Reintroduction Evaluation and Habitat
Assessments of the Virgin Islands boa, Epicrates
monensis granti, to the U.S. Virgin Islands.
U.S. Fish and Wildlife Service
Region 4: Atlanta, GA
Peter J. Tolson, Ph.D.
AZA SSP Coordinator, Mona/ Virgin Islands Boa
Director of Conservation and Research
Toledo Zoological Gardens
P.O. Box 140130
Toledo, OH 43614-0801
(419) 385-5721x 2112 -work
(419) 385-6924- FAX
Judy Pierce, Chief of Wildlife
Division of Fish and Wildlife
6291 Estate Nazareth
St. Thomas, VI 00802
(340) 775-6762 Phone
(340) 775-3972 Fax
The Virgin Islands boa, Epicrates monensis granti, is an endangered, CITES I endemic of
the Puerto Rico Bank (see USFWS 1980) that shows an extremely disjunct distribution
indicative of a long history of population decline and extirpation (Mayer and Lazell 1988;
Nellis et al. 1986; Tolson 1992a). Nonetheless, recovery activities for this snake in the
U.S. Virgin Islands have been ongoing for nearly 20 years due to the efforts of the
Division of Wildlife, USVI and the Toledo Zoo. Nellis and Boulon. (1985) demonstrated
that it was feasible to completely eradicate the black rat, Rattus rattus from smaller cays
in the USVI. This effort spurred searches to find suitable habitats for E. m. granti on the
Puerto Rico Bank that were not necessarily rat free, but had the structural attributes of
vegetation and food resources that the boa required. Tolson (1988) surveyed and
analyzed 23 sites on the Bank as potential E. m. granti reintroduction sites and three sites
were selected- one in Puerto Rico and two in the USVI. One site in the USVI and one
site in Puerto Rico were subsequently poisoned in 1990-1991, but only the Puerto Rico
site remained rat free. In 1993-94 41 boas bred at the Toledo Zoo were released to Puerto
Rico. The population exploded to ca. 500 snakes and is still expanding as of August
2004. This effort demonstrated conclusively that reintroduction was a viable recovery
strategy for the recovery of E. m. granti.
In the U.S. Virgin Islands, repeated rat surveys performed by Peter J. Tolson (most
recently in 2002-03), using removal trapping and chew stick placement, indicated that
Rattus had not re-colonized cays since the initial poisonings in the 1980's. Concurrently,
conservation breeding of the E. m. granti deme (founders from St. Thomas) created a
pool of snakes which could be drawn upon for the first reintroduction to the USVI. The
potential genetic diversity of any reintroduced population was enhanced by the
translocation of snakes from St. Thomas that were confiscated by Division of Wildlife
staff. The survival of these snakes is currently being evaluated by the Division of
Wildlife and staff from the Toledo Zoo and Departamento de Recursos Naturales y
Ambientales de Puerto Rico under the direction of Peter J. Tolson.
Our primary goal for this project was to evaluate the success of the reintroductions
performed during August 2003-04 in the USVI (24 boas). Our second goal was to
evaluate habitat use by reintroduced and translocated boas in the USVI. Data from
sequential recaptures were analyzed to assess age-specific survivorship, growth, activity
patterns, movements, reproductive biology, population numbers, and habitat preferences.
The latter data will be especially useful in selection of target sites for future
reintroductions. The USFWS Recovery Plan mandated evaluation of sites suitable for
release and reintroduction of the boa to predator-free uninhabited islands within its
historical range (USFWS 1986). This was the third goal of this research.
