Chapters 12 Life in the Trees


12. Life in the Trees

Shine your light over the vegetation, concentrating on the tips of the branches. Look for the reflected orange-red glow of the eyeshine of Corallus grenadensis…When you find a treeboa, and you will, watch it for a few minutes…Take some time to appreciate where you are, what you’re doing, and especially Corallus grenadensis.
Robert W. Henderson, 2002

                Treeboas are exactly what the name suggests; everything about them is arboreal, from their compressed, muscular body to the way they coil around a branch as if they were part of the tree. A variety of colors and patterns allows them to blend with leaves and bark, affirming that they belong in the canopy. But they do come to ground, and the first one I found was crossing the Arima Valley Road in Trinidad. The Caribbean Coastal Treeboa (Corallus rushenbergerii) is a large species and the one stretched out on the Arima Valley Road was not record size but well over 1.5 meters in length. When I grabbed it, coils of the body constricted my arm and a few of its exceptionally long, sharp teeth found their way into my fingers. Years later, I was assisting Bob Henderson locate Grenada Treeboas [Figure 12–1], a smaller, more even tempered species. After searching for them unsuccessfully in dense, overgrown, secondary forest, we stopped by a large hotel and walked the perimeter of the parking lot. Searching the crowns of the ornamental trees planted on the hotel grounds revealed eyeshine. I came to appreciate Corallus grenadensis because you can actually see them in their habitat, unlike those mud-dwelling homalopsids I study. Six species of treeboas (Corallus) are endemic to Neotropical forests, and they have been investigated in great detail by Robert Henderson at the Milwaukee Public Museum.


INSERT FIGURE 12-1.
Figure 12–1. The Grenadian Treeboa, (Corallus grenadensis).

                Examination of the variety of arboreal, aquatic, and ground-dwelling snakes provides the distinct impression that arboreal species show more color morphs than snakes living in other habitats. On Grenada, treeboas show the greatest variation of color and pattern found anywhere in the range of the species complex. Henderson found both color and pattern correlated to altitude, rainfall, and sunshine. Dark colored morphs were most abundant above 400 m in elevation while yellow morphs were most common at low altitudes. This led him to propose a thermoregulatory function for the various color morphs, suggesting that the darker snakes at high elevations could absorb more heat than their lighter colored relatives at low elevations.
                Of interest is another group of snakes that are remarkably convergent with the treeboas, the tree pythons of Australasia. Place an Emerald Treeboa (Corallus canius) next to a Green Tree Python (Morelia viridis) and, at first glance most people will assume they are the same species [Figure 12–2]. However they come from unique evolutionary lines which becomes evident with minimal examination; one is live-bearing and the other is an egg-layer, they live on opposite sides of the planet, and they have not shared an ancestor for about 83 MY.

INSERT FIGURE 12-2.
Figure 12–2. A: The South American Emerald Treeboa, (Corallus canius). B: The Australasian Green Tree Python (Morelia viridis).

