Corallus grenadensis

Scientific Name

Corallus grenadensis 
Corallus grenadensis. Photo Bob Henderson
Described and named by Dr. Thomas Barbour (1884-1946), a zoologist at the Harvard Museum.  Barbour was, at one time, the Director of the Harvard Biological Station and Botanical Garden of Soledad, Cuba.

Holotype

Museum of Comparative Zoology (MCZ) no. 7991.  Collected by G.M. Allen 29 August, 1910.  No. 7990 collected 20 August, also in MCZ, is a cotype.

Type Locality

St. George’s Parish, Grenada.

Subspecies

None.

However, morphometric comparison of specimens from different populations on Grenada and some of the Grenadine Islands showed differences between the boas on the islands. See below for the testing methods and results.

Synonyms

(ensis) is a Latin adjectival suffix meaning “pertaining to” or “originating in" Grenada, hence grenadensis.

Common Name

Grenada Treeboa, Grenada Bank Treeboa, Congo Snake.

Description and taxonomic notes

A narrow headed, slender bodied boa.  Maximum snout-vent length (SVL) 1625 mm; maximum number of dorsal scale rows 37-46; ventrals 251-278; subcaudals 100-119; scales between supraoculars 3-9; infraloreals 0-4.  The dorsal ground color is variable depending on the island form (yellowish, orange, gray, taupe, brown).  The dorsal pattern or rhomboid is usually spade or mushroom shaped with the edges either sharply defined or rounded.

Henderson began his studies of the Grenada Treeboas in earnest beginning in 1988.  By this time he had visited St. Vincent twice and Granada once.  During this period he was able to collect boas at the rate of 6 per hour when in ideal habitat.  Some of the boas were caught and preserved for inclusion to the MPM’s collection.  Some were captured for captive breeding studies (Henderson, 1988).

As a  species, C. grenadensis exhibits a great variety of color, much like C. hortulanus . The sites where the boas are collected determines the main color.  It would appear the different islands also present particular color variations; i.e., Mayreau specimens all tend to be red or orange.  On Grenada and several islands in the Grenadines (Bequia, Union and Carriacou) yellow and yellow/orange specimens can be both patternless and near patternless (Henderson, 2015).

Based on live sampling, iris and tongue color often match the dorsal ground color in C. grenadensis.  The yellow boas tend to have yellow irises and pale tongues; the dark brown or taupe boas have dark irises and dark or black tongues .

Henderson conducted sampling and testing of boas on the Grenada Bank to determine if any morphological differences existed between the boas on the different islands.  He chose to sample boas that had a SVL greater than 800 mm for the study.

He found differences existed at the midbody circumference of the boas when comparing them from Grenada and Canouan, Grenada and Carriacou, Grenada and Union and Grenada and Mayreau.  He also found differences in head length between the boas from Grenanda and Canouan and Grenada and Union.  He posits the differences may have foraging and prey capture implications .

C. grenadensis and C. cookii have the shortest and narrowest heads in the genus Corallus, with C. grenadensis being the most slender of the boas.  When morphologically plotted, C. cookii, C. grenadensis, C. ruschenbergerii and C. hortulanus form a cluster due to longer body and tail length with higher ventral and subcaudal scale counts.  C. caninus, C. batesii and C. cropanii form a 2nd cluster, based on their similar morphology.  C. annulatus and C. blombergii form a 3rd cluster, that sits somewhat equidistant between the other two clusters .

Colston et al. and Reynolds et al. (2014), using molecular data, cast some doubt on the validity of both Corallus grenadensis and Corallus cookii at the species-level.  Colston et al suggested additional study was needed to determine its true relationship with C. hortulanus .

An interesting graph depicting the weigths of the Corallus species accessible to the author.  Note C. cookii and C. grenadensis are very much smaller and lighter by weight and volume.

Although C. grenadensis and C. cookii are nested within C. hortulanus, no taxonomic analyses addressing their species validity has been performed with molecular data (i.e. using species tree methods or coalescent species delimitation), and given the strong morphological support for their recognition as distinct , Henderson still considers C. grenadensis a valid species using morphological data such as scale count differences, color, pattern and geographic/reproductive isolation.  .  Based on his data driven arguments, we are inclined to agree with him.