Vegetation of the release site is primarily composed of a closed-canopy subtropical dry
forest, sensu Ewel and Whitmore (1973) 4.28- 9.26 m in height, overwhelmingly
dominated by marble tree, Elaeodendron xylocarpum. However, as one moves to the
west, the Elaeodendron forest incorporates significant numbers of white cedar, Tabebuia
heterophylla, and cotorro, Adelia ricinella. This mixed forest has significant numbers of
large Tillandsia utriculata in the trees, in the understory, and on the ground. The site has
a small grove of tall Acacia macracantha adjacent to a small opening with large numbers
of Opuntia. The western one-third of the site is dominated by a mangrove forest of
buttonbush, Conocarpus erectus, black mangrove, Avicennia nitida, and white mangrove,
Laguncularia racemosa. Littoral vegetation varies from Elaeodendron with an understory
of limber caper, Capparisflexulosa, catch-and-keep, Caesalpinea bonduc, and
Clerodendrum aculeatum on the northeast to isolated stands of sea grape, Coccoloba
uvifera mixed with Capparisflexulosa on the northwest. The eastern shoreline has an
extensive cover of shoreline seapurselane, Sesuvium portulacastrum, with a small stand
of Morinda citrifolia, and isolated living and dead Elaeodendron xylocarpum. The
extreme south end is dominated by panic grass, Panicum maximum, Turk's cap cactus,
Melocactus intortus, and shrubs, most frequently Randia aculeata, Corchorus hirsutus,
and low lying Capparisflexulosa.
No feral cats, Felis catus, have ever been observed on the site. The site once supported a
substantial population of Zenaida doves, Zenaida aurita, in the nesting season but this
has been reduced to only a few birds (Tolson pers. obs.). Smaller passerines, such as the
bananaquit, Coerebaflaveola, yellow warbler, Dendroicapetechia, and grey kingbird
Tyrannus dominicensis, occasionally visit the site but are not commonly encountered.
Various herons, including the snake-predating yellow crowned night heron, Nyctanassa
violacea, nest on or commonly visit the site. Native lizards- the crested anole, Anolis
cristatellus, the barred anole, Anolis stratulus, and the ground lizard, Ameiva exsul are
common on the site. The introduced green iguana, Iguana iguana, has a nesting
population on the site.
Capture- E. m. granti were hunted at night from sunset to 2200 using battery-powered
headlamps and were captured by hand. Searches were performed during the moon
periods of the last quarter through the new moon. Capture data included time of capture,
locality, and perch/ forage site parameters as used by Chandler and Tolson (1990).
Identification- snakes destined for release with snout-vent lengths (SVL) > 600 mm
were implanted with AVID or Trovan passive integrated transponders (PITs) on the left
side 1 cm anterior to the vent. Snakes < 600mm SVL had a digital image taken of the
anterior dorsal body pattern and were implanted with PITs upon recapture after reaching
600 mm SVL.
Meristics- After capture, snakes were sexed with a sexing probe, weighed with a Pesola
scale to the nearest gram, and measured to the nearest mm. Cloacal body temperature was
taken with a quick-recording thermometer. Reproductive condition, e.g. shedding of the
hemipenial epithelium in males, presence of follicles or young in gravid females, was
Habitat analysis- Capture sites were characterized using standard measurements for
terrestrial vegetation (see Bonham 1989). Terrestrial vegetation was identified and
evaluated in two nested plots with the boa capture point used as the center of the plot. A
10 m x 10 m plot was used to calculate tree (> 10 cm dbh) density and frequency data. A
randomly selected 5 m x 5 m plot nested within the larger plot was used to evaluate shrub
and sapling density and frequency. Canopy cover was evaluated using crown
densitometry. Canopy height was measured with a clinometer. Contiguousness of the
canopy was evaluated using nearest neighbor distances of other trees and shrubs. Data
from sequential recaptures was analyzed to assess age-specific survivorship, growth,
activity patterns, movements, reproductive biology, and habitat preferences. The latter
will be especially useful in selection of target sites for future reintroductions.
Population estimates- boa populations were estimated using the Schnabel (1938) and
Schumacher-Eschmeyer (1943) estimates for multiple sampling of marked individuals in
closed populations, using Ecological Methodology v. 6.1 by Exeter Software.
