Scientific Name

Chilabothrus granti 

Described and named by Olive Griffith Stull (1905-1969) in honor of Major Chapman Grant (1887-1983). Stull was an American herpetologist who worked at the Harvard Museum of Comparative Zoology.  Grant was a herpetologist and grandson of Ulysses Grant, 18th US President. Grant started the journal Herpetologica, which became the publishing arm of the Herpetologist's League, an organization he co-founded in 1936.

Holotype

Museum of Comparative Zoology: no. 33847, collected by Major Chapman Grant.

Type Locality

Type locality: Tortola Island, British Virgin Islands.

Synonyms

• Epicrates angulifer var striatus    [6]  [7]
• Epicrates sp.  : 344
• Epicrates inornatus granti   [2]
• Epicrates inornatus granti  : 131
• Epicrates inornatus granti 
• Epicrates inornatus granti  : 150
• Epicrates inornatus granti : 74
• Epicrates inornatus granti 
• Epicrates inornatus granti 
• Epicrates monensis granti    [103]  [104]
• Epicrates monensis granti 
• Epicrates monensis granti  [34]  [35]
• Epicrates monensis  Lazell, 1983: 104
• Epicrates monensis granti 
• Epicrates monensis granti 
• Epicrates monensis granti 
• Epicrates monensis granti  : 23  [24]
• Epicrates monensis granti 
• Epicrates inornatus granti  Liner, 1994: 22
• Epicrates monensis granti  : 21
• Epicrates monensis granti  Tipton, 2005: 46
• Epicrates granti 
• Epicrates granti 
• Epicrates monensis granti 
• Epicrates granti  : 138  [140]  [141]  [144]  [145]
• Chilabothrus monensis granti 
• Chilabothrus monensis granti 
• Epicrates granti
• Chilabothrus granti 
• Chilabothrus granti  : 12  [13]
• Chilabothrus granti  : 38

Common Name

Virgin Islands Treeboa, Virgin Islands Boa.

Description and taxonomic remarks

The first mention of the Virgin Island Treeboa Chilabothrus granti was in the work of Meerwarth. He investigated material collected by Sleebohm from St. Thomas and considered the animal to be an Epicrates striatus (= Chilabothrus striatus), similar to the other striatus samples from Haiti and Santo Domingo (today’s Dominican Republic) that he investigated. However, he did not respect striatus as a valid species and considered it a subspecies or “variety” (var.) of the Cuban Boa, thus naming it Epicrates angulifer var. striatus. .

It was Stull who first recognized the individual characteristics of the Virgin Island Treeboa and described it as a subspecies to Chilabothrus inornatus . This view was maintained until Sheplan and Schwartz revised the species and argued that the Virgin Islands Treeboa more closely resembles, by coloration/pattern and scalation, the Mona Island Boa and thus categorized it as a subspecies to Chilabothrus monensis naming it Chilabothrus monensis granti .

In a study about conservation and risks to the Virgin Islands Treeboa, Platenberg and Harvey elevated the subspecies to the rank of a full species, however this was done without providing explanation or reasoning for their actions . Finally, a molecular study on the Boas from the Virgin and Mona Islands ended the ambiguity of the question whether or not C. monensis and C. granti are two separate species. Rodrígues-Robles and coworkers conducted an analysis based on 10 individuals of Chilabothrus monensis from Mona Island, and 40 specimens of C. granti from Puerto Rico, Cayo Diablo, Culebra, Saint Thomas, and Tortola. They included additional samples of C. fordii, C.gracilis, C. inornatus, C. striatus in their analysis of mitochondrial DNA (two genes) and nucleaer DNA (seven genes). Based on their results, they estimate that the two taxa have been on separate evolutionary trajectories for approximately 3.3 Mya and argue that since they are demographically independent from one another at the present time and that the geographic separation between populations of these snakes constitutes an effective barrier to gene flow, the two populations should be considered as full species .

A second study published slightly prior to the one from Rodrígues-Robles and coworkers came to similar conclusions. They found support for the distinction of Chilabothrus monensis and C. granti and estimated a divergence of the two lineages in the mid Pleistocene (around 2.10 Mya). However, the authors were more conservative in the interpretation of their results, owing to the few animals they based their study on. Therefore, they did not suggest the elevation of C. granti in species rank without further analysis .

