers were painted on larger turtles: one anteriorally, two later-
ally, and one posteriorly. Turtles having a carapace less than 10 cm in length received only two
lateral numbers. Only one adult turtle lost its numbers during the experimental period; this oc-
curred as a result of laminal shedding. Small turtles lost numbers more readily than adults, be-
cause of paint chipping from the small turtle's smooth carapace. Of the marked turtles 99 per-
cent could be identified from the blind during basking periods, the remaining one percent was
small turtles.


\ o


FIGURE 1.-Map of experimental pond.


The natural population in the pond was estimated at 47 Chrysemys s. script and 18 Chry-
semys f. peninsularis, of which 30 C. script were marked. In order to increase the number of
marked turtles, 24 C. s. script, one C. f. peninsularis, and three C. nelsoni Carr (Florida red-
bellied turtle) were trapped in two other duckweed-covered ponds near Gainesville and added
to the study pond. The total number of turtles studied was approximately 93; this comprised
54 marked and 17 unmarked C. s. scripta, one marked and 18 unmarked C. f. peninsularis, and
three marked C. nelsoni. The number of unmarked turtles was estimated from the basking fre-
quency of the marked turtles. It is assumed that 13.2 percent of the marked C. s. script either
escaped from the pond or died, since these adults disappeared from the basking sites early in the
experimental period.
The primary basking areas were (1) four floating plywood sites, (2) the trap, (3) three large
logs,and (4) three buttonbush trunks. The plywood sites measured 3' wide X 4' long X 1' deep.
These were made by nailing a piece of marine plywood to a rectangular wooden frame, to form
a five-sided box which was inverted in the water and held in place by anchor ropes. Two par-
tially inflated tire inner tubes were inserted under the plywood in order to keep it floating when
crowded with basking turtles. Ramps were nailed to three sides of the raft at an angle of 45 de-
grees. It was found that large turtles had difficulty ascending the ramps, repeatedly slipping
back into the water; hardware cloth nailed to the ramp surfaces solved this problem. The ply-
wood sites were arranged in a row from east to west (Fig. 1). As the sun increased its seasonal
southward declination and an increasing percentage of the pond surface was shaded by sur-
rounding trees, this row was moved periodically northward in order to provide more-or-less con-
stant exposure to the sun.
The blind, supported by two 50 gallon oil drums and pilings driven in the pond bottom,
floated on the pond and could be approached via a floating pier (ca. 35 feet long). After re-
moving a few buttonbushes, the entire pond could be seen from the blind. The blind was equipped
with a Yellow Springs 12 channel telethermometer to record air and water temperatures at
two-hour intervals during the day. Air temperatures were taken in the shade at a height of 15 cm,
and water temperatures were taken in the duckweed at a depth of less than a centimeter and
below the duckweed at a depth of 5 cm. Wind velocity was measured at a height of 25 cm with
a three-cup anemometer. A daily record was kept of basking appearances and included identi-
fication of the turtle, time of emergence, time of departure, and site chosen. A short time lag
was unavoidable, especially when 20 or more turtles were basking at once. Observations were
made using binoculars, and a portable tape recorder and stop watch were used to record the
timing of a turtle's movements.
In addition to the above field study, temperature telemetry was used to record a turtle's
changing body temperature during basking in a small concrete-lined pond, by implanting the
probe of a frequency-modulated, temperature-telemetering radio transmitter in a 1901 g female
C. s. script. The receiver used was a Biotronics Model 90 module plus Model 133 Receiver-
Converter. The squegging transmitter, set at 88 megacycles, was accurate to within 1IC.
Audible signals were received approximately 3 m from the turtle, without the use of a trans-
mitting antenna. The transmitter probe was implanted interpleurally through a hole drilled
in the carapace at the junction of the first and second lateral and the second central laminae,
and the probe wire was glued to the top of a plastic dropper bottle affixed to the carapace with
epoxy glue and four small brass screws. The transmitter, sealed in a plastic cylinder measuring
1 cm wide by 3 cm long, was attached to the posterior carapace with wires running through
holes drilled in the llth marginal and a postcentral. The turtle basked the same day she was re-
leased. No postoperative abnormalities were noted. Interpleural temperature, air and water
temperatures, and notes on basking behavior were recorded on five consecutive days, starting
49 days after implantation.



LIGHT.-Light intensity is one factor that determines whether turtles bask.
C. s. scripta and C. f. peninsularis respond to at least two characteristics of the


sun's electromagnetic radiation: the intensity of visible light and the transfer
of heat energy. The former is probably more important in determining fre-
quency and the latter more important in determining duration. Air and water
temperatures also influence basking frequency and duration. With so many
variables involved, an environmental room study is required to quantify the
control of basking frequency and duration; however, some insight into the
relative importance of the environmental factors can be obtained from field
The influence of three factors (minutes of direct sunlight per day, mean
water temperature, and mean air temperature) on the number of turtles bask-
ing per day is shown in Figure 2. Within the mean water temperature range
of 21-290C, the number of basking individuals increased in direct proportion
to the length of time the sites were exposed to the sun. Outside this water
temperature range, longer exposure to sunlight was required to attract the
same number of turtles. In other words, low or high environmental tempera-
tures and overcast skies reduced the arrival rate at otherwise satisfactory bask-
ing sites.


0 120
0 0
9 70- o

20- 0

FIGURE .-Influence of light on number of turtles basking per day. Mean water temperature
30.0-C or above (open circles), 21.0-29.9'C (solid circles), 15-20.90C (stars), 14.50C or less
(open squares). Water temperature mean was calculated from readings taken at two hour in-
tervals each day in the surface duckweed and at a depth of five cm.

0 0 i
o oo doo' i3oo

FIGURE 2.-Influence of light on number of turtles basking per day. Mean water temperature
30.0C or above (open circles), 21.0-29.9C (solid circles), 15-20.9C (stars), 14.5C or less
(open squares). Water temperature mean was calculated from readings taken at two hour in-
tervals each day in the surface duckweed and at a depth of five cm.


Turtles were much more likely to emerge at sites located in direct sun-
light than at those in the shade. During eight warm clear days in September
(mean air temperature =29.2C) only 57 of a possible 973 turtles emerged
(5.9%) on shaded basking sites. At lower air temperatures the percentage de-
creased still more. On eight clear days with a mean air temperature of 16.80C,
only one of 153 turtles emerged (0.65%) in the shade.
Both C. s. scripta and C. f. peninsularis rapidly decreased their rates of
emergence when a cloud blocked the sunlight on partially cloudy days (Fig.
3). The emergence rate dropped almost to zero during cloudy periods. The
departure rate did not increase as much as the emergence rate decreased, al-
though if the overcast period lasted very long many turtles returned to the
water. The increase in emergence rate with clearing skies could be quite dra-
matic, especially in the morning. At 1100 (EST) on August 31, when the sky
had partially cleared, two turtles were basking. After 85 minutes of partially
cloudy skies, the number of emerged turtles had risen to 35.
Basking turtles were not randomly distributed over the pond on clear
days. The distribution of emergences was skewed toward the west side of the
pond in the morning and toward the east in the afternoon. The basking sites

1300 .4RAIN



1000 -

0900 -


0800 "'"'"1"''"1 '""""'"'"" """" """ ""'"I ""'" I """"
1 10 20 30 40 50 60 70 80 90

FIGURE 3.-Effect of intermittent cloud cover on the rate of turtle basking emergence. Each
vertical bar represents the basking time of one turtle on September 20.


on the west side of the pond received direct sunlight first. As time progressed
the forest canopy shaded fewer and fewer basking sites, until almost the entire
pond was in direct sunlight. As the western sites became shaded in the early
afternoon, the turtles progressively shifted to the more eastern basking sites.
Since turtles clearly preferred basking sites located in direct sunlight, pond
shading may be the only reason for the daily shift in sites of emergence. How-
ever, there is some indication that turtles were changing sites of emergence
in direct response to the changing position of the sun (Fig. 4). Sites were ar-
ranged in a row, numbered consecutively from west to east. All four sites were
in direct sunlight between 1000 and 1400. Turtles emerged more frequently
on the western sites between 1000 and 1200, distributed more equally be-
tween 1200 and 1300, and emerged more frequently on the eastern site in the
early afternoon. The low emergence rates at sites two and three may be
due to their close proximity to the blind. One way to determine the influence
of the sun's position on basking site choice would be to observe the distribu-
tion of emergences in a pasture pond, free of forest canopy shading.
The shift of activity from east to west during the day was also seen in the
distribution of surface-floating turtles (Fig. 5). The number of turtle heads

SITE: 1 2 3 4
TIME: 1000-1059

1 2 3 4

1 2 3 4 12 3 4
1200-1259 1300-1359

FIGURE 4.-Influence of the position of the sun in the sky on the turtle's choice of basking site.
Each bar represents the sum of emergences at one plywood site for 10 days in late August and
early September. Sites arranged and numbered as in Figure 1.


Vol. 20, No. 1

50- I--|

5o :

- 30I

20- 5 .