Release site evaluation- Likely candidate cays for reintroduction were selected by
Division of Wildlife staff and were visited by Peter J. Tolson to determine whether a
more detailed evaluation should be performed. The detailed evaluation consisted of 10 m
x 10 m plots to calculate tree (> 10 cm dbh) density and frequency data scaled to patch
size, using a 5% sampling intensity. All trees within the plots were identified to the
species level. A 5 m x 5 m plot nested within the larger plot was used to evaluate shrub
and sapling density and frequency. Canopy cover was evaluated using a crown
densitometry. Canopy height was measured with a clinometer. Contiguousness of the
canopy was evaluated using nearest neighbor distances of other trees and shrubs. Prey
densities were determined by distance measures of Anolis on transects scaled to patch
size, usually one 100 m transect/ ha of habitat. Anolis are also quantified as number of
Anolis seen/ man/ h during hunting episodes. Similarly, rat densities were quantified as
rats/ trap hour captured on transects scaled to patch size. Normally, one 100 meter trap
line, using 1 trap/ 10 m, is employed for each five ha of habitat. Rats were trapped for
three hours/ night, with traps checked each hour and re-baited each hour, for three
consecutive nights. Cats, when present, are quantified as number of cats observed/ man/
Reintroduction- the first group of 11 reintroduced boas from the Toledo Zoo was
released on site 6 August 02. The snakes ranged from 20.2 g in mass and 455 mm SVL to
301g in mass and 1042 mm SVL.
Translocation- 31 boas were translocated to the site from 5 February 96 through 15
September 03. Body masses were not recorded but the snakes ranged from 320 mm to
780 mm SVL. Nine snakes were implanted with AVID or Trovan passive integrated
Captures/ recaptures- During the first evaluation trip to the USVI 28 February- 6 March
03 we made a total of 18 captures of E. m. granti. Four of these were recaptures; three
were from the group released from the Toledo Zoo and one was a Division of Wildlife
snake released on 23 February 96. Of the unidentified snakes, seven were implanted with
PITs. The remaining snakes were too small for implantation and had their dorsal body
patterns photographed as a means of future identification.
During the second evaluation trip to the USVI 19-29 August 03 we made a total of 53
captures of E. m. granti. This makes a total of 52 individual snakes captured in 2003.
There were eight snakes captured twice during August and three snakes captured three
times. Twenty four additional unidentified snakes were implanted with PITs.
During the third evaluation trip to the USVI 10-23 February 04 we made a total of 103
captures of E. m. granti. We captured 81 individual snakes on this trip. There were 16
snakes captured twice, five snakes captured three times, and one snake captured four
times in February. Forty-two additional unidentified snakes were implanted with AVID
transponders or photographed.
During the fourth evaluation trip to the USVI 10-20 August 04 we made a total of 51
captures of E. m. granti. We captured a total of 45 individual snakes on this trip. Six
snakes were captured twice. Twenty-two additional unidentified snakes were implanted
with AVID transponders or photographed.
Sex ratios- seven males and three females were captured in February-March 03, a sex
ratio of 2.3:1. Three snakes were too young to sex. There were 13 males and 24 females
captured in August 03, a sex ratio of 1:1.85. Three snakes were too young to sex.
Sex ratios were skewed toward females in 2004. There were 23 males and 47 females
captured in February 04, a sex ratio of more than two females for each male. Eleven
juvenile snakes were too young to sex. There were 15 males and 27 females captured in
August 04, a sex ratio of nearly two females for each male. Four juvenile snakes were too
young to sex and one snake escaped before being sexed.
Age distribution- Tables I-III show the age distributions of snakes captured from August
2003 through August 2004.
Table I. Age distribution of snakes captured on site August 2003.
Age Class Number of snakes
Newborn < 400 mm SVL 3
Young of year 400-550 mm SVL 2
2 years, 550-650 mm SVL 15
3 years, 650-750 mm SVL 9
4+ years, > 750 mm SVL 14
Table II. Age distribution of snakes captured on site, February 2004.
Age Class Number of snakes
Neonate (born August 03) < 400 mm SVL 7
Young of year 400-550 mm SVL 11
2 years, 550-650 mm SVL 20
3 years, 650-750 mm SVL 19
4+ years, > 750 mm SVL 24
.Table III. Age distribution of snakes captured on site, August 2004.