Distribution

Grenada and Grenada Bank Islands.  Corallus grenadensis is present on almost all of the the various larger Grenadine Islands, consisting of Carriacou (31.6 km²), Bequia (15.6 km²), Union (8.4 km²), Canouan (7.29 km²), Mustique (5.49 km²), Mayreau (1.76 km²), Isle à Quatre (2 km²), Baliceaux (1 km²), Petit Martinique (2.02 km²) and Petit St. Vincent (0.46 km²).  It is possible the boas may also inhabit other small islands on the bank as well.  The exceptions are Tobago Cays and Prune Island (=Palm Island) .  The boas are found from sea level to about 525 meters.

C. grenadensis in situ. Photo Bob Henderson

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Habitat

The annual average temperature in Grenada is around 25°C, with a small variation (one degree) at annual seasonal cycle. The mean maximum temperature is 31.4°C while the mean minimum is 24.0°C.  Annual rainfall  on Grenada ranges from 750 to 1400 mm.  Annual rainfall is measured at approximately 900 mm on Mayreau to approximately 1880 mm on Bequia .  The dry season typically runs from January to May and the rainy season from June to December.  Carriacou and Petit Martinique generally receive lower levels of rainfall and during the dry season can experience severe drought conditions.

Mountain peaks, narrow, deep valleys and very steep ridges are the main features of Grenada.  Mount St. Catherine rises to 833 meters above see level and is located in the northern section of the ridge that runs through the middle of the island from north to south.  There are large coastal plains in the east as a result of the gentle slopes leading down to them.  In the west the terrain is rugged with low hills in the southwest and northeast.   The forested land on the island consists of montane and elfin, rain forest, littoral woodland and dry scrub woodland .

C. grenadensis possibly in ambush posture. Photo Bob Henderson

C. grenadensis are found in a variety of habitat on the islands.  The boas inhabit forest (especially man-made edges of tree lines), rainforest , mangrove and associated cover, vine and thorny shrubbery. On Grenada they can be found in non-native introduced trees such as Mango (Mangifera indica), Breadfruit (Artocarpus altilis), Nutmeg (Myristica fragrans), Cacao (Theobroma cacao), Banana (Musa sp.), Coconut Palm (Cocos nucifera) . They are also found around, in and on housing where trees make contact, heavily disturbed human habitat, fencing and in agriculture areas .

C. grenadensis foraging at night. Note the type habitat. Photo Bob Henderson

Corallus grenadensis still occurs in the Botanical Garden, located in St. George’s, more than a century after it was first collected there (Henderson 1998).  Powell & Henderson describe one such example of urbanization of the Grenada Treeboa:

 "Although we are reluctant to describe any West Indian snake as edificarian, the arboreal boids Corallus cookii and C. grenadensis come closest. When trees abut houses, boas will readily enter the latter using rafters as shaded diurnal perches (Groome 1970; Henderson 2002). On Grenada at a plant nursery, we were shown a C. grenadensis that, by day,  perched in the shade of a greenhouse eave. On a daily basis, it moved into a patch of sunlight and then retreated back into the shade. We assume it left its diurnal retreat at night to forage."

 

The unique rhomboid pattern of Corallus grenadensis.
Variation in the color and pattern of Corallus grenadensis.
More variation in C. grenadensis.

Longevity

It is unknown how long lived C. grenadesis is, though it can be surmised it has the longevity of its sister species if left to its own devices in its natural habitat.

Reproduction

Mating in the wild occurs during the months of February and March with babies born in August and September.  On Grenada breeding appears to parallel the “dry” season (December through May)  with babies born during the “wet” season (June through November).  Gestation for the species is 180 days/6 months; typical of many other species in the genus.  Litter size in the wild has been documented at 23.  Two females were found to have  29 and 40 undeveloped eggs .

In captivity breeding appears to mirror the season on the Island(s).  Mating takes place during the months January through March with babies born in June though September.  Litter size in both Europe and the U.S. appears to number six to thirty babies and (unpublished observations by: Murray 2018-2020, Kjeldgaard 2020, Murray 2021, Versluis 2021, Murray 2022).

Corallus grenadensis mating.
Corallus grenadensis mating.

Behavior

Nocturnal in nature, arboreal in habit.  Foraging duration at night appears to often end around midnight, observes Henderson.  He attributes one possibility as decreasing ambient temperatures at this time.  Heavy rainfall, he noted, reduced the incidence of observation of the boas, though light rain or misting generally had no affect on the boas’ foraging behavior.