1000 1100 1200 1300 1400 1500 1600 1700 1800

FIGURE 5.-Daily shift of surface floating turtles from the western side to the eastern side of the pond. Narrow bars represent heads
counted at one hour intervals west (shaded bars) and east (open bars) of site three, located in the center of the pond. Wide bars
are sums for the entire pond. Bars are sums of data collected on eight clear warm days in September through November.


seen east and west of the site three, which was located in the middle of the
pond, was counted each hour during eight warm clear days. In the morning
and early afternoon, more heads were seen west of the site; later in the after-
noon, more heads were seen east of the site.
It is clear from examination of the location of emergence of individual
turtles (Fig. 6) that the shift in basking activity is not necessarily a result of
the turtle population swimming across the pond to the east each day return-
ing to the west by the following morning. Some turtles (15 of the sample of
32 C. s. scripta) chose basking sites in the western portion of the pond more
than 60 percent of the time with the extreme represented by a turtle emerging
32 times west of site 3 and only 2 times east of the site. A second group of nine
regional baskers chose predominantly eastern basking sites. A third group
(N = 8) emerged a nearly equal number of times in both regions of the pond
(diffuse baskers). Unfortunately, I was unable to track individuals when they
were in the water. Thus, I do not know how much time regional baskers spent
in the portion of the pond where they did not commonly emerge. The small

N 15



0 5 10 15 20 25 30 35
FIGURE 6.-Regional and diffuse basking site choice. Basking emergences east and west of site
three were counted for 32 Chrysemys script script for 33 days of observation. Sites were ar-
ranged as in Figure 1. Dots on the upper and lower diagonal lines represent turtles emerging 60
percent of the times west of site three, 40 percent east of site three, and 40 percent west, 60 per-
cent east, respectively.

Vol. 20, No. 1


size of the pond (125' x 150') and the occasional emergence of even the most
regional basker outside its usual region indicate that most C. s. scripta have the
whole pond as their home range. However, some restriction of movement is
indicated for the majority of the turtles. One result of turtle preference for one
basking region is a partial equalization of numbers among the sites, limiting
crowding on any one site.
WATER AND AIR TEMPERATURES.-Water temperature is a second impor-
tant environmental variable influencing turtle basking. During warm weather
both C. s. script and C. f. peninsularis spent considerable time floating with
heads out of the water. Therefore, except at low water temperature, pond sur-
face water temperature was considered more important in determining bask-
ing frequency than bottom temperature. Mean water temperature in this
paper is the average of measurements taken at two-hour intervals during the
day in the duckweed at a depth of less than a centimeter and below the duck-
weed at a depth of 5 cm.
Figure 7 is a scatter diagram showing numbers of basking appearances per
day of C. s. scripta and C. f. peninsularis vs. mean water temperature. The
peak number appeared at about 28.50C, with a noticeable decline above and
below this temperature level (The two highest numbers of C. s. scripta at a
mean water temperature near 18'C occurred on two days of a warming trend
in early November, when mean air temperature was considerably above mean
water temperature [+ 3.5 and + 4.60C, corresponding respectively to 98 and
114 total baskers per day]. Perhaps turtles were responding to the high air
temperatures by increasing basking frequency). Boyer (1965) stated that Chry-
semys s. elegans did not bask when the water temperature at 5 cm depth was
below 7C; the same minimum was found for Chrysemys s. scripta in the
present study. Chrysemys f. peninsularis stopped such activity at a water tem-
perature of approximately 140C, seven degrees higher than for Chrysemys s.
scripta and C. s. elegans. Perhaps C. f. peninsularis stopped basking at a higher
water temperature than C. s. scripta or elegans because it is a more southern
form that only narrowly overlaps with C. s. scripta and not at all with C. s.
In August, surface water temperature reached as high as 370C during the
early afternoon, partly due to the heat absorbing layer of duckweed. During
this time, many turtles floated quietly at the surface or basked on submerged
sites, with only part of the carapace and head exposed. Basking on submerged
sites increased as mean water temperature increased (Fig. 8), with the peak
for such activity occurring at approximately 31.50C (highest mean water tem-
perature recorded during the observational period); this peak was three de-
grees higher than for individuals both partially submerged and completely
out of the water (Fig. 7). The higher the water temperature, the greater the
number of turtles that selected a partially submerged site, as opposed to a site
completely out of water.


0* 0

0 0

* 0

S 0o



u, 70-

< 60-

I -







30- 0 0

20 0 0

1 0 00 00000

0@1'I 1 1 5 1 20 o 0 1 1 1 1


FIGURE 7.-Influence of water temperature on basking emergence in Chrysemys script script
and Chrysemys floridana peninsularis. Plywood basking sites shaded by clouds for 300
minutes or less per day (solid circles), more than 300 minutes per day (open circles). Water
temperature is an average of readings taken at two hour intervals in the duckweed and at a
depth of five cm.

* *





Vol. 20, No. 1

u1 V




o .
Z 15-

O 0

0 00 .

FIGURE 8-Influence of water temperature on the number of turtles basking on submerged sites.
Chrysemys s. script (Fig. 9). The sample analyzed consisted of 13 males and0

13 females, all of which were in the pond for the duration of the study. All

5 o **

be seen from commonly than females below an
0 O0 0O

than 300 minutes per day (ople s ). Water temperature is an average of readings taken

tw ate r temperature may also have influenced the sexf ratio of basking

basking sites. The male predominance above 30'C possibly resulted from
greater female wariness of the sites or the blind early in the experimental
Chrysemys s. script (Fig. 9). The sample analyzed consisted of 13 males and
13 females, all of which were in the pond for the duration of the study. All
turtles selected had carapace lengths greater than 11 cm, since smaller in-
dividuals could not be accurately sexed using nail and tail characters. As can
be seen from Figure 9, males basked more commonly than females below an
approximate mean water temperature of 20C, and below 14C no females at
all were observed. The number of basking males and females both decreased
with decreasing mean water temperature, with the rate of decline in females
greater than that in males. As the water temperature rose above 14C be-
tween November 30 and December 3, a number of females reappeared at the
basking sites. The male predominance above 30C possibly resulted from
greater female wariness of the sites or the blind early in the experimental
period (which began in August). No explanation of male predominance below
20C can be given.


100 *0 *

90 *


S I2 i' i8 20 22 24 26 28 32
5--------------Iu -W -5

:E 400



10 12 ;4 16 18 20 2224 26 28 30 32
FIGURE 9.-Influence of water temperature on sex ratio of basking Chrysemys scripta scripta.
The numbers of turtles in the samples are 13 males and 13 females larger than 11 cm cara-
pace length. Dots falling on the dashed line indicate that equal percentages of the 13 males
and 13 females basked that day.

Rain decreased the rate of basking emergence and increased the rate of
departure; the rates were dependent on both the duration and strength of the
rain. During 25 periods of rainfall totaling over eight hours on 14 different
days, only 14 turtles emerged, and 98 of 208 baskers (47%) departed soon after
the rain started. The departure rate was greater and the emergence rate less
as rainfall per unit time increased. The rapid departure of turtles during a hard
rain is illustrated in Figure 3.
The question may be asked whether water or air temperature is more im-
portant in influencing a turtle to bask. Body temperature of turtles in the size
range of C. s. scripta and C. f. peninsularis is not much higher than the temper-
ature of the surrounding water. Thus, water temperature is probably more
important in preventing turtles from emerging. At the experimental pond,
the difference between mean air temperature and mean water temperature
usually did not exceed 20C until mid-October, when differences exceeding
20C became more common due to intermittent cold fronts from the north.
Mean water temperature was usually lower than mean air temperature, the
largest difference observed being 5.70C during a warming trend in early No-


member. On four clear days when the mean air temperature reached 16.9C
and the mean water temperature 11.90C, only 56 turtles basked. On four
other clear days during which mean air temperature was 17.70C (0.80C higher
than the first four day sample) and the mean water temperature 16.80C (4.90C
higher), 133 turtles emerged or almost 2.4 times as many. In this case at
least, the warming of the water seemed to be the important inducement for
In order to determine optimum basking temperatures (both air and water)
on clear days, temperatures were analyzed for those days considered to have
had "ideal" basking conditions. Such days were arbitrarily considered to be
those during which 100 or more basking appearances were recorded; using
this criterion 15 of the 65 days of observation qualified. The average of the
daily mean air temperatures for these 15 days was 28.70C (range 25.0-32.40C,
S.D. 1.98), and the average of the daily mean water temperatures 27.90C
(range 22.0-31.20C, S.D. 2.45). Analysis of the 10 days during which the great-
est number of turtles were observed basking at any one time shows a mean of
36.7 individuals (range 33-51, or approximately 35.5-54.8% of the total pond
population; S.D. 5.5). The mean time of occurrence was 1109 (0940-1236), the
mean air temperature 25.7C (range 22.0-29.00C, S.D. 2.5), and the mean
water temperature was 24.30C (range 22.7-26.10C, S.D. 1.1).

Basking duration is defined as the time of a single basking appearance.
In Chrysemys s. scripta this time was found to increase, on the average, dur-
ing the clear days of the observational period. Illustrated in Figure 10 are the
mean basking duration, mean basking frequency (mean number of basking ap-
pearances per turtle for individuals emerging at least once a day), mean air
temperature, and mean water temperature for marked C. s. scripta during 22
clear days. Duration increased slowly during August and September and more
rapidly during October and November, as mean air and water temperatures
fell below 280C and became more variable. This increase probably was caused
primarily by the seasonal decline in daily solar energy flux, decreasing the
rate of heating of the basking turtle. The increasing temperature difference
between the CTM (critical thermal maximum, defined as the body tempera-
ture at which the turtle is no longer able to right itself when placed on its
back) and body temperature at emergence probably is also important. Water
temperature, which is approximately equal to the body temperature of emerg-
ing turtles (Edgren and Edgren 1955), determines the magnitude of difference
at emergence. Other factors being equal, the greater the temperature differ-
ence between body temperature at emergence and the CTM, the greater the
basking duration, assuming that the turtle leaves the site only when the CTM
is approached (Boyer 1965). Minor decreases in duration may be expected



16 20 21 22 23 4


6 15 19


1 2 13 26 27 28


FIGURE 10.-Mean basking duration and mean basking frequency of marked Chrysemys script
script on clear days. Mean basking duration histogramm), mean basking frequency (large
solid circles), mean air temperature (small solid circles), and mean water temperature (open
circles). The number of turtles used for the duration and frequency means is given at the
base of each bar.