Age Class Number of snakes
Neonate (born August 04) < 400 mm SVL 0
Young of year 400-550 mm SVL 5
2 years, 550-650 mm SVL 13
3 years, 650-750 mm SVL 12
4+ years, > 750 mm SVL 16
The largest male captured had a SVL of 1112 mm and a tail length of 237 mm. This
snake was a wild male with no transponder that was born or translocated to the site, date
unknown. The largest female captured had an SVL of 1066 mm and a tail length of 137
mm (stub). This snake was a wild female that was born or translocated to the site, date
Body mass index- one simple way to assess the success of a reintroduced snake is to
examine a crude body mass index that relates the ratio of body mass to snout vent length.
Healthier snakes, with greater muscle mass and higher stores of lipids, will exhibit more
body mass per unit of SVL than snakes that have not been as successful in hunting, and
thus have lower muscle mass and lower lipid reserves. As a basis of comparison, adult
female snakes captured in August, 2004 on site were compared with a population of
female snakes captured in Puerto Rico in August, 1991. An equivalent population of
snakes from St. Thomas was not used as the basis for comparison as no body masses
were recorded for any of the DFW confiscated boas. Eighteen adult female snakes from
the site > 700 mm SVL had a mean body mass of 128 g and a mean SVL of 862 mm. The
mean mass/ SVL ratio was 0.145422. In the equivalent population on Puerto Rico in
1991, 17 adult female snakes > 700 mm SVL had a mean body mass of 69.2 g and a
mean SVL of 785 mm. The mean mass/ SVL ratio was 0.086952. The huge difference in
body mass index was highly significant (p <0.001; U = 10, U' =296) using the Mann-
Whitney U test, indicating that the reintroduced/ translocated boas had much higher body
mass/ unit SVL than an unmanaged wild population of the same taxon. It should be
noted, however, than these two populations are different genetically, and the Virgin
Islands population ofE. m. granti may simply be more robust snakes than their Puerto
Growth- sequential recaptures allowed growth data to be collected on 39 individual boas.
Only one neonate was recaptured; it had a growth rate of 11.13 mm/ month over a growth
period of 11.77 months. No snakes were recaptured in the 450-550 mm size class. Ten
snakes were recaptured in the 550-650 mm size class; the mean growth rate was 5.50
mm/ month. Nine snakes were captured in the 650-750 mm size class; the mean growth
rate was 3.68 mm/ month. Nineteen snakes were captured in the > 750 mm size class;
mean growth rate was 3.48 mm/ month. These data are tabulated in Appendix 1.
Although there appears to be a trend of fast growth for young snakes and slower growth
for adult snakes and the means between these size classes appear to be substantially
different in some cases, the variances of each class, along with the very small sample
sizes (n=l, n=0) for the newborn and young of the year snakes, resulted in no statistical
significance of growth rates between the age classes at the p> 0.01 level using a one-way
ANOVA (STATISTICA v 5.5, StatSoft software).
Habitat preferences- E .m. granti was found most frequently in the dominant tree on
site, the marble tree, Elaeodendron xylocarpum. Other tree species commonly utilized in
2003- 2004 were limber caper, Capparisflexulosa, sea grape, Coccoloba uvifera,
buttonbush mangrove, Conocarpus erectus, and white cedar, Tabebuia heterophylla. In
2003, snakes were also captured in gumbo limbo, Bursera simaruba, and ironwood,
Krugiodendronferreum. In 2004 snakes were also captured in Acacia, Acacia
macracantha, white mangrove, Languncularia racemosa, black mangrove, Avicenna
nitida, yellow torch, Erithalisfructicosa, and tree cacti, Opuntia rubescens, for the first
time. Table IV shows tree capture sites for the snakes captured from February 03 to
August 04. Snakes were also captured in terrestrial bromeliads Tillandsia utriculata,
vines, primarily Stigmaphyllon emarginatum, and on coral rubble. All snakes captured
were active at night; none were found in refugia during the day.