                Lesley Rawlings and Stephen Donnellan used mtDNA and allozyme data to test the differences between populations of the Green Tree Python. Their results support the hypothesis that northern and southern lineages were divided by New Guinea’s central mountain range and strongly suggest M. viridis is composed of two clades, one of which has yet to be named.
                The southern lineage of Green Tree Pythons is present in northern Queensland, where its life history was studied by David Wilson and co-workers using mark-recapture techniques. The authors estimated males reach sexual maturity in 2.4 years, and females in 3.6 years, with a lifespan of about 15 years. Breeding is highly seasonal. Hatchlings appear in late November only, and, since few hatchlings were found in any one year, the authors suggest adults may not breed annually. Hatchling M. viridis are bright yellow or red and they transition to green between 53 and 59 cm, or at about one year old. Wilson and colleagues reported the color change occurs rapidly and is not associated with shedding. They focused on the yellow color morphs and propose two hypotheses to explain its presence. First, the hatchlings may mimic an unpalatable model and, as they grow, they become less susceptible to predation. Second, hatchlings and sub-adults may be using different habitats and the change is required to remain cryptic. In a follow-up paper, Wilson and colleagues demonstrated the color change was likely the result of the shift in habitat. Hatchling pythons used forest-edge habitat as opposed to the adults that inhabited deep forest. The reflectance of young and adults was measured and the disparity for each color against the background calculated. Green was less conspicuous than yellow in the canopy, and yellow and red were less conspicuous against non-leafy backgrounds. Therefore, the juveniles are less likely to be seen by predatory birds or mammals. The color change corresponds to alterations in foraging and habitat use. Small tree pythons foraged for reptiles and insects on the ground in edge habitats during the day. Older snakes hunted birds and mammals in the canopy.
                In a comparison of arboreal boas and pythons to species that live on the ground and in the water, Liga Pizzatto and co-workers found arboreal boas and pythons produce smaller numbers of embryos than their terrestrial relatives, females of arboreal species have little overlap between their right and left ovaries, and arboreal species have lateral compression at mid body with relatively longer tails. Pizzatto and co-workers suggested the non-overlapping ovaries reduce the degree of distention during egg formation and pregnancy to allow the snake to retain climbing efficiency and camouflage. These are basic anatomical adjustments for life in the trees and also occur in arboreal snakes in other lineages.
Arboreal snakes have their heart located closer to the head than snakes that live on the ground or in the water. Roger Seymour and Joachim Arndt compared blood pressure changes in the highly aquatic file snake and a terrestrial python by tilting parts of the snake as well as the whole snake. Blood pressure changes during partial tilts added up to equal-whole-body tilts, and they found the vertical distance to the head had twice the influence on head blood pressure than did blood pooling in the pythons and file snakes. Seymour and Arndt demonstrated that the vertical blood column above the heart is the most important factor in determining head blood pressure in both species. They note that aquatic species are protected from the effects of gravity when they are in water, and, as a result, their heart is positioned near the center of their bodies. Arboreal species face the problem of maintaining blood pressure in their head while they climb. Overcoming the pull of gravity has caused snakes to adapt their elongated bodies in several ways. Harvey Lillywhite described a posterior to anterior shunt that carries blood from the right aortic arch to the dorsal aorta when the snake is in a head-up climbing position. The shunt vessels are richly innervated by nerves and exceptionally small in diameter when compared with terrestrial or aquatic species. Arboreal snakes also have a stereotypical body wiggle when climbing head-up. The undulations start at the head and are transferred down the body with the effect of returning blood from the veins to the heart. The long tail found in arboreal snakes is also involved in preventing blood from pooling at the rear of the snake as it climbs. Exactly how this works is uncertain, but the muscles in the tail probably act on veins to force blood toward the head.
                Arboreal snakes have evolved visually stimulated defense behaviors that are likely to be memorable for a potential predator, usually medium-sized mammals or birds. Cryptic color patterns for arboreal life may work well when they are in a tree or low vegetation. If discovered by a predator, however, arboreal snakes will go to extreme lengths to escape. There are snakes that move easily between trees and the ground, foraging for food in both environments. One of these is the widespread Neotropical Parrot Snake (Leptophis ahaetulla), a rear-fanged snake that hunts frogs and lizards on the ground or in fallen vegetation during the day. Encounters with parrot snakes are always exciting because of their vigorous and spectacular defense displays. When disturbed on the ground, the snake moves into a tree to escape. If followed it confronts the pursuer with a wide-open mouth, multiple strikes, and hissing from an expanded tracheal opening. Interestingly, other unrelated snake species use similar displays. The Neotropical Bird Snake (Pseustes poecilonotus) expands its neck and contorts its mouth in an energetic display. The Tiger Ratsnake (Spilotes pullatus) of the Neotropics has a modified trachea that produces a loud noise while its forebody is reared and expanded to threaten a predator. In Asia, the Red-tailed Ratsnake (Gonyosoma oxycephalum) has a similar visual display of neck flattening, but lacks the volume of some other species. Also in Asia, the treesnakes in the genus Boiga, the rat snakes in the genus Ptyas, the bronzeback treesnakes in the genus Dendrelaphis all have head raised, S-coiled forebody, mouth-open, neck-expanded displays with variations among species. And, of course, Africa has its own species which use similar displays, including the rear-fanged Thelotornis.
                The Forest Vine Snake (Thelotornis kirtlandi) has a spectacular defensive display that was recorded by John and Jenne Goodman. They describe the vine snake as motionless, holding its body in a cryptic posture resembling a branch. Its forebody extends into the air and the only detectable movement is the tongue. Sunbirds and other small species will often discover the snake and mob it, flying in front of it and around it within striking distance. The birds taunt the snake, but eventually the birds give up. If, during the mobbing, a bird makes body contact with the snake, the snake’s behavior changes radically. The Goodmans filmed one incident that lasted 45 minutes in which the snake would inflate and deflate its forebody to reveal a contrasting pattern, taking on the appearance of a begging, fledgling bird. They wrote,

The deception was so perfect that we believe the birds became confused and a female sunbird was almost immediately grasped…immediately before capture this bird was hovering in the air and swinging its body back and forth from side to side in a very peculiar fashion directly in front of the snake.