Only a few specimens had been collected for Museums;  the description by Stull was based on one specimen; Sheplan and Schwartz had a series of six animals from St. Thomas. Their largest Chilabothrus granti male had a SVL of 770 mm, the largest female 470 mm. Chandler and Tolson studied a population of C. granti on Cayo Diablo and reported the average SVL males (N=16) 674 mm and females (N=28) 689 mm .  The largest snout-vent lengths (SVL) recorded for the species were 1,066 mm (42 inches) for females and 1,112 mm (44 inches) for males; total body lengths 1,203 mm (47 inches) and 1,349 mm (53 inches), respectively (Tolson 2005).  Reynolds lists maximum SVL as 1112 mm . Nellis et al. reported six additional specimens from St. Thomas, none of which was larger than 770 mm SVL.  They also describe the coloration as follows: “The natural coloration of the Virgin Islands boa is striking. The blotches are dark chocolate brown with light grey-brown in between. Conspicuous blue-purple iridescence was observed in both live specimens and in the road kill” .

Tolson notes that animals from La Cordillera, Puerto Rico, and St. Thomas, U.S. Virgin Islands exhibit a distinct coloration . According to him (pers. comm.) Chilabothrus granti from Puerto Rico are more dark with cream venters, whereas the USVI Chilabothrus granti have yellowish venters and blotches that are boldly outlined in black; however, pointing out  there may be significant variation within populations.  The small degree of overall genetic variation in C. granti suggests that demes of this snake could be managed as a single unit, a practice that would significantly increase their effective population size .

A short video of a small C. granti can be seen here and here.

* Source 

Lazell measured 6 boas from the British Virgin Islands :

Distribution

Puerto Rico (Districts: Ceiba, Luquillo, Rio Grande, Humacao, Fajardo here on the Cayo Diablo and the Cayo Ratones), British Virgin Islands (Great Camanoe, Guana, Necker, St Thomas, Tortola, Virgin Gorda & Jost Van Dyke), Virgin Islands (US): Culebra, St. Thomas, Steven Key, Tortola, Vieques Island (purposely introduced).  MacLean et al question whether C. granti is found on Guana Island .  According to Dawson, et al (2014), Guana Island also has a population of Anegada ground iguana and the endangered Virgin Islands boa (Epicrates monensis granti) and Lesser Virgin Islands skink (Spondylurus semitaeniatus) .  Jost Van Dyke is a new C. granti extension (Eldon, pers. comm., Mar 2024). There are many satellite islands devoid of C. granti that, geographically and historically, probably had boas as inhabitants when the sea level was ten meters below current levels .

The USVI's were purchased by the United States from Denmark for the sum of $25,000,000 and were formally transferred on March 31, 1917.  See payment here.

Note that according to the the U.S. Fish and Wildlife Service, the population on Puerto Rico occurs in five districts: Ceiba, Luquillo, Rio Grande, Humacao, Fajardo (Cayo Diablo and Cayo Ratones) .  Contrary to this, Reynolds and coworkers claim that the species is found on the main island of Puerto Rico only in the municipality of Río Grande .

Chilabothrus granti has been reintroduced to Vieques Island, being the only Chilabothrus species that has been anthropogenically introduced and become established on an island in the Caribbean .  The introduced population on Cayo Ratones is in doubt since the reestablishment of the rat population in 2004.  2018 surveys were inconclusive, though only cursory in nature.

Chilabothrus granti has been also documented from the U.S. mainland (Florida) with a single animal, considered as introduced and an invasive species . The animal appeared to have been accidentally introduced to Florida and was discovered in a shipment of electronic waste from Puerto Rico. It remains unclear if this animal came from a Puerto Rican population, or if it originated from the Virgin Islands and was shipped with the electronic garbage first to Puerto Rico and subsequently to Florida. While it is conceivable that Chilabothrus granti had been displaced and introduced to Florida prior to this observation, no reports indicate the formation of a population in North America, nor have there been any further sightings of this species.