Vegetation analysis- The subtropical dry forest of the site is overwhelmingly dominated
by the marble tree, Elaeodendron xylocarpum. E. xylocarpum comprised 59.4% of the
canopy species > 10 cm DBH within the 10 m x 10 m forest plots, almost forming a
monoculture in several areas of the site. Coccoloba uvifera, Tabebuia heterophylla, and
Bursera simaruba were the other dominant species of the canopy comprising 9.9%, 8.9%,
and 7.9% respectively of the canopy species > 10 cm DBH within the 10 m x 10 m forest
plots. Total tree density on site equaled 0.018 trees/ m2, or 180 trees/ ha. Densities of
specific trees in the capture plots, expressed as number of trees/ ha, are as follows:
Avicennia nitida, 1.82/ ha; Bursera simaruba, 15.45/ ha; Capparis cynophallophora,
3.64/ ha; Conocarpus erectus 1.82/ ha; Cocos nucifera 5.46/ ha; Coccoloba uvifera
18.18/ ha; Elaeodendron xylocarpum 109.09/ ha; Laguncularia racemosa 3.64/ ha;
Krugiodendronferreum 3.64/ ha; Pisonia albida 5.46/ ha; Tabebuia heterophylla 16.36/
ha. Many species, by virtue of their nonrandom distribution on the cays, were very dense
in some areas, but absent from others, e.g. Acacia, Avicennia, Languncularia, and others.
Table IV. Tree capture sites for the snakes captured in February 2003 through August
Tree species Number of snakes
Acacia macracantha 3
Avicenna nitida 1
Bursera simaruba 2
Clerodendrum aculeatum 1
Corchorus hirsutus 2
Coccoloba uvifera 30
Conocarpus erectus 15
Elaeodendron xylocarpum 93
Erithalis fructicosa 1
Languncularia racemosa 5
Opuntia rubescens 2
Randia aculeata 6
Tabebuia heterophylla 7
The shrubby understory of the site was primarily composed of small trees of the species
tabulated in Table IV. Additional species in the plots included Adelia ricinella, Capparis
flexulosa, Clerodendrum aculeatum, Corchorus hirsutus, and Thespesiapopulnea. Two
species, however, dominated the understory: small Elaeodendron xylocarpum comprised
46.6% of the stems counted in the 5 m X 5 m understory plots and Capparisflexulosa
comprised 24.6% of the stems. Some of the vegetative cover, particularly in coastal
situations free of canopy, was dominated by dense mats of low growing vegetation
primarily composed of viney tangles of Capparisflexulosa overgrown with
Stigmaphyllon emarginatum or Canavalia rosea. We could not quantify stems in these
associations without destroying the vegetation within the plot. In most areas both the
understory and canopy were contiguous. Nearest neighbor distances, i.e. the distance of
the nearest tree or shrub from the capture site, had a value of zero (0) in every case but
one, meaning that every three or shrub in which a boa was captured had another tree or
shrub touching it at some point. The lone exception was a snake capture in a small tree
Opuntia; this cactus was 31 cm from its nearest neighbor.
Foraging heights- There was wide overlap in the foraging heights and perch diameters
of boas of different size classes. Table V presents the data for range and mean capture
height for the different size classes of boas.
Table V. Foraging height and perch diameters of boas captured on site
Size class Mean Range BRDA Range
height in m in mm BRDA
< 450 mm 1.87 m 0.89-2.93 m 8 mm 2-44 mm
450-550 mm 2.14 m 0.89-3.74 m 8mm 5-13 mm
550-650 mm 1.78 m 0.72-3.80 m 13 mm 3-84 mm
650-750 mm 1.36 m 0.41-2.58 m 11 mm 3-39 mm
> 750 mm 1.54 m 0.33-3.58 m 15 mm 2-58 mm
Although there was a slight trend of larger boas foraging at lower heights and larger
branch diameters, these differences were not significant using a two-way ANOVA.
Prey base- the prey base continues to be sufficient, with large numbers ofAnolis
cristatellus. While we have known for years that the common green iguana, Iguana
iguana, inhabits the site, this is the first year that we have seen hatchling iguanas. They
were very numerous in August 04. Seven adult snakes were observed feeding on
hatchling iguanas or regurgitated a hatchling iguana shortly after capture.
Reproduction- Thirteen of the female snakes captured in August 04, which is within the
season of parturition, were of reproductive size; two of these (15.4%) had recently given
birth. However, we found no newborn snakes with umbilical scars in August 04.
