Milwaukee Public Museum

Corallus enydirs in the West Indies
by Robert Henderson

EDITOR'S NOTE: This article is reprinted from LORE magazine (vol. 38, no. 4, (Winter, 1988), p. 25-30), a benefit of museum membership. ©1996 Milwaukee Public Museum, Inc.

The abandoned control tower and exoskeleton of a Soviet cargo plane stood in mute testimony to recent history as my wife Rose and I drove down a badly rutted and pock-marked road that paralleled the runway of the old Pearls airport near the northwest coast of Grenada. A new jetport had been constructed near St. George's, the capitol of Grenada, and Pearls had become superfluous and, so it seemed, deserted. But we heard someone shouting and glimpsed a boy perusing us on foot. We continued along the cracked, deteriorating runway, still being followed, and Rose suggested we stop and see what was on his mind, if for no other reason than to reward him for his determination.

I stopped the car and the boy, gasping for breath, poked his head in the window and asked if we'd like to buy some pre-Columbian artifacts that he had found in the Pearls area. We examined them, assumed they were genuine, but declined his offer. Using the local vernacular name for tree boas, I asked if he could find "sarpints" for us.

"Sarpints?"
"Yeah, snakes that live in trees."
"Yeah, yeah, I know where to find sarpints."

And with that our new friend climbed into the back seat of the car and directed us to the sleepy village of Upper Pearls. At the insistence of our guide, we stopped near a young man who had been working along the roadside. After a rapid conversation the young man also got into the back seat. I decided that the situation was getting out of control.

"This isn't a taxi, and I'm not providing transportation for all of your friends," I sternly informed our 12-year-old guide.
"It's O.K., this guy is a hunter and he can find sarpints."
I turned to the smiling, solidly-built hunter:
"How long will it take you to find a sarpint for me?"
"Two minutes."
"No, seriously, how long? Twenty minutes? An hour?"
"Two minutes."

At his direction, a five-minute car ride took us out of the village to a pleasant area of cultivated fields, a stream bed, fruit orchards, giant trees that were remnants of forests long- gone, and lots of smaller trees and bushes. In less than two minutes the sharp-eyed hunter pointed upwards. I needed binoculars, but there it was, a tree boa coiled about 65 feet up in a tree. That was not the only tree boa he located for me that afternoon; others were pointed out, and I searched on my own while the boys hunted for iguanas to eat. (They caught one, but I purchased it from them and later released it.)

That night, Rose and I hunted in the same area. We swept the beams of our headlamps over the vegetation looking for the tell- tale, red-orange eyeshine of a tree boa. Always an exciting moment, the eye-shine is visible from over 150 feet, indicating that a snake is nearby. Some eyeshines were 60 feet or more up in the trees, impossible to collect. We became adept at using long cut branches to collect boas; we picked them off of high branches or induced them to crawl onto the branch being held. Until the snake is in hand, we never know if it's going to be yellow, gray, red, black and white, or something else.

Within two hours that first night we had observed a dozen tree boas. Over the next week, Pearls remained an exciting hunting ground, yielding dozens of tree boas in a bewildering array of colors. Most were examined and released.

Tree boas belong to the snake family Bonidae, which also includes pythons, anacondas, and the boa constrictor. Three species of tree boas are recognized: Corallus annulatus, patterned in various shades of brown, occurs from Nicaragua south to Ecuador. Corallus caninus, the spectacular "emerald tree boa," is frequently a vivid green with enamel-white markings, found in the Amazon Basin of Columbia, Venezuela, Brazil, Ecuador, Peru, and Bolivia, and the Guianas. Corallus enydris can be whitish, gray, black, brown, red, orange, yellow, or most likely, a combination of those colors. It can be found from Costa Rica and Panama into South America (Colombia, Venezuela, and the Guianas, Peru, Ecuador, Brazil, and Bolivia), and also the islands of Trinidad and Tobago off the cost of Venezuela, and the Lesser Antillean islands of St. Vincent, Grenada, and some of the Grenadines.

Corallus enydris reaches a total length of about 6 feet. Like other tree boas, the body is laterally compressed, an indication of its highly arboreal life-style. The head is large and chunky, giving rise to the vernacular name of "dog-headed snake," very distinct from the slender neck. The mouth is equipped with long, recurved teeth that can deeply penetrate the body of a prey item and lessen the likelihood of losing it to the ground below. They can also inflict nasty bites to would-be-collectors, and I have never collected a Borallus enygris that didn't try to bits. All tree boas have heat-sensitive pits on the labial scales bordering the mouth which allow the snakes to detect temperature changes as slight as 0.026 degrees C in their environment, and to locate and capture endothermic prey (small birds and rodents). Tree boas are nocturnal, and spend daylight hours sleeping, resting, and basking in sun-shade mosaics.

My enthusiasm for tree boas was kindled when friend, collaborator and colleague Albert Schwartz, the dean of West Indian herpetologists, suggested that perhaps more than on taxon of tree boa occurs in the West Indies.