U 2.3



1 1.9
-35 Z
-25 I
:25 1.6

-20 1.5 Z

S 14


. 1.1

3 9, 23- 3
20 26

17l181 6 P19 23 6 23 1


Vol. 20, No. 1


from a decrease in the CTM in response to decreasing seasonal water temper-
ature and photoperiod (Hutchison and Kosh 1964). Mean basking frequency
generally decreased during the observational period from a peak which oc-
curred during the first half of September. Basking duration appears to be in-
versely related to atmospheric basking frequency, except during the warm
month of August, when aquatic basking was most common.
In C. s. scripta body size was found to be directly related to basking dura-
tion, as can be seen in Figure 11. All regression lines show a significant upward
trend (0.1% level). As body size increases, the surface to volume ratio de-
creases, resulting in decreased heating and cooling rates and presumably an
increased range of possible basking durations. Data for Figure 11 were care-
fully selected in order to minimize the influence on basking duration of shad-
ing, cloud cover, environmental temperature, time of emergence, and dis-
turbance. Except in four cases in which basking exceeded 200 minutes, turtles
were in the sun the full time. Turtles commonly returned to the water before
body temperature increased appreciably. This is undoubtedly due in part to
increased turtle wariness, caused by trapping and marking.
Although large turtles usually basked longer than small ones, this did not
always hold true. Of the 102 basking sessions lasting 180 minutes or longer
that I observed, turtles smaller than 10 cm carapace length accounted for 27
(26.5%). It is surprising that the body temperature of some of these small
turtles did not exceed the CTM. The smallest basking C. s. scripta caught had a
volume to carapace area ratio of 0.35 (carapace length = 3.9 cm). This turtle
could not have basked between 1000 and 1400, according to the position of
the regression lines of Figure 11.
Some small basking turtles had a large percentage of their carapaces
covered with duckweed, whereas in larger turtles only scattered clumps were
usually present. Little algal growth was present. Duckweed may decrease the
rate of body temperature rise in three ways; (1) by increasing the reflection
of sunlight, (2) by increasing insulation, providing a so-called "private zone"
(Guyton 1966:992) that must be heated before the shell can be heated, and
(3) by increasing heat loss by evaporative cooling. The water holding capacity
of duckweed is large, a 744 g wet sample having a dry weight of 35.2 g. In an
experiment designed to test the insulating properties of duckweed, 8 g of wet
duckweed were evenly distributed on top of a black metal cube, which in turn
was exposed to a 150 watt incandescent bulb located directly above. In 40
minutes the surface temperature of the metal had increased from 25 to 400C.
In contrast, when only a thin film of water was initially present on the cube,
the same temperature increase required 6.4 minutes. A dry cube required
only 4.3 minutes.
The majority of turtles returned to the water before the basking sites on
which they were resting became shaded. Some turtles, however, spent as
much, or more time on the shaded basking site as on the same exposed one.


150 I I I I I I I I I
T: 1001 -1100 *
r- 0.671
100 y- 2721x-5.81

0@* 0
0 0
15 280 T 1101 -1200 0

100 y 5111x-65.32

280 T :1201 -1300
Sr 0.487
Y 43979x-3277

I* I *

200 -0 2.5 3.0 3.5 40 4.5

100 *

0 0.5 1.0 15 20 2.5 3.0 3.5 4 0 4.5


FIGURE 11.-Relation of basking duration to turtle size. Each dot represents the time of a single
basking appearance of a Chrysemys script script on one of six warm clear days in Sep-
tember. The time span of 1000 to 1400 is divided into four one hour segments and each seg-
ment is graphed separately. T is the time of turtle emergence, r is the coefficient of linear
regression. Regression lines were determined by the method of least squares.


Amount of time spent in the shade was related to size of the turtle, time pre-
viously spent basking in the sun, and air temperature (Fig. 12). Time spent in
the shade, as well as time spent in the sun, increased directly with size of the
turtle. For turtles smaller than 1500 ml, longer sun-basking times appear to be
correlated positively with time in the shade. For turtles larger than 1500 ml,
this does not appear to be the case. However, some of these large turtles were
out of water for more than five hours at a time, and thus one may ask if a time
limit may be reached beyond which it is no longer profitable to stay on the
basking site, even though a temperature difference between the body and the
water exists. As might be expected, on cold days turtles quickly returned to
the water when the basking sites became shaded. As air temperature fell, the
shade times of all turtles decreased, regardless of sun basking time or body

S N:12 I 11 26 1 7 I 12 I 14 I 2 I
0-500ML 500 1500 >1500
0 90MN 91-180 0-90 91-180 0-90 91-180 >18
FIGURE 12.-Relation of shade duration to turtle size and sun basking time. The number of turtles
used for the mean is given at the base of each bar. Data were collected on 15 clear days with
a mean air temperature of 16.1-31.00C.


Turtle basking frequency is defined as the number of basking appearances
per unit time. Basking frequency will be examined in several ways: on the pop-
ulation level by a simple count of baskers per day (Fig. 13) and by mean

>- I I

(3o .

Z 0
o o>

0 0 0*

-o 00

t 'b 2 3 2 1 IS 25 lbi I Y b b5 I A

FIGRE 13.-Record of number of turtles basking (solid circles) and departing due to disturbance per day (open circles). Lines P
connect the number of basking turtles for consecutive days of observation. The horizontal line across the graph is the estimated
total number of Chrysemys script script, Chrysenys floridana peninsularis, and Chrysemys nelsoni in the pond.


basking frequency per day (Fig. 14), and on an individual level by the bask-
ing records of specific individuals (Fig. 15).
The total number of turtles basking per day for the four-month observa-
tional period is shown in Figure 13. September was the peak basking month,
although over 100 turtles emerged as late as November 3. A similar graph
extending over a twelve-month period is bimodal, with basking peaks in the
spring and fall, a lower number of baskers during the summer, and the least
during the winter. Low basking numbers occurred during the October 26-30,
November 13-26, and December 4 cold periods, on overcast and rainy days,
and during the high water temperatures of August. Minor deviations in the
line estimating the pond population in Figure 13 are due to trapping and ad-
ditions of turtles and the assumed escapes of marked C. s. scripta. The large
number of turtles scared off the basking sites (29.9 percent of the total basking
departures) is undoubtedly positively correlated with the number of baskers
per day as well as with basking frequency. If the sky remained clear and the
turtle was not scared into the water late in the afternoon, it commonly
emerged again after a time. I have taken care to avoid using the basking fre-
quency data collected on days when many disturbances interrupted turtles
that may have basked only once under normal circumstances.
How the mean percentage of the total marked C. s. scripta population
basking per day and the mean basking frequency (mean number of appear-
ances per day for those C. s. scripta basking at least once a day) are related
to different weather conditions and to each other is illustrated in Figure 14.
The weather conditions are as follows:
1) clear and warm (mean air and water temperatures above 200C);
2) partly cloudy (total plywood site sun time reduced more than 200
minutes by cloud cover) and warm;
3) overcast and warm;
4) clear and cool or cold (mean air or water temperature equal to or below
5) partly cloudy and cool or cold; and
6) overcast and cool or cold
(5 and 6 will be used in two subsequent figures). The mean percentage of the
total C. s. scripta population basking per day and the mean basking frequency
were greater on warm-clear and partly cloudy days than on warm-overcast
and cold-clear days (i.e. an increase in percentage of the population basking
and in the number of times individuals emerged when the temperatures were
warm and the skies clear). At such times, it was common for turtles to bask
two or three times a day, and occasionally four and five. The percent of the
total marked script population basking per day was usually considerably
below the maximum observed value of 92. It is clear that basking is not an
absolute daily requirement for C. s. script, because even during the most
favorable weather not all individuals will emerge.





0 -

Z 30-

^ 0-

1 2 1,2 5 A




3 -



1,2 12 3 4

FIGURE 14.-Mean percent of total turtles that bask per day and mean basking frequency in dif-
ferent weather conditions. A total of 26 marked Chrysemys script script were used. Mean
(horizontal line), range (vertical line), and one standard deviation from either side of the
mean (open rectangle). Number of days of data used in the calculations is at the top of the
range. Weather: 1) clear and warm, 2) partly cloudy and warm, 3) overcast and warm,
4) clear and cool or cold.