[intergeo id=”wN0ETM”][/intergeo]

Habitat

The Virgin Islands and Passage Islands differ from Puerto Rico in climate and vegetation. Thomas described the Life Zones on the Virgin Islands, stating that beyond Puerto Rico, the other islands of the area are more xerophytic. Scrub to dry woods with the most mesic areas being found in ravines and on the highest parts of the islands. The Passage Islands and U.S. Virgin islands are climatically capable of supporting Subtropical Moist Forest and  range from 72% on St. Thomas to 0% on Culebra. Tortola has a substantial amount of Subtropical Moist Forest habitat. St. Croix, although larger than any of the eastern bank islands, has only 16% in the Moist Tropical Forest Life Zone .

A study on Mount Resaca in northern Culebra Island, where the species was known to occur yielded no visual encounters. The study site is a dry subtropical forest type, which fosters constant vegetational changes depending on the seasonal temperature, humidity and rainfall. The Virgin Islands Treeboa was encountered in the Culebra National Wildlife Refuge but outside of the study area during the time of that study. During the day hours the mean humidity was 82.0% with a mean temperature of 27.78°C; night mean humidity was 84.8% (only 2.0% higher), and the mean temperature was 26.05°C (only 1.73% higher) .

In contrast to this, Nellis et al. report that all specimens from Saint Thomas were collected on the eastern part of the island, the most xeric part of the island . Tolson confirms this observation and considers subtropical dry forest with closed canopy as the habitat where C. granti  is most commonly observed, particularly St. Thomas. This habitat type is characterized by small (< 5 meter) deciduous trees with small, coriaceous or succulent leaves and thorns, spines, and secondary defensive compounds. He found that the common tree species on St. Thomas included Burseria simaruba, Cephalocerekts royenii, Pictetia aculeata, Bucida buceras, Guaiacum officinale, Leucaena glnuca, Tamarindis indica, Melicoccus bijugatus, Acacia ssp., and Capparis cynophallophora. .  Much of eastern Saint Thomas is privately owned.

The US Fish and Wildlife service lists “forested areas” and “rocky soils” as habitat types . Harvey and Platenberg found that C. granti habitat was associated with low elevation (<150 m), non-stony soils, and particular vegetation/land use communities (e.g. mangrove, thicket/ scrub, hotel/resort, waterfront/marine, low density residential).  They found no snakes above 150 meters  elevation and more than half (57%) of the boas were found within 50 meters of roads .

On Cayo Diablo, the boas concealed themselves in refuges such as Cocos and Sabal axils, termite nests, rocks and debris during the day and forage at heights of up to 4 meters at night .  Both termitaria and debris may be important refuges on Saint Thomas given their greater abundance near observations. C. granti occupied areas with trees as tall as 10 meters on Saint Thomas; however, snakes may forage on lower branches or shorter trees within these areas .  Tolson (1996) found foraging boas on Cayo Diablo are not restricted to just trees; they also use salt-tolerant shrub lands just above the high tide line.  C. granti are rarely encountered where the populations of Rattus are not held in check by introduced predators; this is especially so on the cays (Tolson 1988).

Longevity

The longevity of this species in captivity often exceeds 20 years, with a small percentage exceeding 30 years.  They can also reproduce at greater than 20 years of age.  Tolson also found the VI boa has a biannual reproductive cycle.  As a result, female VI boas have the potential to produce 50-75 offspring during their lifetime (Tolson 1986, 1996).

Reproduction

Slavens reports the following reproduction data :

1991 TOLO 0.0.7 born during 1991.
1992 FORT 0.3.1 born during 1992.
1992 SANT 0.0.7 born during 1992.
1993 TOLO 0.0.10 born during 1993.
1996 TOLO bred during 1996.
1997 FORT 0.0.9 born during 1997.

The only knowledge about litter size is derived from captive breeding. Litter sizes range from two to ten young and the young are colored but more contrasting than the adults .  Litter size increases with the body size of the female .  Neonate SVL is 200 mm-350 mm  with weights of 2.0-7.2 grams (Tolson 1992).

A study on the herpetofauna of  the island of Culebra found environmental conditions such as precipitation, temperature and humidity on the study site at Mount Resaca were relatively constant throughout the year.  This prompted the authors to suggest that the constant climate might cause herpetofaunal populations not to reproduce seasonally .  However, they did not show any evidence supporting their idea, such as newborn, young and half grown animals encountered at the same time, mating activities observed at different time points during the year, etc.

