notes on the natural history of the caicos islands dwarf ...

NOTES ON THE NATURAL HISTORY OF THE CAICOS ISLANDS DWARF BOA, TROPIDOPHIS GREENWAYI J OHN B. I V E R S O N Department of Biology, Earlham College Richmond, Indiana 47374

A

BSTRACT

The Caicos Islands dwarf boa is known from only six islands in the Caicos archipelago. Females average larger than males, but have relatively shorter tails. Size distributions of juveniles suggest growth rates of nearly 9mm/month. Maturity is reached at between 250 and 265 mm total length in females and about 225 mm in males. Peak activity, (and possibly mating) apparently occurs at the end of the wet season and beginning of the dry season (December-March), and females are more commonly found than males. At least two to three offspring are live-born early in the rainy season (June). Anoles and geckos are the primary food items. Other aspects of behavior are discussed.

specimens until 1971. Sufficient material is now available (74 specimens) to permit this study of the biology of T. greenwayi, t h e first such treatment of any tropidophid.

I NTRODUCTION ost of the attention given to the dwarf boas of the genus Tropidophis has conM cerned their higher taxonomic position (e.g. Underwood, 1967, 1976; Dowling, 1975; Stimson, 1969; McDowell, 1975). Very little is known about the biology of the genus, and the Caicos Island dwarf boa, T . greenwayi, is one of the least known species. The Caicos boa was described as T. parda/is greenwayi in 1936 by Barbour and Shreve on the basis of two specimens from “Ambergris Cay”, Caicos Islands, British West Indies, and elevated to a full species by Schwartz and Marsh in 1960. Based on the collection of 14 additional specimens from South Caicos (12 specimens) and Long Cay i n 1 9 6 1 , Schwartz (1963) described another subspecies, T. greenwayi lanthanotus. The species was known from only 16

M ETHODS Field work in the Caicos Islands included 11 visits totaling 25 weeks between 15 September 1973 and 15 June 1976 (coincident with the study of the Turks and Caicos rock iguana, Cyclura carinata; I v e r s o n 1979), 10-17 March 1978, and 3-17 December 1979. Fourteen T. greenwayi were collected during this study and either preserved immediately for autopsy (2 specimens) or maintained in captivity for behavioral observations. Museum specimens were also examined and most were dissected for gonadal and diet information. As there were no apparent differences in diet or asynchrony of breeding cycles for snakes from different islands, these data were lumped. Means are followed by +/- one standard deviation.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -t Carib. J Sci. 22(3-4): 191-198 (1986)

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SPECIMENS E XAMINED North Caicos-ASFS 23589-90, 2356963, 23689, 27127-31, 27162-63, 37177-84, 27233, 27236-39, 27243, 27259, 27282; HWG 11 specimens (alive); JBI 7 specimens (sent to HWG alive). Middle Caicos-ASFS 27002-06, 2701 618, 27100, 27107. South Caicos-UF 32655, 33285,4239293, ASFS 27333-34, JBI 3 specimens (sent to HWG alive). Middleton Cay-UF 32693. D ISTRIBUTION Tropidophis greenwayi is known from only six of the Caicos Islands: South Caicos, Middleton and Long Cays (off South Caicos), North Caicos, Middle Caicos and “Ambergris Cay” (Figure 1 ). The only two specimens known from “Ambergris Cay” (actually two islands; see Schwartz 1963) are the types of T. greenwayi Barbour and Shreve (1936), collected there in March 1936. The species has not been recollected on the Am-

bergris Cays, despite collecting trips by Albert Schwartz in January 1961 (Schwartz, 1963), Donald W. Buden in March 1972 (Schwartz and Crombie, 1975) and Ralph Axtell, Harry Greene and myself in March 1978. Tropidophis greenwayi may also occur on Providenciales; natives of that island frequently told me of snakes they had seen. Their descriptions unquestionably suggest the occurence of the boa Epicrates chrysogaster, but they were uncertain if two different species were represented. Despite the proximity of the Long Cay to both South Caicos and Middleton Cay, and the fact that Schwartz collected two Tropidophis on the Cay in 1961, none was collected there on 16 September 1973 (half day) or 3 March 1974 (half day), or by Walter Auffenberg on 4-5 August 1973, or by Ralph Axtell and Harry Greene on 15 March 1978. Most of my field time (25 of 28 weeks) was spent on the western cays, including Ft. George, Pine, Water, and Little Water Cays. No snakes were collected, and no evidence of their occurrence was noted on any of these islands. Natives of Pine Cay had likewise

FIGURE 1. Map of the Caicos Islands with distribution of Tropidophis greenwayi indicated by stars.