By mimicking a begging, fledgling bird, the vine snake switched the mobbing bird’s behavior from aggression to parental care, and it made the bird more vulnerable to predation.
                North American ratsnakes in the genus Pantherophis are relatively well known; the Corn Snake (P. guttatus) may be the snake most often bred in captivity; but much remains to be learned about their behavior and ecology in the field. Steve Mullin and Robert Cooper found that the Gray Ratsnake (P. spiloides) was visually responsive to flying model birds in a captive experiment. Mullen and Cooper’s observations suggested that ratsnakes may learn the location of bird nests by watching the behavior of parental birds. Ratsnakes are well adapted for climbing, and they have their ventral scales bent at a 90º angle on each side so the edges of the scale can be used to gain purchase on tree bark. By wedging the edge of the scales into crevices on the bark’s surface, a ratsnake can easily ascend a tree. I kept a particularly large Gray Ratsnake for many years that was a true escape artist. Out of its cage, she would find a corner in the room and crawl straight up the wall by wedging herself into the corner and grabbing the wall on each side with the edges of the ventral scales.
                In eastern Texas, Josh Price and colleagues located seven Pantherophis obsoletus 363 times using radio tracking. Snakes selected hardwood trees and snags that contained cavities 95% of the time. The snakes were in the trees after the peak bird-nesting season, and, therefore, not foraging for nestling birds. Instead, they may have been using the arboreal sites in search of nesting rodents, basking sites, or resting sites secure from predators. In an earlier study, Patrick Weatherhead and colleagues examined the prey taken by the Gray Ratsnake and found them to be opportunistic predators, preying primarily on mammals and taking birds when they found them.
                Not all ratsnakes are climbers; the fox snakes (P. vulpinus, P. gloydi) live in grassland burrows as does the Great Plains Ratsnake (P. emoryi). Despite their occasional subterranean habits, they may also climb and one was found in a tree preying on a Golden-cheeked Warbler. Jinelle Sperry and Christopher Taylor radio tracked the Great Plains Ratsnake at Fort Hood, Texas and found the population had an affinity for using human-made structures like rock gully plugs, human constructed bundles of rock used for erosion control in road construction. They also preferred to be near patches of wooded vegetation, specifically in edge habitat. The open characteristics of edge habitats may allow the snakes to more effectively regulate body temperature.
                Before changing the topic, I want to continue a theme started earlier. It is of interest that hatchling rat snakes often have a distinctly different pattern than the adults. This variation has been exploited by reptile hobbyists who breed these snakes for color and patterns. However this polymorphism, regardless of whether it is ontogenetic or occurs throughout the life of the snake, plays an important role in survival.
                There are ratsnakes in Asia and Europe and for many years most of them were placed in the same genus as the North American species, Elaphe. The Lampropeltini is now the New World ratsnake clade, a group of snakes that also includes the kingsnakes, gopher snakes, and other relatives (Arizona, Bogertophis, Cemophora, Lampropeltis, Pantherophis, Pituophis, Pseudoelaphe, Rhinocheilus, Senticolis, and Stilosoma).  A few of these snakes are strictly ground-dwelling, but most will climb into low vegetation or, in some cases, the forest canopy in search of food or basking sites. New World rat snakes had an Old World ancestor and the question of how and when they arrived in the New World, is a question Frank Burbrink and Robin Lawson investigated using sequences of mtDNA and cDNA. Burbrink and Lawson recovered the smooth snakes in the genus Coronella as the sister to the Lampropeltini. Three species of living smooth snakes range from the UK, across Europe to Western Asia, and east to India. The living species tend to use dry habitats with sparse vegetation, and feed on other snakes, lizards, and small mammals. The mammals are usually killed by constriction.
                Burbrink and Lawson found evidence suggesting that the ancestor of the Lampropeltini arose in the Eastern or Western Palearctic. The authors estimate the date of Coronella-Lampropeltini divergence at 28.0–26.5 MYA, and a date for the most recent common ancestor of the Lampropeltini as 25.9–24.9 MYA. They proposed the Coronella-like ancestor dispersed to the New World via the trans-Beringia land bridge (Asia to Alaska) because the De Geer Land Bridge (Europe-Greenland-North America) was too cold, a chain of islands, or possibly inaccessible because it was covered by cold ocean. On the other side of North America, Beringia was covered with coniferous forest or mixed deciduous forests 25.9–24.9 MYA. They also note that modern ratsnakes included two cold-tolerant species, the black ratsnake species complex (Pantherophis obsoletus) and the milk snake complex (Lampropeltis triangulum). Their scenario may very well be correct. However, in the last chapter, the submerged hibernation abilities of garter snakes was discussed, and it is known that the Western Fox Snake (P. vulpinus) [Figure 12–3] can also spend the winter submerged. A snake capable of slowing its metabolism and heart rate, absorbing oxygen through its skin, not having to fill its lungs with air for five months, may have been able to disperse across either of the high-latitude land bridges, even if it was cold and inhospitable to snakes for more than half the year.