Whereas all snakes do exhibit some color and pattern variation, C. enydris shows an extraordinary range of colors and, at first glance, patterns. I am interested in determining whether or not color phases could be correlated with some environmental variable, such as rainfall, altitude, habitat, geography, etc. In other words, does the variety of colors and patterns have any adaptive value? I borrowed preserved specimens from various institutions, but quickly realized that more material was needed, especially fresh material from which accurate color and pattern descriptions could be made.

Since then, thanks to the support of Albert Schwartz and the Friends of the Milwaukee Public Museum, Rose and I have been to St. Vincent twice and to Grenada once. We have found Corallus enydris to be a locally abundant species. At good localities on both islands, we have collected them at the rate of about 6 snakes/hour, a remarkable rate for a tropical snake species. Some of the snakes we collect are preserved and incorporated into the permanent scientific collections at the Milwaukee Public Museum, and some are collected and kept alive for captive breeding. Many snakes seen are examined, weighed, measured, sexed, body and adjacent air temperature recorded, structural aspects of habitat noted, and color and pattern descriptions take -- and released at the site of capture. Preserved material at MPM and from other museum collections is utilized for recording data on scale characters, color and pattern, diet information from examination of digestive tracts, and reproductive status (when reproduction occurs, number of offspring). Slowly, between field and laboratory work, we are piecing together the natural history of Corallus enydris in the West Indies, and will eventually be able to elucidate the color and pattern variation.

Searching for tree boas has proven adventuresome. It is sometimes necessary literally to go out on a limb in order to make a capture. Since I weigh considerably more than even the largest tree boa, I have more than once crashed to the ground in a most alarming fashion after a bough has broken while I shinnied or brachiated towards the distal portion of a snake-occupied branch. We have driven for hours on the twisting, narrow highways of small mountainous islands in our pursuit, always hoping that everyone else was driving as defensively (fearfully) as I. We have become friendly wit forestry and wildlife officials, national park rangers, and many people trying to make a living from the land. All have been helpful.

Although many people we meet perhaps can't appreciate our desire to find snakes, most try to offer constructive advice on how or where to locate them. One night in St. Vincent's verdant Layou Valley, we encountered local opossum hunters. We explained why we were wandering about the hillsides with lights strapped to our heads, and they intimated that the smell of fish burning on a fire always attracts "congo snakes," the vernacular name for tree boas on St. Vincent.

We have seen beautiful beaches but rarely had the time to enjoy them, and palm trees silhouetted by calendar-perfect sunsets; sampled exotic West Indian dishes (and exotic Kentucky Fried Chicken!); we have wandered the streets of West Indian ports and backwoods villages; been on more boats than I ever wanted to be, and experienced hospitality in many forms.

We have spent hours wearing headlamps, scanning trees and bushes for the tell-tale nocturnal eyeshine of tree boas, visible even in the gleam of car headlights. Occasionally we are fooled by moths, rats, and small opossums, but not often. In the mountainous Vermont Nature Reserve on St. Vincent, the last stronghold of the endemic, endangered and uniquely beautiful St. Vincent parrot, on a hillside lush with dripping foliage, we was the fog-shrouded eyeshine of a large tree boa that was slowly foraging in a tall tree fern. The image of that scene is one of the most evocative in my memory, for it conjures up all that is exciting and romantic about West Indian field work.

In the West Indies, Corallus is found in a variety of habitats ranging from lush mountain rain forests to less mesic regions in heavily disturbed agricultural areas. Indeed, we have found the highest concentrations of tree boas in or adjacent to groves of fruit trees, especially mango, but also cacao, nutmeg, coconut, lime, and banana. Corallus enydris will spend daylight hours coiled in a tight ball at the distal end of a branch, and usually in shade or a sun-shade mosaic; it could be anywhere from seven to seventy-five feet above the ground. They become active at night, and seem to spend the night actively foraging for food at heights from ground level to nearly 100 feet above ground. We have evidence that an ambush, or sit-and-wait foraging mode may also be used. Because of their conspicuous eye-shine, it is possible to track their movements through tree tops and bushes, and they seem to rarely stop moving.

Contrary to previously published information which suggests C. enydris preys almost exclusively on Anolis lizards in the West Indies, we have found that West Indian tree boas frequently take small rodents and occasionally birds in addition to anoles. Lizards are more often taken by smaller tree boas, whereas larger ones take mammals, but even small tree boas (up to 78.5 cm snout- vent length) will prey on rodents. Elsewhere in its wide geographic range, Corallus enydris is known to feed on frogs, small opossums, and bats. Prey is killed by constriction, which suffocates the meal-to-be; it is then swallowed whole, and usually head-first.