Population estimate- a Schnabel (1938) population estimate performed on the February-
August 04 data gives an estimate of 168 snakes on the site, an increase of 126 snakes
over the 42 snakes originally released. The population has increased fourfold, with a
density of 202 boas/ ha. The new Schnabel population estimate eclipses the old 2003
estimate of 98 snakes for the site.
Reintroduction site selection
Initial evaluations were performed on five cays in the U.S. Virgin Islands in 2003-04:
Dutch Cap Cay, Henley Cay, Salt Cay, West Cay and Whistling Cay. Neither Dutch Cap
Cay, Henley Cay, nor Whistling Cay seemed to be optimal forE. m. granti because of the
lack of adequate forest cover for a large boa population, but Salt Cay and West Cay both
seemed suitable. An intensive four-day evaluation was performed on West Cay In
August 04. Attempts were made by DFW to gain access to Great St. James Cay and
Little St. James Cay without success. Both are privately owned.
Feral mammal densities- No cats were seen on any of the cays, but evidence of the black
rat, Rattus rattus was present on every offshore cay visited in 2003-04. Intensive three-
night trapping was carried out on West Cay from 11-13 August 04 to obtain an index of
rat populations. The index for West Cay was 0.155 rats/ trap/ hour. Data for 110 trap/
hours were recorded. These data indicate a very heavy rat density for West Cay, in fact
the highest rat density we have recorded in the U.S. Virgin Islands. In the U.S. Virgin
Islands we have previously recorded densities of 0 (Saba Cay- coastal forest; Steven Cay-
dry forest) to 0.09 (Buck Is.-grassland) rats/ trap/ hour. Recent goat or deer droppings
were also present on West Cay but no goats or deer were observed.
Prey densities- Anolis densities on West Cay were the lowest observed on any cay in the
U.S. Virgin islands. Only three Anolis were seen in nine hours of observations; two
individuals of A. stratulus and one individual of A. cristatellus. This dismal observation
of an anole density of 38 Anolis /ha contrasts sharply with the normal observations of ca.
5000 Anolis/ ha.
The subtropical dry forest of West Cay is far more diverse than that of the site. Dominant
tree species > 10cm DBH of 20 randomly selected 10 m x 10 m forest plots included
marble tree, Elaeodendron xylocarpum, gumbo limbo, Bursera simaruba, sea grape
Coccoloba uvifera, and white cedar, Tabebuia heterophylla, as on the site, but several
other tree species were very common, including bastard mahogany, Andira inermis, gree-
gree, Bucida buceras, silver palm, Coccothrinax alta, black manjack, Cordia rickseckeri,
Eugenia axillaris, black mampoo, Guapira fragrans, manchineel, Hippomane
mancinella, and frangipani, Plumeria alba. E. xylocarpum was the dominant dicot tree
and comprised 20.4% of the canopy species > 10 cm DBH within the 10 m x 10 m forest
plots, but the silver palm, Cocothrinax alba, was the tree most frequently found in the
plots, comprising 27.8% of the trees with the forest plots. Coccoloba uvifera, Guapira
fragrans, Capparis cynophallophora, and Bursera simaruba were the other dominant
species of the canopy comprising 16.7%, 9.3%, 7.4%, and 7.4% respectively of the
canopy species > 10 cm DBH within the 10 m x 10 m forest plots. Tree density on West
Cay equaled 0.017 trees/ m2 or 170 trees/ ha. Densities of specific trees in the evaluation
plots, expressed as number of trees/ ha, are as follows: Andira inermis, 10/ ha; Bucida
buceras 5/ ha; Bursera simaruba, 20/ ha; Capparis cynophallophora, 20/ ha;
Coccothrinax alba 75/ ha; Cocos nucifera 10/ ha; Cordia rickseckeri 5/ha; Coccoloba
uvifera 25/ ha; Eugenia axillaris 5/ ha; Elaeodendron xylocarpum 55/ ha; Guapira
fragrans 25/ ha; Hippomane mancinella 5/ ha.