CAICOS ISLANDS DWARF BOA

never seen snakes on these islands. The apparent absence of Tropidophis from these islands is indeed suprising in light of their occurence on nearby North Caicos, and their proclivity for moist microhabitats; these cays have the only fresh water lakes and/or marshes in the Caicos islands. Of the six Caicos islands (Ambergris Cays here included) from which Tropidophis is known, all are also inhabited by the lizards Anolis scriptus and Sphaerodactylus caicosensis (all herpetological records: Iverson, unpublished). In addition, the skink Mabuya m a b o u y a is known from all but Big Ambergris Cay, where the snake likely also occurs. The curly-tail lizard Leiocephalus psammodromus and the boa Epicrates chrysogaster are known from all but South Caicos, where they may have been extirpated by feral mammals, and Middleton Cay, where they possibly never occurred. The gecko Aristelliger hechti is sympatric with T. greenwayi on at least Little Ambergris Cay and North Caicos. The rock iguana, Cyclura carinata, is known from all of these islands except Middle and South Caicos, where they were probably extirpated by humans and feral mammals (Iverson, 1978). The frog Eleutherodactylus planirostris is the only amphibian that occurs sympatrically with Trepodophis greenwayi, but only on North Caicos. The occurrence of T. greenwayi o n Middleton Cay is interesting, since its maximum elevation is only ca. 3 m; all other islands on which the snake occurs reach above 9 m.

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S EASONAL O CCURRENCE Specimens have been collected in December (11), January (24), February (23), March (20; includes 11 collected by Harry Greene and Ralph Axtell, but not seen by me), and late May-early June (8). Although this highly seasonal distribution of collection records undoubtedly reflects bias due to the timing of field trips, it may still adequately represent this snake’s usual annual activity cycle. I found no snakes on Middleton Cay on 17 September 1973, 19 July 1974 (half day), or 16 March 1978 (half day), or on South Caicos on 16 September 1973, 19 July 1974 (half day), or 13 March 1978, even though the same microhabitats were checked that produced snakes in December 1973 and March 1974. The apparent increase in activity from December to March may be associated with mating rather than seasonal rainfall patterns (Figure 2). SIZE Maximum size for Tropidophis greenwayi is apparently near 340 mm snout vent length (SWL), 40 mm tail length (TL), and 380 total length (TOL). Adult females average significantly longer than males in SVL and TOL (Table 1). The relationship between TOL and body mass (BM in g) for seven snakes weighed when caught is BM - 0.066TOL -7.016 (r = 0.93; p = 0.002 ). There is considerable dimorphism in relative tail length in T. greenwayi, with males having relatively longer tails (Table 1).

HABITAT

G ROWTH

The Caicos dwarf boa appears to be most common in areas of Rocky Coppice or Dense Scrub (habitat types after Iverson 1979), although it is known from all habitat types (including vacant lots near towns), except Beach, Marsh, Lake and Mixed Woodland communities. It is extremely secretive; all snakes were taken from under limestone rocks lying on substrates of rocks or soil. A single juvenile collected by Walther Auf fenberg was found under a small board. No snakes were found under rocks imbedded in the soil, and no burrows or evidence of burrowing activity in the field or captivity were noted. Hence, there is no indication that T. greenwayi is truly fossorial.

Based on the mean size of two nearly full term embryos (between 28 May and 4 June 1971; 57 and 68.5mm SVL and 78 and 79 mm total length, respectively) and the assumption that all early age classes are represented by specimens, average growth rates for a 15 December unsexed juvenile (1 17 mm SVL), a 24 December female (1 18 mm SVL), a 6 February male (138 mm SVL), and two 13-22 January males (both 140 mm SVL: Schwartz, 1963) were calculated to be 7.6, 7.4, 9.2, 8.8, and 8.8 mm/month, respectively. If the mean of these growth rates (8.4) were maintained to adulthood, sexual maturity in males at about 195 mm SVL and females at between 220 and 235 mm SVL

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F IGURE 2. Seasonal changes in maximum diameter of largest follicle (open circles) or largest oviducal egg (solid circles) in mm in female Tropidophis greenwayi (top), maximum testes length (either lobe; squares) in mm in male T. greenwayi (middle), and mean monthly precipitation (1900-1968) in cm at Grand Turk Airfield, Turks Islands (bottom). Asterisks on testes graph mark observed captive copulations. Each symbol indicates at least one observation. Question marks indicate anomalous data (see text). Hypothetical curves are drawn on top two graphs: they are not statistical fitted to data.