We have taken body temperatures of tree boas on St. Vincent and Grenada. This is accomplished by inserting a slender, quick- recording thermometer into the snake's cloaca. Not surprisingly, since tree boas are nocturnal, their body temperatures when active closely approximate the adjacent air temperatures. Body temperatures ranged from 23.9-27.6 degrees C, adjacent air temperature ranged from 22.5-27.1 degrees C. Higher body temperatures (average 27.0 degrees C) were recorded at a lower- elevation locality than at one at a higher elevation (average 24.8); this suggests that tree boas living at higher elevations may be able to accomplish the same things at reduced body temperatures as tree boas at lower, warmer elevations.

The puzzle of color and pattern in Corallus enydris is still unsolved, and will remain so for some time to come. I have expanded the project to include the entire range of the species, and I have many specimens to examine. Nevertheless, some interesting patterns have already emerged from the initial phases of the investigation.

1. Tree boas on St. Vincent are remarkably uniform in color and pattern. There is one basic pattern composed of shades of brown, gray, black, and off-white, usually in a series of irregular white or pale gray-outlined dark gray to black rhomboidal blotches, usually with a paler center, on a pale brown to gray ground color. Juveniles frequently have salmon orange in their patterns, but that fades or is lost completely in adults. The color and pattern stability observed on St. Vincent may be due to "founder effect." A new population on an island may be derived from single individual (a pregnant female) or a limited number of immigrants. The founder or founders represent a very small sample of the genetic pool to which it or they formerly belonged.
2. Tree boas on Grenada exhibit a wide variety of colors and pattern variants. The dorsal ground color may be pale grayish white, gray to almost black, yellow, orange-red, or brown. Generally they may: (a) be of uniform coloration (no pattern, usually yellowish), or with hints of a pattern or with a suffusion of yellowish rhombs on a rose ground color; (b) have a typical St. Vincent type pattern; (c) have large dark brown, black or blue- black paramedian blotches outlined in white or cream on a pale yellow to gray-white ground; or (d) red-orange or brown ground with cream-outlined rhombs that are a shade of brown.
3. On the Grenadines, color and pattern on individual islands appear stable. Islands lying closer to St.Vincent (Bequia) have a St. Vincent-like pattern; those lying closer to Grenada (Petite Martinique) have patterns close to one of the Grenada variations.
4. There are significant differences in certain scale characters (ventrals + subcaudals) between St. Vincent and Grenada tree boas.
5. Corallus enydris on Trinidad and Tobago, lying just off the coast of Venezuela, are invariable uniformly colored pale brown to mustard yellow. Mustard yellow is a common color variant on Grenada. c. enydris occurring near the mouth of the Orinoco River in Venezuela is also frequently patternless mustard yellow (R. Pawley, pers. commun.). It is not unlikely that tree boas occurring along the mouth of the Orinoco were swept with the currents outward and westward on rafts of debris and ultimately landed at and colonized Grenada.
6. Tentatively, and more data are needed to verify this, it appears that on Grenada, the darkest color pattern (black or very dark brown blotches on a white to gray ground color) is most common (and color variability rare) in high, wet forest habitat. Most color diversity occurs in lower, warmer, drier, areas. The dark pattern is present in the Pearls area, but pale gray, yellows and reds predominate.
7. A female C. enydris may produce offspring that are all colored and pattered as she is, or may produce a variety of color variants in a single litter, only a fraction looking like the mother.

Because Corallus enydris has a proclivity for rodents and for fruit orchards, and because they occur at relatively high densities in orchards, I believe that they may be of significant economic importance. Rodents probably cause serious damage to a wide variety of tropical fruits, including some that may be of significance in a local economy. It is likely that some kinds of fruit trees (mango) in some areas will harbor one or more tree boas. It is not known how many mice and rats a single large tree may support, or how much fruit is damaged by each rodent, but the presence of one or more tree boas must have a significant impact on maintaining rodent populations at a more economically advantageous level, than a mango tree lacking a rodent predator.

For this reason, it would be economically beneficial not to destroy tree boas. The usual fate of snakes in the West Indies is to be chopped by a machete or killed with a rock. Since there are no venomous snakes co-occurring with tree boas on any island in the West Indies (this does not hold true in Central and South America, or Trinidad), there is no "practical" reason for killing snakes on these islands. Tree boas reach high densities in disturbed situation (fruit orchards) and, like several other snake species in the West Indies, have probably thrived because of man's destructive encroachment. Through education, locals can be taught that snakes play an economically vital role in their daily lives, and that sparing the lives of snakes is to their advantage

Field work in the Lesser Antilles had been funded by Albert Schwartz and the Friends of the Milwaukee Public Museum. The hospitality and cooperation of personnel in the West Indies cannot be overestimated. On S. Vincent, Calvin Nicholls and his staff in the Department of Forestry, and on Grenada, George Vincent, Manager of National Parks and Wildlife, were extremely helpful. Allan Winstel had provided information on the results of captive breeding, color patterns, and photographs. Richard Sajdak of the Milwaukee County Zoo has been an exuberant, highly competent, and good natured field companion; I hope we go island hopping again. Rose Henderson is a superb hunter of amphibians and reptiles, able to see snakes at night that I would have missed; she's the best collector I've ever worked with. I can't wait to go tree boa hunting with her again.