Defensive posture in Corallus grenandensis is a ball position.  They display this behavior at all ages.

Corallus grenadensis baby in a defensive ball.
Defensive posture in a young Corallus grenadensis.
Defensive posture in a subadult Corallus grenadensis.

Diet

C. grenadensis is one of only two species in the genus whose diet consists mainly of lizards  (Anolis richardii and Anolis aeneus).  The boas also feed on Iguana (Iguana Iguana), introduced mice (Mus musculus), rats (Rattus rattus) and small birds. Notably, 2 recent introductions to the island have started to show up in the boas’ diet.  They are the frog (see below) and the marsupial Marmosa robinsoni (Rush & Henderson 2014).  Remarkably, gekkonids are missing from the Treeboas’ diet.  The gecko has not been reported as a stomach content food, nor has the boa been seen in the field taking one as a meal  .  Lizards are prey items for all classes of C. grenadensis; juveniles, sub-adults and adults alike.  This could be the result of the lizards being geographically imposed as a prey item on the boas.

The treeboas exist where there are no readily available supplies of rodents.  Henderson and Pauers suspect, because lizards are geographically imposed on the boas, they developed a true specialization as a result of the lack of bird and rodent prey.  The incredible numbers of Anolis have probably reduced any dependence of young boas on  bird prey items .

C. grenadensis appears to be the most euryphagic of the genus; in its lifetime it consumes frogs, lizards, and birds as juveniles.  As adults they consume anoles, birds and introduced  rodents . (Henderson 2002)

Pendlebury found, in a study from 1974, the treeboas from Carriacou contained 62% ectothermic prey and 14% endothermic prey.  Conversely, the mainland treeboas’ diet consisted of 0% ectothermic prey and 67% endothermic prey .

The diet of Corallus grenadensis undergoes an ontogenetic shift from ectotherms (primarily native species of Anolis) to endotherms (largely introduced Mus and Rattus). Contrary to its mainland relative Corallus hortulanus, birds are rare in its diet .  The type of prey is more size dependent in the Island boas.  In the smaller boas, only ectothermic prey was found in their stomachs.  Both ectothermic and endothermic prey were found in the larger boas.  On the mainland, the prey type was not size dependent.  C. grenadensis appears to be an opportunistic feeder, as borne out in the Pendlebury study.  If one had to affix a general rule of thumb for how the ontogentic shift in diet takes place, Henderson came away from a study of two sites with the following data:

  1. Boas <600 mm SVL preyed exclusively on anoles
  2. Boas 600-1110 mm preyed on both anoles and endotherms
  3. Boas >1110 mm preyed exclusively on endotherms

Rush et al (2013) reported the predation of the introduced frog, Eleutherodactylus johnstoni, by a young C. grenadensis on two separate occasions.  The Grenada Treeboas’ primary food prey items are the readily available Anolis aeneus, Anolis richardii and introduced rodents.  The one native frog is found where the treeboas are either absent or rare.  Finding the boa feeding on the frog, which is just as abundant as the native anoles, surprised the researchers since sleeping anoles are a much more lucrative target .

Ex-situ diet, in the USA, consists of the invasive brown anole as a food source for neonate  and young C. grenadensis.  The babies generally require assist feeding for the first 90 days or so, until they exhibit an interest in feeding on their own. It is unknown, at this time, whether the babies will voluntarily feed on small geckos or skinks. Henderson (pers. comm.) has never found a gecko in the stomach of C. grenadensis.  It is important to verify this in captive bred and born treeboas as well.

The young boas, at approximately 8 – 18 months of age, will start feeding on hot pink mice covered with chick fuzz.  After four or five meals the scenting with chick fuzz can be discontinued and the boas will continue to feed.  Growth, once on rodents, is exponential.  Adult boas can occasionally become finicky feeders, especially after breeding season, and may require an assortment of different prey items to get them feeding again.

Newborn Corallus grenadensis. Note the forward looking eyes, useful for finding sleeping Anolis on branches.

Of interest is the parallel of the island Chilabothrus and island Corallus-the young of both genus’ have developed a specialization for lizards as juveniles (C. angulifer is the lone  exception).  Whereas most of the Chilabothrus quickly outgrow this dependence on lizards (except for C. fordii, C. gracilis and C. granti), C. grenadensis feeds on lizards its entire life cycle.