The captive breedings reported by Tolson were stimulated by yearly fluctuations in temperature and humidity.  He considered these factors as very important in maximizing reproduction of the species .  The animals in their breeding group were adults.   Tolson observed thermoregulating gravid females in the wild; measurements of cloacal temperatures showed elevations to over 36°C.  In August and early September they routinely observed wild gravid females basking on a daily basis in the morning (8.00h-11.00h) or afternoon (15.00h-17.00h), but not at midday.  They found the females on or within sun-baked termite nests and debris.  They provided the females of the captive breeding program the opportunity to bask regularly.  The young are supposedly extremely easy to raise, given the availability of anoles as a prey food item.  If fed heavily, they grow very rapidly and can exceed 100 gram mass during the first year.  Tolson estimates, based on field data as well as their experience with the animals in captivity, that C. granti can reach sexual maturity within two years .

The captive breeding program at the Toledo Zoological Gardens is a case study in point.  The USFWS 5 year review (2005) of the boa states:
“The captive breeding for the VI boa started in 1985 with a cooperative breeding program between the Service, the DNER, the DFW and the Toledo Zoological Garden.  At that time, ten VI boa were collected from wild populations in Cayo Diablo and Saint Thomas for
the purpose of breeding in captivity.  The objective was to release the offspring into the
wild to establish additional populations thereby increasing the distribution of the species
in Puerto Rico and USVI.  In December 1990, the American of Zoological Parks and  Aquariums Association (AZAA) developed the Species Survival Plan (SSP) for the VI
boa and the Mona boa (Tolson 1991b).  According to Tolson (1996), the environmental
and social parameters necessary to induce reproduction can be duplicated fairly easily in
captivity, the husbandry of the species is not difficult, and the survivorship of captive
neonates is extremely high, exceeding 83% at the Toledo Zoological Garden.

Two populations of VI boas were established by the reintroduction of the species from
the captive breeding program.  The first reintroduction was conducted in Cayo Ratones in
Puerto Rico in 1993.  Twenty eight captive born boas were released from seven different
zoological gardens (Buffalo, Denver, Ft. Worth, Milwaukee, San Antonio, Virginia and
Toledo). An additional six boas were released later for a total of 34 individuals.  The boas released were offspring of adults captured by Tolson (1986a) in Cayo Diablo.

From August 2002 to September 2003, a total of 42 VI boas were released at Steven Key,
USVI.  Eleven boas came from the captive breeding program from the Toledo Zoo and
31 boas came from St. Thomas as part of the translocation program conducted by the
DWF.  Nine boas from the DFW translocation program were implanted with AVID or
Trovan passive integrated transponders for easy identification in the future.  Surveys
conducted by Tolson et al. (2008) on the Key determined that the release was successful and documented recruitment.

Behavior

Several studies on the behavior of the Virgin Island Treeboa Chilabothrus granti have been conducted.  The species is an active forager as the results of a study by Chandler and Tolson indicate.  They studied a population of C. granti on Cayo Diablo and found that the Virgin Islands Tree Boas become active shortly after dark and move slowly along branches or through tangles of vegetation.  Sleeping anoles were relatively conspicuous on branches and foliage in the same general areas where boas were encountered .  Gravid females actively thermoregulate by basking in the morning or evening .  Foraging and resting  are two different behaviors that require two different habitats.  Foraging is accomplished through tree density and connectivity; resting is accomplished through the use of tree axils, termite nests. tree holes and under rock and debris (Tolson 1988).

Diet

The boa feeds mainly on Anolis cristellus in the wild , but had been successfully offered rodents as well as Anolis carolinensis in captivity .  The greatest concentrations of Virgin Islands boa are found where Anolis densities exceed 60 individuals/100 m² (Tolson 1988).  One of the specimens studied by Sheplan and Schwartz (WPM 1569) had eaten a Mus musculus.  Small birds and  ground lizards (Ameiva exsul and iguana hatchlings) are also taken (Tolson 1989).