The basking records of 37 C. scripta over a 61-day period can be seen in
Figure 15. Variability is evident, with one individual emerging on only 11%
and another on 74% (X = 44%) of the 61 days. A high percentage of the individ-
uals were not seen on the sites for long periods of time, although some un-
doubtedly basked on days I was not in the blind (50.4% of the total days). Fe-
males tended to appear less frequently than males, especially when water
temperature fell. One may also note the wariness of the 108 mm turtle, which
emerged for brief periods on 11 of the 21 total days it appeared.
An estimated 35 percent of the C. s. script present in the pond had a
carapace length of 100 mm or less. The estimate was obtained from the bask-
ing frequency of the 11 small marked C. s. script (since the total numbers of
appearances per day of marked and unmarked small C. s. scripta [Am and A,]
were known, as well as the number of small marked turtles in the pond
[Nm= 11], the number of small unmarked turtles [NJ] could be estimated
from the equation: N, =(NmA)/Am). Before 0900 and after 1400, 53-57%


Vol. 20, No. 1

2211 1112111311 31 2 12 11 332311 13444444464r 4344454433314
em 1 0 17;
13 *0 21
N5 i 222t
16 6* 0 ** em *1* Mm M
< 25 ~.237f

2 8 126 L,
Z 29 _* *234 V)
31- 250
33 : 225
a M 224

S43 l Cie iam Ic"T
Z *203m
OrA6 c179lM c
48 .152m
51 200aM
52 74 U
0 52 *.9 4
53 8 aC
- 54 .192
< 59 *
O 59 1 I1m U C
63 65
63 71
65 75m
73 iem 7
74 120m

FIGURE 15.-Basking records of individual Chrysemys script script. Turtle basked 10 minutes or more per day (black square), O
less than 10 minutes per day (solid circle), not present in the pond (horizontal line). Consecutive days of observation are indicated
by horizontal bars on the abscissa. Weather: 1) clear and warm, 2) partly cloudy and warm, 3) overcast and warm, 4) clear and cool
or cold, 5) partly cloudy and cool or cold, 6) overcast and cool or cold.


Vol. 20, No. 1

of the basking C. s. scripta were smaller than 100 mm, 18-22% more than ex-
pected (Fig. 16). This may have resulted from physical competition for space
on the basking site, because small turtles could not make room by pushing
aside larger turtles. Another possible explanation has to do with body tem-
perature rise during basking, which is slower in early morning and mid-to-late
afternoon than at midday (when environmental radiations are maximal); this
presumably is a more important factor with regard to smaller than larger
turtles and may somewhat restrict small turtle basking to the cooler portions
of the day. A third possibility is that small turtles start activity earlier in the
morning due to spending the night floating at the surface, whereas larger
turtles stay on the bottom. Thus, if the start of daily activity is correlated with
a specific light intensity in the pond water, small turtles would become active
and start basking sooner than large turtles.
For specimens of C. s. scripta weighing over 500 g, there seems to be little
correlation between basking frequency and body size, as can be seen from
the following data (taken on 15 warm clear days in September): 51-499 g, 1.65

55; -280

40 -200

35: --- ----------- -- ---- ---180
30 -160


2 100 o

15 80

5- 20

0801- 0901- 1001- 1101- 1201- 1301- 1401- 1501- 1601-
0900 1000 1100 1200 1300 1400 1500 1600 1700

FIGURE 16.-Relation of turtle size to time of basking in Chrysemys scripta scripta. Small turtles
(carapace length less than or equal to 10 cm) basking per hour (open circles), larger turtles
basking per hour (solid circles), percent of baskers which are small turtles (open squares),
expected percent of baskers which are small turtles (dashed horizontal line). Data collected
on 16 warm clear days in September and October.



mean basking appearances per day, 500-999 g, 1.36, 1000-1499 g, 1.36, and
2000-2651 g, 1.38. C. s. scripta smaller than 500 g may either be forced to
leave a basking site more frequently than larger turtles due to high body tem-
perature, or they may more often be physically forced off a site by larger in-
dividuals. The latter was occasionally observed.
As seen in Figure 15, nine C. s. scripta that were trapped at two other
duckweed covered ponds near Gainesville were added to the pond during the
observational period. An additional six turtles, part of the original pond pop-
ulation, were trapped a second time. These 15 turtles were kept in a dry room
from 3 to 11 days (mean of 7.3 days). They were released, usually on warm
clear mornings (13 of 15 releases) prior to the appearance of the first basking
turtle. A high percentage (13 or 87%) basked the same day. On the average,
the new releases basked longer and at a higher frequency than the rest of the
C. s. scripta population both on the day of release and on the second day ob-
servations were made (For 9 of the 15 releases, there was a variable break in
continuous daily observations between the first and second days) (Fig. 17).
Subsequent declines in basking frequency and duration on the second and
third days were not due to unfavorable weather. Turtles defecated repeatedly

2-12 TA

2 5



: 05-

FIGURE 17.-Mean basking duration and frequency of newly released Chrysemys script script
(N = 15). Newly released turtles (solid circles), rest of population (open circles). Turtles were
released on five different days, most of which were warm and clear.



while in captivity and were thus probably released with relatively empty
guts. Perhaps, the turtles basked more intensively in response to duckweed
eaten the day of release.


Daily basking started at about the same time in August and September
(Fig. 18). The first individual usually emerged on the western plywood site
just before or soon after the site was in full sunlight. Overcast skies delayed
the time of first emergence. During October and November, basking activity
started at later times during the day and was not correlated so closely with
the illumination of the western site. The delay in basking activity was prob-
ably due to decreasing environmental temperatures and photoperiod. Except
for August days, which usually had two maxima, a single peak in basking ac-
tivity occurred daily, as determined from the maximum number of turtles
basking simultaneously. This maximum number usually occurred during mid-
morning from August to October, but like the time of first emergence, became
more variable and occurred at later times in October and November. The
maximum number of basking turtles observed at any one time during the
period of study was 51 (an estimated 55% of the total population), this occur-
ring on 3 November. The late date is correlated with the fact that basking
durations averaged longer later in the year.
Two variables (the mean number of basking turtles and the mean bask-
ing duration per appearance) along a common daily time axis for the four
months of the study are compared in Figure 19. The graphs will be discussed
separately and then compared. Three types of basking number curves are
shown in the upper graph. The typical August curve for a clear day was bi-
modal, with a midday decline in basking number due to high surface water
temperature and insolation. During the early morning and early afternoon,
the rate of emergence was greater than the rate of departure, resulting in an
overall increase in the number of turtles. The reverse was true as midday and
evening approached. The basking number curves for warm, clear days in
September through early November were asymmetrical and unimodal, with
a midmorning peak and generally decreasing numbers of individuals during
the rest of the day. The third type of basking number curve was unimodal and
symmetrical, occurring on warm, partially or completely overcast days and on
clear, cool to cold days in November. The number of baskers declined after
1400 in all cases. If basking has a thermoregulatory function, the declines
would be expected based on declining environmental temperatures and in-
solation. Also, most of the turtles that will emerge have already done so by
midafternoon. In the lower graph, the increase in basking duration from
August to November can be seen. Basking durations for each month are rela-


Vol. 20, No. 1

o o

0800- ?e o0

0 g. 3
1000 232 9 o o


1300 2 20

1400 29 0
o 0
:19 l 0 8 o 28

1500I o I L II II hla lll lt hel l !ii'
1 0 2 14 0 20

u --- No DE

FIGURE 18.--Time of first basking emergence (solid circles), time of first basking emergence prior to arrival of the ob-
server at the pond (open circles; time given is observer arrival at the pond, not time of turtle emergence), time of max-
imum number of turtles basking at once (open squares; maximum number of turtles basking at once given under each
open square), approximate time the first western basking site was sunlit (varying horizontal line), consecutive days
of observation (horizontal bars on the abscissa). Weather: 1) clear and warm, 2) partly cloudy and warm, 3) overcast
and warm, 4) clear and cool or cold, 5) partly cloudy and cool or cold, 6) overcast and cool or cold.
FiUR 1-im O irt akig mrgnc soidcrcestie f istbakngemrene rortoarialo te b
serer t he on (oencirles tme ivn i osererarrvalatthepod, ottim o tutleemrgece, tmeof ax
imm ube o urls asig tone oensuaes axmm ume o urle asig t ne ienune ec

and warm, 4) clear and cool or cold, 5) partly cloudy and cool or cold, 6) overcast and cool or cold.


FIGURE 19.-Monthly rhythms of basking number and duration. Values are means for the fol-
lowing days: 8 clear days in August, 10 clear days in September, 10 clear days in October,
and 14 clear or overcast days in November.

Vol. 20, No. 1




12 I



S6 2

2 3

I, ~ NOV,

S 16I

1 21

FIGURE 20.-Number of baskers per hour in terms of number of times the turtle has basked. Total
turtles basking per hour (total curve), turtles basking per hour for the first time (curve 1),
turtles basking per hour for the second time (curve 2), turtles basking per hour for the third
time (curve 3), etc. Data were collected on five warm clear days in September and five
clear warm or cool days in November and December. Only Chrysemys script script are


tively constant from 1000 until 1400. Late afternoon declines resulted from
the paucity of turtles remaining on the basking sites after the sun had set. The
high October 0800-0900 duration mean was due to a few large turtles that
stayed nearly all day on a western basking site. These large turtles emerged
early in the morning, attained high body temperatures, and spent the after-
noon on the site in the shade. A comparison of the two graphs shows that
the number of basking turtles was high in September, although basking dura-
tions were relatively low, whereas the reverse was true in November. In
August both basking numbers and duration were low, and in October values
were intermediate.
In Figure 20, the basking number curves for five clear days in September
and five clear days in November were broken down according to the number
of basking appearances per day. During the September period, 46% of the
total number of turtles emerging basked once a day, 34% twice, 13% three
times, 5% four times, and 2% five times. In November, mean basking fre-
quency was lower, with 75% of the baskers basking once a day, 21% twice,
and 4% three times. It was found that most turtles either emerged during the
morning or not at all, those appearing in the afternoon being, with few ex-
ceptions, repeaters from earlier in the day. During November, however, no
such preponderance of early morning emergences was noted, a high percent-
age of individuals basking during the afternoon having come out for the first

The basking act may be divided into four stages, based on changes in turtle
body orientation, head and limb posturing, and movement on the basking
site. The present description is based on an adult male and female C. s.
scripta situated on a horizontal site out of water on a clear warm day, with no
disturbances and no other turtles close enough to restrict the subject's move-
ment. Numbers in parentheses correspond to numbers in Table 1.
ARRIVAL AT THE BASKING SITE.-The prospective basker usually was first
seen less than a foot from the basking site, but on occasion individuals were
observed as much as 30 feet away. After a period of stationary floating, the
turtle swam to the site with its head above water. Some turtles disappeared
below the duckweed, reappearing momentarily at another ramp or farther
along the log. This occurred most commonly when the first site approached
was occupied by a larger turtle. Turtles often surfaced their front legs,
paddling laterally while simultaneously surfacing the head. The leg movement
temporarily cleared duckweed from the water surface, resulting in the turtle's
eyes being fairly free of the plant. It seems clear that C. s. scripta was locat-
ing the basking site visually, by surface and, likely, subsurface exploration.
The turtles commonly returned day after day to the same basking site.