Captive management

The Grenada Tree Boa was kept in terraria of European and American private breeders since the early second half of the last century. German breeder Karl-Heinz Progscha reports a litter of six young, born from parent animals that originated from Grenada . In the same article the author pointed out that the animals are referred to as Corallus enydris cooki after Werner – while they are referred to as Corallus enydris grenadensis according to Barbour. Blumberg obtained several animals from Grenada and reported a litter of thirty young. In this article he states that all island forms of Corallus belong to the supspecies Corallus enydris cooki, wheras all mainland forms are C. e. enydris . These nomenclatorial uncertainties together with the problems feeding the young animals – both authors stated that they had to force feed the small animals – might have led to a strong decline of the population in captivity. In addition to this, not all keepers seemed to have been aware of where their animals originated from as the picture from the publication of Kreutz indicates .

Corallus grenadensis picture from the publication

The ball position puzzling  Kreutz as unusual for Corallus enydris is perfectly normal for C. grenadensis. In addition, the mushroom like patttern is indicative for C. grenadensis but rather untypical for C. hortulanus.

Today, fortunately, a small number of animals are kept in European and US private collections and reproduction has occurred successfully several times. J. Murray reports that typical boid breeding strategies work with this boa.  A cool down and misting season for 4-8 weeks followed by a slow warm up period is sufficient to elicit mating behavior from the boas.  Mating is accomplished in the typical fashion of all the Corallus species. Spurring and tail wrapping are obvious signs of having correct environmental parameters.

Once a female ovulates, she will bask under a heat lamp or radiant heat panel.  The body temperature of the gravid females can sometimes reach temperatures in the the 89°-91°F (31.7°-32.8°C)  range for short periods.  Heat sources are not provided at night other than the NTL, usually 78°-80°F (25.6°-26.7°C).  The females took the occasional heated medium mouse (MM) on the small side or a rat pup (RP) while gravid; these were offered on the average of once a month.  Once the females are within several weeks of parturition the offer of food is discontinued.  The females start to bask less, eventually avoiding the heat entirely.  Parturition for this species is about 180 days from the post ovulation shed (POS).

Gravid Corallus grenadensis resting in the open.
Gravid C. grenadensis alleviating her weight by soaking in a water bowl; she spent nine days in the bowl.

The young have proven difficult to start feeding on their own.  Murray has had to resort to assist feeding the babies sections of rat pup tails or small anoles for approximately 90-120 days.  At this point the babies start to feed voluntarily on small brown Anolis; growth once feeding on their own quickly accelerates.  Babies that refuse to feed on their own past the 100-120 day point start a downward spiral and and rarely recover.  Once they are feeding solidly on anoles, growth is constant and they shed more often.  Around the 8 – 18 month mark they can be converted to pink mice by offering them super heated and covered in chick fuzz.  Within 2 weeks all youngsters had been converted to unscented pink mice.  At this point they are considered “bullet-proof”, a common reference to the maintenance state of a juvenile boid.

Assist feeding a two month old C. grenadensis.

10 youngsters from 2 litters born in 2018 are all doing very well for this author.  6 of the boas went out on long term loan to experienced private collectors.  2nd and 3rd generation breeding is needed to determine the long term viability of of this species and what affect, if any, subsequent generations will require as first foods.

Newborn C. grenadensis.

The outlook for the species ex-situ appears somewhat promising. Combined efforts between private and public collections could help secure the species ex-situ and establish a stable, diverse and self-reproducing population.

Newborn C. grenadensis.
Young of the year with many hiding places.
C. grenadensis born 21 July 2020; first pink mouse taken at day 161.
Yearling Corallus grenadensis.
Hemipenal epithelium from a 4 month old male C. grenadensis.

Conservation status, threats and population size

CITES:  Appendix II
Grenada joined CITES on 30 August, 1999; entry into force on 28 November, 1999.

IUCN Red List:  None

Catalogue of Life:  (click here)

The National Center for Biotechnology Information:  (click here)

CITES import/export data:  (click here)  **

**  Statistics from the Cites Trade database for Corallus exported from Grenada between 1975 and 2017. Note that out of the 141 animals exported from Grenada, 75 originated from Guyana.