Captive management and population in captivity

To our knowledge, the Virgin Island Treeboa is only kept in zoos where also a successful breeding program under the lead of Dr. Peter Tolson was established. The initial reproductive program has proven very successful, producing more than 100 offspring from ten founders.  Representatives of two populations of the Virgin Islands boa, one from Puerto Rico and one from St. Thomas, USVI, were managed in captivity as two separate genetic units .  We lack any additional information about the continuation of the program and current breedings of the Virgin Islands Treeboa in captivity.  We are grateful for any information.

Conservation status, threats and population size in nature

CITES:  Appendix I
The Virgin Islands Treeboa was listed by the USFWS on 13 October, 1970.

IUCN RED List: EN Endangered  (click here)

Catalogue of Life:  Unavailable

The National Center for Biotechnology Information:  (click here)

CITES import/export data:  Unavailable

USFWS status:  (click here)
-The USVI Boa has been proposed for down-listing from Endangered to Threatened by the USFWS .

FWS Focus:  Yes

By all appearances, the towel had been thrown in with regard to the conservation of C. granti.  Nellis et al., close out their 1983 paper with the following:  “Besides endangered species status and the protection that confers, the establishment of critical habitat status for the approximately one square mile of St. Thomas east of 64°52′ is probably all that can be  done to preserve the St. Thomas population of E. m. granti.  Additional protection through local ordinance with a fine for molesting or harming specimens and an education program might be worthwhile.  Enforcement personnel should be especially watchful for collectors seeking this rare boa.”   On Guana Island these populations are threatened by predation from feral cats and dogs and habitat modification by feral sheep .

Historically the boa might have been much more widely distributed on the islands off the Puerto Rico Bank. Tolson notes that vegetation on Puerto Rico and the Virgin Islands has been severely disrupted, and 17th-18th century land use patterns on the U.S. Virgin Islands may partially explain the limited distribution of the boa on the east end of St. Thomas and its absence from St. John .

This is also indicated by the habitat suitability analysis by Platenberg and Harvey who found suitable habitats outside the known distributional range of the species indicating that the boa might have once been distributed in much wider parts of the island.  Aside from habitat loss, threats for the species are introduced non-native species.  Tolson found that those islands with heavy rat densities (ca. 20 rats/hectare~-Buck Is., Cas Cay, and Salt Cay-have lower Ameiva and Anolis densities and apparently no boa populations, despite suitable habitat. Even on islands with suitable habitat, strong Anolis populations and only low rat densities the boas were absent (the exception is St. Thomas).  This is explicable by a behavioral change in the Anoles. They are resting lower to the ground, where boas do not find them .

Tolson concluded that rats and feral cats are major factors affecting animal community composition of islands on Puerto Rico and the Virgin Islands. The complexity of a food web in which several predators are also prey is illustrated by his work. Cats in particular are a double edged sword for the boas, since they could provide benefits to the boa by preying on Borikenophis portoricensis and Margarops fuscatus as well as introduced rats. All of which are boa predators and competitors for Anolis prey. His results suggest that habitats infested with rats (and or cats) are amongst the most serious threats to Virgin Island Tree Boa survival, either due to direct predation, or to predation of the prey species and induced behavioral change in the prey species.

Additionally, introduced reptiles and amphibians were listed by Platenberg as potential threats. She found a diverse array of introduced herpetofauna on St. Thomas among which she considered that Corn Snakes (Patherophis guttatus), a North American snake species, might pose a threat to the endangered Virgin Island Tree Boa by direct predation and competition . Given the situation at present and in the recent past, the measures for reintroduction and habitat restoration are necessary to secure the existence of the species.

Luckily, the Virgin Islands Tree Boa interested a skilled biologist, driven to the conservation of the species.  The history of conservation of this boa is a true success story.  Upon its conservation start in the early 1980’s, establishment of the SSP in 1990, and the eventual involvement of 13 Zoos boas from different populations have been kept and successfully bred in captivity .  The release strategy of captive bred boas included the clearance of the release sites of non-native predators (Black Rats Rattus rattus), health tests of the boas and tests for their ability to capture their natural prey, Anolis cristatellus.  In addition, three different age classes were released from newborn animals to sub adults and reproductively mature adults.  This lead to an increase of the populations of this boa from the original 41 snakes to nearly 500 snakes on Puerto Rico and from 42 snakes to nearly 170 snakes in the U.S. Virgin Islands, as a 10 year survey concluded .