Vol. 20, No. 1




Time of Occurrence

1. Extension

2. Rotation

3. Bobbing
4. Protracted Eye Closure
5. Extension and Retraction

6. Eye Watering
7. Eye Blinking
8. "Smelling" the Substrate

9. Bending Toward a Disturbance
10. Yawning
11. Pumping of the
Hyoid Apparatus

12. Walking Position

13. Extension

14. Rotation and Partial
15. Kicking

16. Retraction

17. Head Rubbing

18. Walking Position
19. Extension (Not Touching
20. Digit Spreading

21. Kicking
22. Extension (Relaxed on
23. Rotation and Partial
24. Retraction

25. Relaxation and Retraction

26. Rotation (Sun Orientation)
27. Rotation

Characteristic of Stage I and early
II, occurs less often in other stages.
Especially common during a disturbance.
Characteristic of I, occurs less often
in other stages.
I-III, most commonly in II and III.
Characteristic of later III.
Characteristic of later III, commonly
occurs in IV.
Characteristic of later III.
I-IV, least in I, most often in III.
Occasionally in any stage, least
commonly in II.
Occasionally in any stage.
Occurs in all stages, with highest
frequency in Stage III.

Characteristic of I, observed less
often in other stages. Occurs before
and after a walk, before departure,
and when disturbed.
Intermittently in II and III, sometimes
in IV.
Intermittently in stages when legs
are extended.
Characteristic of III, occurs less
often in IV, infrequently in I and II.
Characteristic of III, sometimes occurs
in IV.
Occurs in all stages, most commonly
in III.

Same as for front legs.
Intermittently in II, III, sometimes in IV.

Characteristic of II, occurs sometimes
in IV.
Same as for front legs.
Intermittently in II, III, sometimes IV.

Same as for front legs.

Same as for front legs.

Alternates in all stages. More commonly
retracted in III and IV.

Characteristic of IB, III.
Occasionally in IV.


28. Walking Fairly common in III, especially
among smaller turtles, less common
in IV, sometimes occurs in IB.
29. Departure Characteristic of the end of III, IV,
and during disturbance, but occurred
voluntarily at any time.

proached the site, usually at an unoccupied spot, grasped the site with its
front legs, and pulled itself partially out of the water. Large individuals
sometimes experienced considerable difficulty in climbing out. For example, a
large C. f. peninsularis female (4,743 g) and two large C. nelsoni females
(3,054 and 4,838 g) sometimes struggled two to three minutes, trying all three
ramps of a plywood site, before pulling themselves onto the horizontal sur-
face. One or two strokes with a front leg (17) removed any duckweed clinging
to an eye. The head was fully extended (1), with the eye level above the level
of the site. Small turtles commonly did not climb the ramp far enough to see
the surroundings directly ahead. Turtles repeatedly rotated their heads in all
directions, moving in small increments with a pause between each movement
(2). The maximum rotation was only occasionally greater than a 900 angle
with the long axis of the body. Rotation and maximum head extension con-
tinued throughout Stage I.
Stage IA usually lasted less than 10 minutes. Turtles with carapace length
longer than 20 cm commonly bypassed this stage, crawling immediately to a
horizontal position.
STAGE IB: STABILIZATION OF POSITION.-After crawling completely out of
the water, the turtle rested with its legs either placed on the substrate (12,
18), dangling down toward or into the water, or in a nearly retracted position.
The turtle either paused a few minutes or oriented itself immediately with re-
spect to the sun. If the turtle's initial position happened to be the most advan-
tageous one with regard to location of the sun, the animal made no further ro-
tational movement. Otherwise, it moved so that the orientation angle (Fraen-
kel and Gunn 1961:107) was approximately 1800, with the turtle's head di-
rectly away from the position of the sun (26). If the sun was near its zenith in
a summer declination (i. e. almost directly overhead), orientation was random.
Having achieved the desired position, the turtle either extended its rear legs
(thus entering Stage II) or remained for a time with its legs resting on the sub-
It is clear from the extension and rotation of the turtle's head and the posi-
tion of the legs on the substrate that Stage I represents a period of high aware-
ness, during which the surroundings are scanned for possible danger and the
turtle can depart with maximum efficiency. Turtles were most likely to de-
part during this stage, either due to cloud cover, wariness, or disturbance.


Vol. 20, No. 1


Stage I behavior occurred more consistently than behavior of later stages, and
was repeated under all light and temperature regimes. The only exception
was body orientation, which was random on heavily overcast days.
STAGE II: EXTENSION OF THE REAR LEGs.-The rear legs were either ex-
tended simultaneously or one at a time (19). They were held above, and ap-
proximately parallel to, the substrate and long axis of the body. The webbed
digits were spread apart to a variable degree (20) and partly pronated, with
the medial side held higher off the substrate than the lateral side. Occasion-
ally, the rear legs were rotated slightly, or partially retracted and extended
again (23). Subsequently, tension in the interdigital skin was reduced and the
legs rested on the substrate (22).
The front legs were curled back along the bridges of the carapace or ex-
tended parallel to the long axis of the body (13), with the medial edges of the
legs resting on the plastron. However, the legs were also occasionally in in-
termediate position.
The head was extended less than in Stage I, but still was held above a
horizontal position. Head rotation decreased and eye blinking (7) became
more common. The turtle occasionally bobbed its head during Stages II and
III (3). The head was quickly moved vertically, diagonally, or in a circular
path, the motion lasting less than a second. Bobbing possibly is a response to
drying of the head skin or some other irritation. Turtles often bobbed when
insects flew about their heads.
CREASING ACTIVITY.-This was usually the longest stage, although Stage IV
was sometimes longer. Stage III consisted of periods of relative quiescence,
alternating with shorter periods of activity. As body temperature continued
to increase, the periods of quiescence shortened and activity increased.
During early periods of quiescence, the head was horizontal and partly
retracted. The front legs were usually curled back along the bridges of the
carapace or resting on the substrate in a walking position. The rear legs were
usually partly to completely extended and relaxed on the substrate. During
later periods of quiescence, the turtle retracted its head, limbs, and tail into
the shell (5, 16, 24, 25). On occasion, the head was quickly and fully extended,
as in Stage I. After a short period of rotation, the turtle retracted its head
again. The elevation of the head may be adaptive, since with its head com-
pletely retracted a turtle is vulnerable to attack. If a turtle is alone on a bask-
ing site and has its head retracted, one may sometimes approach closely
enough to pick the turtle up. However, since C. s. scripta usually basks in
groups, at least one turtle will usually have its head out. The vibrations
caused by departing turtles is usually a sufficient stimulus to cause any Stage
III baskers to raise their heads.
During periods of activity, turtles kicked their legs, rotated their bodies,
walked about the basking site, and rubbed their heads with a front leg. A back-


leg kick (15, 21) started with the leg extended and relaxed on the substrate or
retracted into the shell. The leg was moved quickly anterolaterally, bent at
the knee, sometimes so far forward that the webbing extended over the dorsal
surface of the marginals. The leg was then smoothly extended posteriorly.
One fore or rear leg was kicked at a time, up to 25 kicks in quick succession,
followed commonly by kicking of one or more other legs. The time between
kicking bouts decreased and the number of kicks per leg increased as the stage
The turtle occasionally rotated its body before or after kicking, usually
from 90 to 3600. A change of orientation angle from 180 to 900 resulted in
partial shading of two legs and half the carapace, provided the sun was not
directly overhead. Thus, the movement possibly could decrease heat intake.
Most turtles rotated at least once after the Stage I orientation, with some ro-
tating four or five times during Stage III. The probability of rotation increased
with time.
Walking for short distances (28) increased (especially among small indi-
viduals) as density of turtles on the site increased. This usually resulted from
physical displacement by larger individuals although occasionally a turtle
moved voluntarily for distances up to a foot, returning to a Stage III posture
after stopping. Voluntary walking was more common late in the stage.
The turtle commonly rubbed its head with its front legs (17) during Stage
III. Rubbing was a response to irritation caused by watering eyes, clinging
duckweed, insects flying near the head, and drying of the skin. Eye watering
(6) started approximately halfway through the stage, with a corresponding
increase in eye blinking. The eyes were closed (4) for longer periods, especially
during head retraction.
Yawning (10) occurred occasionally during any of the basking stages. It
consisted of a maximum lowering and then closure of the lower jaw, lasting
two to three seconds.
Pumping of the hyoid apparatus (11) commonly occurred throughout the
basking appearance, with greater frequency at higher body temperature.
Gaping, common in Panamanian C. scripta (Moll and Legler 1971), was
not observed at the Gainesville pond.
At the end of Stage III, the turtle returned to the water (29). Turtles
often departed earlier, before the CTM was approached. Departure was usu-
ally preceded by a period of activity, rather than a quiescent period of head
and limb retraction. Some turtles simply rotated toward the water and pushed
off. Others remained balanced on the log or ramp edge, raised the plastron
off the substrate as though preparing to depart, but sitting down again on the
substrate. The turtle extended its head toward the water or into the water
several times before finally pushing off. Turtles usually took the shortest route
to the water.