C. grenadensis is one of the few species that can be found in in dense numbers (40-125 per ha) .  In spite of all the agriculture, the treeboas are probably more abundant now than in pre-Colombian times .  C. grenadensis is one of the best studied boa species in situ, in large part due to the restless, consecutive and creative work of Robert W. Henderson and his collaborators.  According to their work, the population size is estimated to consist of 7000 to 8000 animals in total on Grenada . Yet, the overall population size might be shrinking due to urbanization projects which lead to habitat loss and a decline in the population of the preferred prey items Anolis aeneus and Anolis richardii.  All forests are now secondary, due to human encroachment and sustainment agriculture.

Corallus grenadensis, Mayreau Island. (Internet photo).

On Mayreau the once common Congo Snake is now seen less frequently; this could possibly be attributed to the large feral cat population and the introduced manicou (Didelphis marsupialis insularis).  The manicou may also be responsible for the identical boa situation on Mustique, where fauna is officially protected .

The boas are currently listed under Cites Appendix II. While this grading would make legal exportation within a quota system possible, significant exports for leather/fashion or the pet trade seem non-existent. Few animals might be poached illegally, but lack of interest on the side of hobbyists makes this threat unlikely to affect natural populations negatively. Other threats such as exploitation for skin leather etc. are unknown to be relevant for the conservation of Corallus grenadensis.

While introduced domestic cat/feral cat populations are a major threat to the boas, the  greatest threat to the boas is the unabated development of land in the Bank’s islands for the tourism industry .

Politically, the islands of the Bank are divided between Grenada and Saint Vincent.  Carriacou, Petit Martinique and Isle de Ronde are under the control of Grenada.  Protections and enforcement differ by island, dependent upon on the governing authority.  Differing layers of bureaucracy concerning the management and conservation of the herpetofauna has been unnecessarily complicated by the division of governing parties.

Currently, the major concerns for the survival of the Grenada Tree Boas are habitat loss, introduced predators, loss of available prey items and climate change.  The maps below illustrates the loss of habitat for C. grenadensis on the main island, Grenada.  The islands of the Grenada Bank are under the same economic pressures.  Two of the islands are exclusive resorts.  For now, the Grenada Treeboa seems to be holding its own.

Current research by Henderson et al. (2021) attempt to determine the diet, size and transience on its demography on Carriacou using a one km stretch along a road marked off in 50 meter sections (x20).  The study ran from 5 June, 2015 through 26 October, 2019 and consisted of 254 individual captures for a total of 680 times.  The smaller boas, generally less than 800 mm SVL,  tended to stay in the study area for years-this was attributed to the high density of prey items (Anolis).  The larger boas, larger than 800 mmm SVL, were transient and moved quickly, or eventually, to habitat that supported rodent populations in a higher and more sustainable density.  The selective pressures brought upon the larger boas as they transition from ectothermic prey (Anolis) to endothermic prey (mus & rattus) demonstrates their transience as a result of their increase in size and change of diet.  One can infer there is little concern about populations numbers from from an immediate conservation standpoint; it would appear the boas are in no danger of disappearing in the short term.  The authors conclude this type of research can be the start point for additional research on the demographic and ecological importance of other boas in the West Indies (Henderson et al. 2021).

The CIA World Factbook lists the following environmental threats for Grenada: deforestation causing habitat destruction and species loss; coastal erosion and contamination; pollution and sedimentation; inadequate solid waste management. For St. Vincent and the Grenadines: pollution of coastal waters and shorelines from discharges by pleasure yachts and other effluents; in some areas, pollution is severe enough to make swimming prohibitive; poor land use planning; deforestation; watershed management and squatter settlement control .

Grenada (Corallus grenadensis)
Partial view of the Grenada Bank. (Corallus grenadensis)
Partial view of the Grenada Bank. (Corallus grenadensis)
Early map of Grenada. 1775.

On display in these Zoos

According to the website zootierliste.de Corallus grenadensis is kept in Horní Pěna (Zoopark Na Hrádečku) in the Czech Republic.

Grenadensis Galore

More information:

The Grenada Tree Boa has been extensively studied in the wild. Through the authoritative academic work of Robert W. Henderson as well as others, a wealth of information about the natural history, habitat use and behavior of this boa is present. Henderson summarized the current knowledge about the Genus in three books dedicated to the Genus Corallus . We strongly recommend the reading of these books to the dedicated Grenada Treeboa afficionado.

Continue to Corallus ruschenbergerii

 

Citations