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mm (264). These data suggest that maturity is reached at between about 220 (250) and 235 mm SVL (265 TOL). Only one female over 237 mm SVL had follicles less than 1 mm diameter, but she was collected between 28 May and 4 June and may have been postpartum. This implies that most or all females reproduce each year. However, three February females did have follicles between only 1 and 1.5 mm (Figure 2). Whether they were non-reproducers or just late reproducers in the year of their collection is unknown. The smallest mature male was 195 mm SVL (223 TOL). All larger males had enlarged testes and were considered mature. The largest immature male was only 138 mm (155 TOL). The lack of specimens between 138 and 195 mm SVL makes any assessment of size at maturity in males difficult.

would be possible in about 15 and 18-20 months, respectively. R EPRODUCTION Sex ratio The sex ratio of 48 dissected specimens (not including two nearly full term male embryos) is 2.2 females: 1 male; Chi square = 6.75; p = 0.009. Perhaps the preponderance of females is the result of differential predation on the males. A general (but not significant) ontogenetic increase in the female-male sex ratio supports this hypothesis, as does the slightly (not significant) higher frequency of tail injuries in males (13.3%) than females (6.1%), which may reflect predation pressure. Increased vulnerability of males in search of females for breeding might contribute to the low male survivorship.

Based sion for between parently

Maturity Sexual maturity was determined by the presence of enlarged follicles (> 1 mm) or embryos in the female and enlarged testes (> 5 × 1.5 mm) in the males. By these criteria, the smallest mature females were 220 mm SVL (248 mm TOL), 234 (263), and 237 mm (262), and the largest immature females were 204 (228), 233 (262), and 235

on the body size-body mass regreslive specimens, females mature at 9.5 and 10.5 g, whereas males apmature by approximately 8 g.

Our dissections revealed that the right ovary was larger than the left in 15 (65%) of 23 mature females and approximately equal in four (17%) others, Likewise, the right testis was larger than the left in ten of 13 males (77%). Gonadal asymmetry (with right or-

T ABLE 1.– Size parameters for mature Tropidophis greenwayi. Range appears in parentheses below means. Male and female means are compared by a T-test for each variable. Only snakes with complete tails are included under the latter three parameters. Abbreviations are SVL (snout-vent length), TL (tail length), and TOL (total length).

Males

Females

Variable

N Mean S.D.

N.

SVL

14 240.2 32.5 (195-291)

28259 .221.5 (220-332)

2.26

0.014

TL

12

26

30.8 2.5 (25-38)

1.24

0.111

TOL

12 267.8 35.7 (223-327)

26289 .324.2 (248-370)

2.18

0.017

TL/SVL

13 13.550.58 (12.3-14.4)

26 11.920.66 10.4-13.3)

7.45

4.0 mm diameter (4.00 +/-2.00; N = 8; range 1-8; mode = 4 or 5) or ovarian follicles l.5 to 4.0 mm (3.33 +/- 1.18; N = 12; range 2-6; mode = 3). The grand mean of all these measures is 3.43 +/- 1.56 (N = 23; mode = 3). The higher values based on follicular counts suggest that not all enlarged follicles are ovulated. Brood size (BS) is positively correlated (5% > p > 1%) with body length (SV), whether brood counts included follicles greater than 1.5 mm (BS = 0.037SVL 6.179; r = 0.525; N = 23) or greater than 4 mm (BS = 0.098SVL - 21.975; r = 0.632; N = 11 ), along with oviducal eggs and embryos (Figure 3). The latter, more conservative measure indicates that brood size increases by one for about each 27 mm increase in SVL.


Reproductive Comparisons Ovoviviparity has now been confirmed in two of the four tropidophid genera (Tropidophis and Trachyboa; Barbour, 1937; Petzgold, 1969; Lehmann, 1970); reproductive mode for the other two {Exiliboa and U n g a liophis) is unknown. Preliminary reproductive data for Tropidophis haetianus (Iverson, unpublished), T . m e l a n u r u s and T. semicinctus (Petzgold, 1969; Grant, 1957), and T. greenwayi (this study) suggest that T. semicinctus produces young of nearly twice the relative length (relative to female size) of young of the other three species, More data are needed before the significance of this difference can be interpreted. F OOD

AND

FEEDING

Of the 48 dissected snakes, only ten contained identifiable food items in the gastrointestinal tract. Anolis scriptus was represented in eight tracts (two male and six females snakes) and Sphaerodactylus caicosensis in two (both female). The absence of

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Leiocepha/us may reflect an actual preference for the other two species. In captivity, dwarf boas typically captured Anolis within five minutes after the latter were placed in their containers; they initially showed little interest in L e i o c e p h a l u s of the same size, although they would usually eat them within hours or days. A NTI -PREDATION BEHAVIOR When disturbed in the field, Caicos dwarf boas coiled in a ball, hiding their heads, and smeared a foul-smelling cloacal discharge on their coils. The bright yellow to orange tail was usually exposed, but whether it is an important part of this antipredator display remains uncertain. This behavior has also been reported for many other species of Tropidophis (Hecht et al., 1955; Petzgold, 1969) and other snakes (Greene, 1973). Autohemorraging during handling has previously been noted in at least seven species of Tropidophis (Mertens, 1946; Hecht, et al., 1955; Petzgold, 1969), but never in T . greenwayi. Of the 14 snakes I collected, two