Vol. 20, No. 1


basking site was shaded in the afternoon or when the sky became overcast,
turtles usually did not return immediately to the water, but rather sat in the
shade for a time. The duration of Stage IV was dependent on at least five fac-
tors: (1) previous length of time in the sun (Fig. 12); (2) size of the turtle
(Fig. 12); (3) time of day the sky became overcast or the site shaded (turtles
departed sooner from sites shaded in the late afternoon than in midafternoon);
(4) air temperature; and (5) solar energy flux.
If a turtle was previously in Stage I, departure usually occurred after a
short time. If previously in Stage II, the rear legs remained extended and the
digits spread for a few minutes, then relaxed more of the time. If previously
in Stage III, activity soon decreased, the eyes stopped watering, and the turtle
kicked its legs less often. Retraction of the head and limbs was common after
the first few minutes in the shade, whether the turtle was previously in Stage
II or III. The head was occasionally extended and rotated, then retracted
"Smelling" the substrate (8) was observed occasionally in all stages, but
was most common in Stage IB when the turtle first emerged, Stage III, and
late Stage IV. "Smelling" consisted of a slow dirolent movement of the head,
with the nostrils approaching the substrate.
A few minutes before departure in Stage IV, the legs were returned to the
walking position. Activity sometimes became more common, including short
walks, kicking, and rotation of the body (27). Departure movements were the
same as for Stage III.
C. s. scripta displayed little aggressive behavior while basking. Turtles
usually remained near the periphery of the plywood sites, the new arrivals
emerging at unoccupied spots. As density increased, more turtles emerged
where others were basking. If the resident basker was smaller, the emerg-
ing turtle soon nudged the resident, which moved inward away from the
water. If the emerging turtle was smaller than the resident, it (the former)
either ceased its efforts entirely or swam to an open place farther along the
site. A strict correlation with size was evident, regardless of species. For ex-
ample, the largest C. s. scripta, a 2,651 g female, was nudged aside by the two
female C. nelsoni (3,054 and 4,838 g) and the large female C. f. peninsularis
(4,743 g). The size of the turtle was also important during body rotation.
Smaller turtles were pushed off the site by larger ones in 88 percent of the 43
cases observed; the reverse occurred only 12 percent of the time. On rare oc-
casions a resident individual would snap at an intruder, thus causing the in-
truder to move quickly out of the way. Bury and Wolheim (1973) have re-
ported aggressive behavior during basking in Clemmys marmorata, but at a
much higher frequency (145 aggressive acts in 37 hours of observation) than in
Chrysemys scripta.
During only four of 4,441 basking appearances was a turtle found to be
balancing itself on the carapace of another. Each time this involved small


C. s. scripta (less than 10 cm carapace length) balancing on larger individuals
of the same species.
Turtles basking on submerged sites did not perform some of the Table I
movements. Eye watering, protracted eye closure, leg kicking, and Stage III
orientation, all characteristic of high body temperature, were not observed.
However, head extension and rotation, "smelling the substrate," bending the
head toward a disturbance, yawning, head rubbing (duckweed removal), front
and rear leg extension, rear leg digit spreading, and Stage IB orientation all
were seen. Leg extension was observed only in turtles sitting fairly high in the
water. Moll and Legler (1971) saw C. script in Panama fully extend their
legs while floating in surface water. In the present study, however, the duck-
weed coat made observation of the legs impossible, unless the turtle was partly
out of the water. Stage IB orientation also occurred in stationary floating
turtles, but not when feeding on surface duckweed nor when moving across
the pond. Although one may assume that Stage IB orientation increases heat
intake, it is interesting to note that orientation occurred in turtles in water,
in which the temperature was little different from the animal's body tempera-
ture. This suggests that Stage IB orientation is a sun compass reaction, whereas
the Stage III orientation is a response to both the position of the sun and the
turtle's increasing body temperature.
The orderly sequence of basking movements also did not occur on com-
pletely overcast days. Stage I remained the same, except that IB body orien-
tation was random. Stage II leg extension and digit spreading occurred much
less often. Extension and spreading were more frequent during lightly over-
cast than heavily overcast periods. Activity characteristic of the higher body
temperature of Stage III was also less frequent. Eye watering did not occur.
Stage III usually lasted longer than Stage II, which in turn was longer
than Stage I. Stage IV was of variable length, ranging from shorter than Stage
I to longer than Stage III. In general, the mean duration of Stages I-III in-
creased and Stage IV decreased as air temperature and the sun's energy flux
decreased. Large turtles still reached the end of Stage III when air temper-
ature was below 150C, if the turtle emerged early enough during the day.
On cold days, the movements of Stage I were slower, due to the turtle's low
body temperature. Also, retraction of the legs and tail occurred earlier in
Stage IV.
Turtles with carapace length less than 15 cm voluntarily walked farther
and more frequently than larger turtles. During Stage II, small turtles held
their legs off the substrate more often and spread their rear leg digits more
consistently than larger turtles. C. s. script from near hatchlings to old
adults were seen basking. The smallest C. s. script caught while basking had a
carapace length of 3.9 cm, slightly less than a centimeter longer than would
be found in a hatchling (Carr 1952:246). Brattstrom (1965) reported that C.

Vol. 20, No. 1


picta hatchlings were not seen basking. He suggested that hatchlings reach suf-
ficiently high body temperatures by floating in the warm surface water.
Chrysemys f. peninsularis and C. scripta basking movements were simi-
lar, except for leg extension. C. f. peninsularis held its legs off the substrate
more of the time and spread its rear leg digits more extensively during Stage II
than C. scripta, on both clear and overcast days.


Illustrated in Figure 21 are three days of basking by the C. s. scripta fe-
male with the implanted interpleural temperature probe. Included are data on
interpleural, air, and water temperatures (water temperature is a mean of sur-
face and 5 cm depth measurements); time the turtle was in the sun, shade,
and water; and the basking stage. The turtle's core temperature on emer-
gence was about IC below water temperature, because water temperature
was measured at the surface, whereas the turtle had been in the colder bottom
water prior to emergence. By basking, the turtle raised its body temperature
as much as 100C above water temperature on all three days. On October 22,
which was partly cloudy, air and body temperatures were about the same.
But with longer periods in direct sunlight on 20 and 23 October, body tem-
perature was also considerably above air temperature during basking. On 22
October, when the turtle was temporarily shaded, core temperature con-
tinued to increase for a time. The lag time of continued core temperature
increase was probably due to the large temperature difference between the
surface of the carapace and the core. On 23 October core temperature showed
an almost immediate leveling off or decline with shading (1200-1300), due to
the smaller temperature difference between the core and the carapace sur-
face. As Mackay (1964) suggests, the "driving force" that changes the rate of
heat movement between shell surface and core is the difference in tempera-
ture between the two; the greater the difference, the greater the rate of heat
flow. The bone of the shell may not be so good an insulator as first appears,
since the bone is well supplied with blood vessels (Spray and May 1972). There
is little doubt, however, that the scutes are insulative (Rose 1969). On 20
October the turtle moved short distances from shade to sun seven times, as
the shadow of a building advanced over her carapace (partial shade in the
figure). Movement from shaded to sunlit parts of the basking sites was never
observed at the experimental pond. Note that the rates of heating on 20 and
23 October are greater than the rates of cooling in the afternoon shade. The
main reason for the rapid rise in body temperature is, of course, the high heat
input from solar radiation. However, differences in basking behavior of Stage
II vs. Stage IV may also be important; i.e. extension of the head and legs and
sun orientation in Stage II vs. retraction of the head and legs in Stage IV. Also,


9 10 I I 12 13 14 15 16

OCT 22 V

2 s oooo.******
2 iII~



35- "/,/0* .

3.- . ... o .o 0

3 ..*/iiiii II iii

... "*..

20 6000..* BASKING STAGE
ol "II I I I V

OCT 20
-- T IN SUN ....*...
3 ..... T-IN SHADE L. "
ooo T IN WATER '',
30 *** TO "00o
..... T. IN SHADE / ........., ..
2 ..... ............

0io0.p 0I0I

_ ..

9 I 1 12 1'3 14

15 16


FIGURE 21.-Summary of three days of temperature-telemetry data. Tb is interpleural body tem-
perature, Ta is air temperature, and T, is water temperature.

blood supply to the carapace may be greater during heating than during cool-
ing (Spray and May 1972).
Body temperature ranges can tentatively be assigned to C. s. scripta bask-
ing stages, based on field data, temperature-telemetry data, and information
from the literature. Body temperature at the start of Stage I varies with am-

No. 1







S I Iv

I II 1 5 I


bient water temperature. C. s. scripta basks atmospherically within a water
temperature range of 7C to an unknown maximum, somewhere above
28.50C mean water temperature (Fig. 7). The body temperature at the start
of Stage II is probably quite variable, from just above 70C to 31-330C, the
body temperature at the start of Stage III. Stage III ends near the CTM, which
is approximately 410C in C. s. scripta (Hutchison, Vinegar, and Kosh 1966).
Note that on October 22 (Fig. 21), when body temperature did not reach
310C, Stage III basking movements were not observed. The body temperature
at the start of Stage IV is highly variable, from just above 70C to near 410C.
The turtle cools in the shade to near the ambient air temperature (Fig. 21)
before returning to the water. The body-air temperature difference at de-
parture is probably greater when the air temperature is low.