The Virgin Islands Treeboa was also introduced to Vieques Island, where it is believed that C. granti had previously been found.  The boa is now established there .  In regards to the population size, the newest data comes from Rodríguez-Robles et al.  They report that in most surveyed populations, the snake densities are so low that mark-recapture studies are not the approach of choice. Therefore a different strategy was taken to estimate the population density.  They provided unpublished data from Peter J. Tolson and Miguel A. García to describe the abundance in terms of unit effort to find animals in the wild.  They report 0.14 snakes/person/hour on Puerto Rico, 0.22 s/p/h on Culebra, and 0.025 s/p/h on Saint Thomas .  Given the low population density, the conservation status of these boas is still listed as Endangered (EN).  The species might be in a much more desolate state, had the conservation measures not been undertaken.

Tolson’s observation that the boas might reach sexual maturity very early (within two years) when conditions are favorable instills hope that the continued conservation measures bear fruit. However, additional studies on distribution and abundance are necessary. The viability of the populations needs to be monitored continuously to determine how many additional individuals are needed to make up a healthy, sustainable population – this will be also of great value for other West Indian Boas, to establish captive breeding and release programs. Data for population densities on the British Virgin Islands are deficient .

MacLean describes the boas as exceedingly rare in 1982.  Lazell believes granti is the most common snake on Tortola .  Grant (1932) stated the boa was found also on Guana Island but never provided a specimen.  Lazell states he has never personally seen one there; however, is aware of accounts by  people who claim to have seen them (Randall 1980), and Righter (mid-1980’s).  Lazell also states there are at least 20 salvaged road kills or electric-fence kills bottled at the H.L. Stoutt Community College on Tortola .

The USFWS, in 2009, estimated the boa population to be 1300-1500 using  “all” available data.  Reynolds et al., recommended VI boas should be immediately evaluated for potential elevation to critically endangered status .  As of 2018, the Service admits the populations on Puerto Rico and the USVI are “declining, potentially declining, considered rare or unknown and most populations are small or considered rare.” (FWS 2018, 2020).

Cayo Ratones, is an offshore island that is host to a boa population that was reintroduced-that population had significantly grown over the 10 years since release. Sadly, between 2004 and the most recent survey in April 2018, the island was again recolonized by rats, and in 2018 no boas were found.  The boa may or may not now exist on the Island; while the boas are notoriously difficult to find, the burgeoning rat population may have won out.  A significant investment of time and energy went into populating Cayos Ratones with C. granti.  One wonders why the rat population was allowed to grow unchecked between 2004 and 2018.  Despite this enormous loss, the USFWS plans to lower the threat level of C. granti from Endangered (EN) to Threatened (TH).

The CIA World Factbook lists the following environmental threats for The Virgin Islands: BVI: limited natural freshwater resources except for a few seasonal streams and springs on Tortola; most of the islands’ water supply comes from desalination plants; sewage and mining/industry waste contribute to water pollution, threatening coral reefs. For the USVI: lack of natural freshwater resources; protection of coral reefs; solid waste management; coastal development; increased boating and overfishing .

The maps below illustrate the extent of habitat loss and alteration due to development and agriculture.

On display in these Zoos

North Carolina Zoo; The Toledo Zoo transferred their Chilabothrus granti breeding group in order to build a bio-secure facility in the renovated museum. It is unclear if the animals will return on display (Tolson pers. comm.).

Gorgeous Granti

We are not in possession of any additional photos of Chilabothrus granti, however we assembled a link-out collection for you:

Joel Sartores pictures of Chilabothrus granti see links below. His website has more spectacular pictures of many endangered species: click here!

Chilabothrus granti scalation
Chilabothrus granti scalation
Chilabothrus granti overview
Chilabothrus granti overview
Chilabothrus granti head
Chilabothrus granti head
Chilabothrus granti overview
Chilabothrus granti overview

Other sources

Chilabothrus granti
Chilabothrus granti
Chilabothrus granti
Chilabothrus granti
Chilabothrus granti
Chilabothrus granti
Chilabothrus granti

Citations