FIGURE 3. Relationship between brood size and body size (SVL in mm) in Tropidophis greenwayi. Solid circles based on counts of follicles > 4 mm diameter; open circles, follicles 1.5-4 mm diameter; solid squares, oviducal eggs or embryos. See text for regression equation,

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autohemorraged. The function of this behavior is unknown (see discussion in Hecht et al., 1955). S TATUS Because of its restricted island distribution, Tropidophis greenwayi is extremely susceptible to extirpation. Islanders are not particularly fond of these “rock pythons” and frequently kill them. The impact of introduced (and now feral) mammals is unknown. However, the greatest threat to the existentence of this species may be the pet trade. Specimens have recently appeared in great numbers on pet trade price lists. Unless the existing wildlife protection laws are enforced, the relatively secretive nature of this snake may be its only protection against extinction. A CKNOWLEDGMENTS Albert Schwartz generously allowed the examination of specimens in his care (ASFS) and provided comments on an early draft of the manuscript. Ralph Axtell and Harry Greene vigorously assisted during our 1978 field work. Collecting permits were provided by C. W. (Liam) Maguire of the Turks and Caicos Island Ministry of Resources. The research was supported by grants from the Florida State Museum and New York Zoological Society. Sheila Iverson typed portions of the manuscript. L ITERATURE C ITED BARBOUR, T. 1937. Ovoviviparity in Trachyboa. Copeia 1937 (2):139. , AND B. SHREVE, 1936. New races of Tropidophis and of Ameiva from the Bahamas. Proc. New England Zool. Club 16:1-3. D OWLING , H. D. 1975. A provisional classification of snakes. HISS Yrbk. Herpetol. 1974:167-170.

F OX H. 1977. The urogenital system of reptiles. pp. 1157. In C. Gans. Biology of the Reptilia. Vol. 6. Morphology E. Academic Press, New York and London. GRANT, C. 1957. The black tailed Tropidophis (Reptilia: Serpentas). Herpetological 13:154. G REENE , H. W. 1973. Defensive tail display by snakes and amphisbaenians. J. Herpetol. 7(3): 143-161. HECHT, M. K., V. WALTERS, AND G. RAMM . 1955. Observations on the natural history of the Bahaman Pigmy boa, Tropidophis pardalis, with notes on autohemorrage. Copeia 1955:249-251. IVERSON , J. B. 1978. The impact of feral cats and dogs on populations of the West Indian rock iguana, Cyclura carinata. Biol. Conserv. 14:63-73. 1979. Behavior and ecology of the rock iguana Cyclura carinata. Bull. Florida State Mus. Biol. Sci. 24(3):175-358. LEHMANN , H. D. 1970. Beobachtungen bei der Haltung und Aufzucht von Trachyboa boulengeri (Serpentes, Boidae). Salamandra 6:32-42. M CDOWELL , S. B. 1975. A catalogue of the snakes of New Guinea and the Solomons, with special reference to those in the Bernice P. Bishop Museum. Part Il. Aniliodea and Pythoninae. J. Herpetol. 9(1):1 -79. M ERTENS , R. 1946. Die Warn- un Droh-Reaktionen dar Reptilien. Abh. Senckenberg. Naturf. Ges. 471:188. MURPHY , J. B., D. G. BAKER, AND B. W. TYRON, 1978. Miscellaneous notes on the reproductive biology of reptiles. 2. Eleven species of the family Boidea, genera Candoia, Corallus, Epicrates and Python. J. Herpetol. 12(3):385-390. PETZGOLD, H. G. 1969. Zur Haltung und Fortpflanzungsbiologie einiger kubanischer Schlangen im Tierpark Berlin. Salamandra 5:124-140. S CHWARTZ , A. S. 1963. A new subspecies of Tropidophis greenwayi from the Caicos bank. Breviora, MCZ 194:1-6. , AND R. 1. CROMBIE . 1975. A new species of the genus Aristelligar (Sauria: Gekkonidae) from the Caicos Islands. Proc. Biol. Sot. Washington 88(27):305-314. , AND R. J. MARSH . 1960. A review of the pardalis - maculatus complex of the boid genus Tropidophis of the West Indies. Bull. Mus. Comp. Zool., Harvard 123(2):49-84. STIMSOM, A. F. 1969. Liste der rezenten Amphibian und Reptilien: Boidae. Das Tierreich 89:1-49. U NDERWOOD, G. 1967. A contribution to the claasification of snakes. British Mus. (Nat. Hist. ), London. 1976. A systematic analysis of boid snakas. pp. 151-175. In A. d’A. Bellairs and C. B. Cox (eds. ) Morphology and biology of reptiles. Linnean Sot. Symp. Ser. 3:1-290.