The question may be asked whether a turtle's elevated body temperature
during basking is an unavoidable consequence of sitting in the sun (Pritchard
and Greenhood 1968) or a desirable result, achieved through active behav-
ioral and physiological control. Boyer (1965), Moll and Legler (1971), and
Spray and May (1972) suggest that turtle basking is primarily thermoregu-
latory. I also have concluded that basking in Chrysemys scripta scripta and
Chrysemys floridana peninsularis, which results in a body temperature that is
higher than either the ambient air or water temperature for significant periods
of time, is thermoregulatory.
C. s. scripta and C. f. peninsularis basked less frequently on shaded bask-
ing sites and during cloudy and rainy weather. Basking in these situations did
not produce an appreciable body-environmental temperature difference. If
basking were thermoregulatory, basking frequency should decrease under
these "unfavorable" conditions. One may ask why C. s. scripta basking fre-
quency should also decrease at low temperatures and cease below a water
temperature of 70C, even though basking results in an elevated body temper-
ature. Goodman (1971) found that the brown water snake, Natrix taxispilota,
did not emerge from the water, no matter how high the air temperature, if
the water temperature was below 10C. He suggested that emergence at low
water temperature would be futile, since activity and thus food capture are
minimal. Adult C. s. scripta and C. f. peninsularis were often seen feeding on
duckweed, using a seining movement similar to the South American turtle,
Podocnemis unifilis (Belkin and Gans 1968), but neither of these species ate
duckweed at low water temperatures. It may be hypothesized that basking
would serve no purpose at low water temperature, assuming that basking is
primarily a means of digesting food. On the other hand, atmospheric basking
at high water temperatures would be unnecessary, since body temperature
can be kept in an optimum range by aquatic basking.


By basking up to five times a day during warm weather, C. s. scripta in-
creased the time its body temperature was above environmental temperature.
Turtles returning to the water at the end of Stage III often reemerged quickly,
before body temperature had fallen to near the water temperature. Basking
behavior during the second to fifth appearance was often different from the
first appearance, Stage I being abbreviated and Stage II being abbreviated
or absent. This presumably resulted from the fact that the turtle's body tem-
perature was already elevated upon emergence from the water.
There was an inverse correlation between basking duration and frequency.
At warm environmental temperatures and relatively high solar energy flux,
basking frequency was high and durations short. As environmental tempera-
tures and solar energy flux decreased, basking frequency decreased but dura-
tion increased. Thus, the body-environmental temperature difference was
maintained over a wide range of environmental temperatures for a longer
time each day than if basking frequency and duration were constant.
C. s. scripta and C. f. peninsularis usually did not emerge at a shaded site,
but sometimes sat in the shade for long periods after basking in the sun (Stage
IV). In the first case, body temperature is not far above ambient air tempera-
ture, but in the second case, body temperature may be at least 100C above
ambient air temperature, especially if the turtle is in Stage III when the site
is shaded. Return to the water would result in a fairly rapid loss of the heat
gained by basking. By remaining in air, the elevated body temperature is
maintained for a longer time. In smaller turtles and low air temperature, the
rate of heat loss during Stage IV is more rapid. Return to the water, there-
fore, is also more rapid.
Some of the movements of the basking act appear to be thermoregulatory.
Extending the legs and spreading the rear digits, common in emydids during
basking, has been suggested as a means of drying the skin (Boyer 1965) and
for synthesis of vitamin D from skin sterols under the influence of solar ultra-
violet light (Pritchard and Greenhood 1968). Another possibility is that the
sequence of limb and head movements in C. s. scripta and C. f. peninsularis
are a means by which the animals can more rapidly approach and maintain
an optimum basking temperature range. Extension of the head and limbs, and
digit spreading of the rear legs during Stage II may increase body temperature
rise, due to the greater vascularity and heat conductivity of the head and limbs
than the carapace.
The mean water temperature above which atmospheric basking begins to
decrease (28.50C), the mean water temperature of maximum aquatic basking
(31.50C), and the body temperature at the start of Stage III (31-33C) are
about the same. This may mean that the high body temperatures of Stage III
are above the optimum temperature range. Stage III body orientation, leg
kicking, and retraction of the limbs and head under the carapace may be
means of slowing down body temperature rise. Leg kicking increases as Stage


Vol. 20, No. 1


III progresses. Kicking, alternating with retraction of the limbs under the cara-
pace, may decrease the rate of body temperature rise by increasing radiation
heat loss, assuming that kicking increases blood flow to the extremities. In
Chrysemys floridana, blood flow to the extremities is known to increase with
heating (Weathers and White 1971). Heart rate also increases with increasing
body temperature (Akers and Damm 1963) and with voluntary activity in
Chelodina longicollis and Emydura macquari (Webb and Johnson 1972). Re-
traction of the head, limbs, and tail into the shell later in Stage III may de-
crease temperature rise by reducing exposed surface area. The same behavior
in Stage IV may reduce heat loss, increasing the time a body-environmental
temperature difference is maintained.
Chrysemys scripta scripta and Chrysemys floridana peninsularis appear to
have some physiological control of body temperature rise during basking.
Control appears to be exerted primarily through changes in blood flow to the
periphery of the body with changes in body temperature. Spray and May
(1972) found that Chrysemys scripta becomes heated more rapidly than it
cools in both air and water, and they stated that the turtle may actively con-
trol conductance by altering blood flow. Spray (1972) found that shifts in body
weight during heating and cooling occur in opposite directions in Chrysemys
picta and Terrapene ornata. He interpreted this to be a shift in blood volume,
which in C. picta moved toward the periphery during heating and away from
the periphery during cooling, and in the reverse direction in T. ornata. In
Chrysemys floridana the half time of 33xe clearance from the carapace and
subcutaneous area of the hindleg during cooling was significantly greater than
during heating, indicating that blood flow to these areas was greater during
heating than during cooling (Weathers and White 1971). Stitt, Semple, and
Sigsworth (1971) found a plasma sequestration (blood flow cut off from a re-
gion of the body) in Graptemys geographic during cooling from 20 to 50C.
The sites of plasma sequestration were the skin, shell, kidney, gut, and muscle,
whereas blood flow to the liver and lungs were maintained during cooling
(Stitt and Semple 1971). Rewarming resulted in recirculation of sequestered
plasma. Although the above studies were performed on restrained turtles in
the laboratory, basking turtles in the field may also change blood flow pat-
terns during heating and cooling.
The anterior hypothalamus is the center of thermosensitivity in the turtle.
Localized hypothalmal heating in Chrysemys scripta resulted in a significant
rise in arterial blood pressure and cooling resulted in a decrease (Rodbard,
Samson, and Ferguson 1950, Heath, Gasdorf, and Northcutt 1968). The ap-
parent contradiction of vasodilation of blood vessels in the carapace and limbs
during heating and increased blood pressure during hypothalmal heating are
perhaps resolved by hypothesizing concomitant visceral vasoconstriction
(Heath et al. 1968). Surgical removal of the sensory nerve supply to the cara-
pace of Chrysemys picta resulted in equivalent heating and cooling rates, in-


stead of the faster heating rate found in controls (Spray and May 1972). Per-
haps, the change in temperature of peripheral receptors, as well as heating of
the hypothalamus, is important in blood shift control.
Does basking facilitate more rapid or efficient digestion of a meal? Rates
of substrate digestion by the enzymes pepsin, trypsin, erepsin, and amylase
increase with temperature in various turtles (Kenyon 1925, Chesley 1934,
Wright, Florey, and Sanders 1957). However, no one has investigated the pos-
sible correlation of turtle basking and digestive efficiency (the animal's ca-
pacity for decreasing caloric content of its food by digestion). Boyer (1965)
found no correlation between basking frequency and feeding time in Chry-
semys scripta elegans. Moll and Legler (1971) did find a basking-feeding cor-
relation in C. scripta from Panama. In a group of juveniles, basking was most
frequent within 48 hours after feeding. After 48 hours the turtles distributed
themselves more equally between illuminated and nonilluminated sites. In
the present study, basking numbers were high on the first warm clear day
after a series of cloudy days. On subsequent clear days, basking numbers de-
creased. The same phenomenon was reported for C. scripta in Panama (Moll
and Legler 1971), and in the present study C. s. scripta returned to the pond
after confinement in a dry room. It is not known whether the increased
basking is correlated with increased feeding. Gatten (1974) found that C. s.
elegans increased its thermal referendum from 24.6 to 29.10C in fasting vs.
recently fed animals placed in a thermal gradient. The body temperature
29.1C correlates well with the mean water temperature in the present study
above which atmospheric basking decreased (28.50C).
Clearly, considerable work is still required to explain why some turtles
bask atmospherically and others do not. Comparative studies in the following
areas would probably clarify the situation considerably: (1) telemetric moni-
toring of hypothalmal, interpleural, cloacal, and carapace surface tempera-
tures of unrestrained aquatically and atmospherically basking turtles under
varying controlled air and water temperatures and heat inputs; (2) determina-
tion of digestive efficiency in turtles allowed to bask, aquatically and atmos-
pherically, and those prevented from basking, again under varying controlled
air and water temperatures and heat inputs; (3) determination of digestive
enzyme temperature optima; and (4) more extensive analysis of diets in
aquatically and atmospherically basking turtles.