The shrubby understory of West Cay was primarily composed of small trees of the
species listed above for the site. Additional species in the plots included Andira inermis,
Capparisflexulosa, Corchorus hirsutus, Eugenia sp., and Hippomane mancinella. Two
species, however, dominated the understory: Elaeodendron xylocarpum comprised 25.5%
of the stems counted in the 5 m X 5 m understory plots and Coccoloba uvifera comprised
26.8% of the stems. Some of the vegetative cover, particularly in exposed rocky coastal
situations free of canopy, was dominated by low growing vegetation primarily composed
of Corchorus hirsutus with dispersed Melocactus intortus. These areas appeared to be
heavily grazed by goats. There were also areas of viney tangles ofPithocellobium dulce,
Caesalpinea bonduc, and Clerodendrum aculeatum.
The population structure on the site continues to be stable, with a continued skew towards
adult snakes. This could have easily been caused by the large number of adults that were
translocated to the site, as Virgin Islands boas may easily live more than 25 years, but
may also be due in part by the importance of hatchling iguanas in the diet of E. m. granti.
Using only the available capture numbers (102 individuals) the snakes existed in a
minimum density of 123 snakes/ ha in August 04. Using the Schnabel (1938) population
estimation for closed populations I calculated a population of 167.5 snakes (95%
confidence interval from 144.9 to 237.2); using the Schumacher and Eschmeyer (1943)
population estimation calculated a population of 165.9 snakes (95% confidence interval
from 129.4 to 237.2) for August 04. The density of boas on the site, 202 boas/ ha
exceeds the densities of 125 snakes/ ha calculated for Puerto Rico and selected habitat
patches on Isla Mona.
The age structure of the snakes is indicative of a healthy population, with a
preponderance of snakes < three years of age. This is coupled with a healthy population
of older animals of indeterminate age. In fact, the snakes > four years of age were the
most numerous age class of the population. I believe that this indicates substantial food
resources for adult snakes (predominantly hatchling/ juvenile Iguana) and low predation
pressure. The only predator currently on the island is the yellow crowned night heron,
Nyctanassa violacea. Only the slight skew of the population indicates that the island is
near carrying capacity for boas. As the adult snakes are able to utilize large prey, such as
birds and hatchling green iguanas, they need not eat as often as the smaller boas.
Nothing about the population of boas on site suggests that it is different in any respect
from any healthy population of boas. Capture data suggest that boas on site have activity
patterns, reproductive timing, and prey selection similar to, if not exactly like, other
populations of Virgin Islands boas. The boas on site seem more flexible in their selection
of foraging heights. On Puerto Rico, Chandler and Tolson (1990) found that boas SVL
was correlated with foraging height. This association did not hold for the site, possibly
due to structural differences of the vegetation between the two sites. There were
statistically significant differences in the perch height selection ofAnolis cristatellus on
Puerto Rico. Large males tended to perch high, while females and juveniles tended to
perch lower. Large snakes, which preyed on the male Anolis, were most frequently found
higher in the trees, while smaller snakes, which fed on smaller Anolis, were most often
found in the understory. Differential perch height selection by different sexes and age
classes ofAnolis cristatellus was not observed on the site. This factor may explain why
there were no significant foraging height differences between different size classes of
Habitat is excellent for the boas. The site has a very dense closed canopy forest, which is
apparently very important for this species (Tolson 1988). The density of the forest seems
more important than the relatively low diversity of tree species, as boas were often
captured in areas that were essentially a monoculture ofElaeodendron, Conocarpus, and
Laguncularia. West Cay seems to be an excellent site for an additional population of
reintroduced boas. The low Anolis densities there are doubtless due to the high densities
of Rattus. The situation could be reversed with rodent eradication on the cay.
The most important activity that the DFW can continue for the site is the continued
enforcement of the non-visitation policy of the site. Despite this policy, however,
occasional visitors continue to flagrantly disregard the posting of the site. We observed
five different instances when visitors ignored the restrictions during the course of our
study. In two instances, non English-speaking visitors were collecting hermit crabs for
use as fish bait. In one instance, a visitor allowed his dog to run free on the site. Fire
from illegal visitation or poaching of boas are serious concerns.