1) Light intensity has considerable effect on basking behavior in Chry-
semys scripta script and Chrysemys floridana peninsularis. Both the num-
ber of basking turtles per day and the number of times individual C. s. scripta
bask per day decrease on overcast days. Turtles respond rapidly to clouds
screening the sun on partly cloudy days by decreasing their rate of emergence

Vol. 20, No. 1


from the water. Turtles rarely emerge on shaded basking sites, but commonly
remain on shaded sites after basking in the sun. In C. s. scripta, and probably in
C. f. peninsularis, the time spent sitting on a shaded site is positively corre-
lated with body temperature, body size, sun basking time, and air tempera-
2) Turtles respond to the position of the sun in the sky by orienting their
bodies away from the sun early in the basking appearance. Orientation is
random on heavily overcast days, and when the sun is near its zenith during
August and September. Orientation is away from the sun, even when the sun is
near zenith during October and November, as the southward declination of
the sun increases. The daily movement of the sun in the sky from east to west
results in a shift in basking and surface activity from the west side to the east
side of the pond. The turtles may respond directly to the sun's position as well
as to the daily cycle of basking site illumination and shading. Turtles are
found to be either regional or diffuse baskers, emerging on sites in one region
of the pond or showing no preference.
3) Atmospheric basking reaches a maximum at a mean water temperature
of 28.50C, but decreases at higher and lower mean water temperatures.
Aquatic basking increases with increasing water temperature to the maximum
mean water temperature recorded during the experimental period (31.50C).
Chrysemys s. scripta females bask less frequently than males as water temper-
ature decreases, no longer basking when mean water temperature falls
below approximately 140C. C. s. scripta males still bask at a mean water
temperature of 10.5C, though at a much reduced rate, whereas C. f. penin-
sularis does not bask below 140C. No C. s. scripta bask below a water tem-
perature of 7C. Water temperature is probably more important than air
temperature in determining whether a turtle does or does not bask.
4) Mean basking duration in Chrysemys s. scripta increased on clear days
from August to December, because of decreased sunshine and generally de-
creased air and water temperatures. Basking duration and body size are di-
rectly related in C. s. scripta. The relationship is not precise since many fac-
tors besides body size influence basking duration as, for example, time of day
of emergence, sun energy flux, air and water temperatures, amount of duck-
weed on the carapace, variation in turtle wariness, and frequency of dis-
turbances. Many turtles leave the basking sites before the CTM is approached.
5) The number of atmospherically basking turtles increased during
August, reached a peak in early September, decreased toward October, and
subsequently fluctuated between relatively low and high numbers as cold
fronts alternated with warming trends. Basking is not a daily requirement in
C. s. scripta, even when air and water temperatures and light are most favor-
able. Basking frequency is, roughly speaking, inversely proportional to bask-
ing duration from September to December. Basking frequency and duration
are greater for the C. s. scripta released after a period of confinement than


for the undisturbed population. The newly released turtles may bask in order
to digest recently eaten duckweed.
6) Chrysemys s. scripta less than 10 cm carapace length bask more often
per day than larger individuals of the species, comprising a greater percent-
age of the total baskers before 1000 and after 1400 than expected (based on
their estimated percentage of the population) and reach a basking number
peak earlier in the day.
7) The time of first emergence is correlated with the illumination of the
first major basking site during August and September, but this correlation
weakens during October to December as basking activity commences progres-
sively later during the day. The maximum number of turtles basking at one
time occurs between 1000 and 1100 during August and September, shifting
to as late as 1400 as photoperiod and air and water temperatures decrease in
October and November.
8) Three curves of hourly change in basking numbers are described:
(1) the bimodal August curve, with morning and afternoon peaks in basking
numbers and a midday decline; (2) the unimodal asymmetrical curve, with
a midmorning maximum of basking numbers and a decline in numbers during
the rest of the day; and (3) the unimodal symmetrical curve, with a midday
maximum in basking number. The importance of individual basking fre-
quency per day in determining the shape of these curves is elucidated.
9) The basking act is described for an adult C. s. scripta basking on a
clear warm day. The act is divided into four stages, based on changes in body
orientation, position of the head and limbs, and activity with changing body
temperature. Variations in the act correlated with environmental conditions,
turtle body size, and species are discussed. Basking stages of Chrysemys s.
scripta are assigned body temperature ranges based on field data, tempera-
ture telemetry, and the literature.
10) Many aspects of turtle basking behavior appear to be thermoregula-
tory: body orientation, leg and head extension and retraction, rear leg digit
spreading, and leg kicking during basking; sitting on the basking site in the
shade after basking in the sun, but rarely emerging to bask in the shade; in-
verse correlation of basking duration and frequency with changes in air and
water temperatures and sunshine (shown for C. scripta only); and sitting on
submerged basking sites or floating in warm surface water.

Akers, T. K., and M. G. Damm. 1963. The effect of temperature on the electrocardiograms of
two species of turtles. Copeia 4: 629-634.
Belkin, D. A., and C. Gans. 1968. An unusual chelonian feeding niche. Ecology 49: 768-769.
Boyer, D. R. 1965. Ecology of the basking habit in turtles. Ecology 46: 99-118.
Brattstrom, B. H. 1965. Body temperatures in reptiles. Amer. Midl. Nat. 73: 375-422.
Bury, R. B., andJ. H. Wolheim. 1973. Aggression in free-living pond turtles (Clemmys marmorata).
BioScience 23:659-662.

Vol. 20, No. 1


Cagle, F. R. 1944. Home range, homing behavior, and migration in turtles. Misc. Publ. Mus.
Zool., Univ. Mich. 61: 1-33.
1950. The life history of the slider turtle, Pseudemys script troostii (Holbrook). Ecol.
Monogr. 20: 31-54.
Carr, A. 1952. Handbook of turtles. Cornell Univ. Press, Ithaca, N. Y. 542 p.
Chesley, L. C. 1934. The influence of temperature upon the amylases of cold- and warm-blooded
animals. Biol. Bull. 66:330-338.
Edgren, R. A., and M. K. Edgren. 1955. Thermoregulation in the musk turtle, Sternotherus
odoratus Latreille. Herpetologica 11: 213-217.
Fraenkel, G. S., and D. L. Gunn. 1961. The orientation of animals. Dover Publications, Incorpo-
rated, New York, N. Y. 376 p.
Gatten, R. E. 1974. Effect of nutritional status on the preferred body temperature of the turtles
Pseudemys scripta and Terrapene ornata. Copeia 4: 912-917.
Goodman, D. E. 1971. Thermoregulation in the brown water snake, Natrix taxispilota, with dis-
cussion of the ecological significance of thermal preferenda in the Order Squamata. Ph.D.
Thesis. Univ. Florida. 85 p. (Diss. Abstr. 32: 6122-B).
Guyton, A. C. 1966. Textbook of medical physiology. 3rd. ed., W. B. Saunders Company, Phila-
delphia, Pa. and London, England. 1210 p.
Heath, J. E., E. Gasdorf, and R. G. Northcutt. 1968. The effect of thermal stimulation of anterior
hypothalamus on blood pressure in the turtle. Comp. Biochem. Physiol. 26: 509-518.
Hutchison, V. H., and R. J. Kosh. 1964. The effect of photoperiod on the critical thermal maxima
of painted turtles (Chrysemys picta). Herpetologica 20: 233-238.
Hutchison, V. H., A. Vinegar, and R. J. Kosh. 1966. Critical thermal maxima in turtles. Herpe-
tologica 22:32-41.
Jackson, C. G., Jr. 1964. A biometrical study of form and growth in Pseudemys concinna suwan-
niensis Carr. Ph.D. Thesis. Univ. Florida. 76 p. (Diss. Abstr. 26:4126-4127).
Kenyon, W. A. 1925. Digestive enzymes in poikilothermal vertebrates, an investigation of en-
zymes in fishes, with comparative studies of those of amphibians, reptiles, and mammals.
Bull. U.S. Fisheries. 51: 179-200.
Lagler, K. F. 1943. Methods of collecting freshwater turtles. Copeia 1: 21-25.
Mackay, R. S. 1964. Galapagos tortoise and marine iguana deep body temperatures measured by
radio telemetry. Nature 204: 355-358.
McDowell, S. B. 1964. Partition of the genus Clemmys and related problems in the taxonomy
of the aquatic Testudinidae. Proc. Zool. Soc. London, 143: 239-279.
Moll, E. O., and J. M. Legler. 1971. The life history of a neotropical slider turtle, Pseudemys
script (Schoepff), in Panama. Bull. Los Angeles Co. Mus. Nat. Hist., Sci. 11. 102 p.
Pritchard, P. C. H., and W. F. Greenhood. 1968. The sun and the turtle. Int. Turtle and Tortoise
Soc. J. 2: 20-25.
Rodbard, S., F. Samson, and D. Ferguson. 1950. Thermosensitivity of the turtle brain as mani-
fested by blood pressure changes. Amer. J. Physiol. 160: 402-408.
Rose, F. L. 1969. Desiccation rates and temperature relationships of Terrapene ornata following
scute removal. Southwest Nat. 14: 67-72.
Spray, D. C. 1972. Weight shifts in the intact turtle during heating and cooling. Comp. Biochem.
Physiol. 43A: 491-494.
Sand M. L. May. 1972. Heating and cooling rates in four species of turtles. Comp. Biochem.
Physiol. 41A: 507-522.
Stitt, J. T., and R. E. Semple. 1971. Sites of plasma sequestration induced by body cooling in
turtles. Amer. J. Physiol. 221: 1189-1191.
Stitt, J. T., R. E. Semple, and D. W. Sigsworth. 1971. Plasma sequestration produced by acute
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Physiol. 3: 704-710.
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Chrysemys. Tulane Stud. Zool. 14: 63-73.
Webb, G. J. W., and C. R. Johnson. 1972. Head-body temperature differences in turtles. Comp.
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tilia. Quart. J. Exp. Physiol. 42: 1-14.

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