For the time being, boas confiscated by DFW biologists from St. Thomas can be released,
but it is essential that at least two additional sites on offshore cays be located for
additional reintroductions. At least three cays appear to be suitable: Congo Cay, Salt
Cay, and West Cay. However, all three are densely populated with Rattus rattus; these
must be eradicated before boas are released on these islands. Although boas on Puerto
Rico weathered Hurricane Hugo quite well (pers. obs.), the low elevation of the site
makes the boa population there potentially vulnerable to a high storm surge. Although
there are some concerns about maintaining small isolated populations of boas that are
reproductively isolated, such a strategy spreads the risk of having a single stochastic
event, such as disease or a predator introduction, doom all of the reintroduced boas. It is
likely that the boas on St. Thomas are genetically similar, and yet there are no signs of
inbreeding depression. This indicates that it is unlikely that boas would have to be
genetically augmented. However, such an augmentation could be implemented easily by
translocating as few as one or two boas to each site from St. Thomas every ten years.
This is a far greater interval of translocation than what already occurs from DFW
The author would like to thank Judy Pierce and the staff of the Division of Wildlife,
Department of Planning and Natural Resources, Government of the Virgin Islands, for
logistical support during the project. This project was funded by State Wildlife Grant T-1
from the U.S. Fish and Wildlife Service to the Government of the Virgin Islands,
Department of Planning and Natural Resources, Division of Fish and Wildlife.
Bonham, C.D. 1989. Measurements for terrestrial vegetation. John Wiley & Sons, Inc.,
New York, New York. 338pp.
Chandler, C.R. and P.J. Tolson 1990. Habitat use by a boid snake, Epicrates monensis,
and its anoline prey, Anolis cristatellus. J. Herpetology 24(2):151-157.
Mayer, G.C. and J.D. Lazell, Jr. 1988. Distributional records for reptiles and amphibians
from the Puerto Rico Bank. Herpetological Rev. 19(1):23-24.
Nellis, D.W. and R.H. Boulon. 1985. Roof rat eradication on small tropical cays. Final
Report. US Fish and Wildlife Service, P-R Wildlife Restoration Project FW-3-
11D, Division of Fish and Wildlife, St. Thomas, USVI.
Nellis, D.W., R.L. Norton, and W.P. MacLean 1983. On the biogeography of the Virgin
Islands boa, Epicrates monensis granti. J. Herpetology 17(4):413-417.
Schnabel, Z.E. 1938. The estimation of a total fish population in a lake. American
Mathmatical Monthly 45: 342-358.
Schumacher, F. X. and Eschmeyer, R. W. 1943. The estimation of fish populations in
lakes and ponds. J. Tennessee Acad. Sci. 18:228-249.
-----------. 1992a. The conservation status of Epicrates monensis (Serpentes:Boidae) on
the Puerto Rico Bank. Pp. 11-19. In: J.A. Moreno (ed.) Status y Distribucion de
los Reptiles y Anfibios de la Region de Puerto Rico. Publ. Cientifica Misc. No. 1,
Depto. Recursos Naturales de Puerto Rico.
-----. 1992b. The reproductive biology of the Neotropical boid genus Epicrates
(Serpentes:Boidae). Pp. 165-178. In: W.C. Hamlett (ed.) Reproductive Biology of
South American Vertebrates. Springer-Verlag, New York.
------------. 1990. Captive breeding and reintroduction: recovery efforts for the Virgin
Islands boa, Epicrates monensis granti. Endangered Species Update 8(1):52-53.
------------. 1989. Breeding the Virgin Islands boa, Epicrates monensis granti, at the
Toledo Zoological Gardens. Inter. Zoo Yrbk., 28:163-167.
------------. 1988. Critical habitat, predator pressures, and management of Epicrates
monensis on the Puerto Rico Bank: a multivariate analysis. U.S. Dept. Agr. Forest
Serv. General Tech. Rpt. RM-166:228-238.
U.S. Fish and Wildlife Service 1980. Status of Virgin Islands boa clarified. Endangered
Species Tech. Bull. 5:12.
---------------- 1986. Virgin Islands Tree Boa Recovery Plan, U.S. Fish
and Wildlife Service, Atlanta, Georgia. 23pp.