INSERT FIGURE 12-3.
Figure 12–3. Two cold adapted North American rat snakes of the Lampropeltini clade. A: The Western Fox Snake, (Pantherophis vulpinus), a prairie species, that shows little interest in climbing. B: The Eastern Ratsnake (Pantherophis alleghaniensis) a forest dwelling species, well known for foraging in trees.

Life in the Vines
                The Tabonuco Forest has a high canopy, a rich diversity of trees, and little light reaches the ground. Twenty-four Puerto Rican Boas (Epicrates inornatus) were radio tracked in this low-light forest by Joseph Wunderle and colleagues between 1996 and 2001. Males moved more than females and had an average home range of 8.5 hectares, and females had an average home range of 5.6 hectares. The snakes were located in the canopy, on the ground, and in the ground, with gravid females spending more time below ground than non-gravid females or males. Additionally, females generally spent more time on the ground than males. The trees used by these snakes were large with many lianas (woody vines). These larger trees were presumably preferential in that they provided larger perches for bigger snakes, were more likely to have cavities than smaller trees, and had large crowns contacting the crowns of other trees, allowing the snakes to move from tree to tree without going to ground. The vines were also important in allowing the snakes access to the trees, and provided cover for foraging and resting. During the study, Hurricane George struck the island in 1999 and significantly damaged the forest. The damage opened the vegetation enough that it increased the movement of snakes, making them more visible, more subject to mobbing behavior by birds, and more subject to predation. The Puerto Rican Boa depends upon trees and the habitat they provide, but there are snakes even more specialized for life in the trees.
                One group of arboreal snakes share a peculiarly similar morphology: a slender build, an elongated head, elongated vertebrae, large eyes with binocular vision, a long tail, and day-active behavior. Virtually all these snakes feed on lizards via an ambush strategy, and they often extend their tongue with the two tips held together for extended periods of time [Figure 12–4a]. Collectively, they are called vine snakes and they are scattered around the tropical world. In the Caribbean, there is Uromacer; in Central and South America, there are Oxybelis and Xenoxybelis; in Southeast Asia there is Ahaetulla; and in Africa there is Thelotornis. The Southeast Asian flying snakes in the genus Chrysopelea also approach this body form. The slender build and elongated vertebrae allow them to bridge gaps between branches of small diameter; binocular vision gives them depth perception for striking prey; and they are able to use exceptionally small diameter branches because of their low body weight to length mass. Several hypotheses have been proposed to explain the extension of the tongue, including fascinating prey, luring prey, disrupting the outline of the head, and maintaining olfactory contact with the prey. Robert Henderson and Mary Binder reviewed these hypotheses and observed Oxybelis stalking prey with its tongue continuously extended for 17 minutes. They proposed the tongue acts as an extension of the eye line, and that the snake may use it as a reference point during hunting.
                Walking along a road with a closed canopy in Belize, one of my companions spotted the coils of a bright green snake overlapping a branch above our heads. The snake was about 5 m above the ground and catching it seemed unlikely.  In the time it would take me to reach it, the snake could easily move away through the branches from one tree to the next or ascend higher into the canopy well out of reach, but I had to try. Using a dead branch, I prodded the snake and encouraged it to crawl onto the branch in my hand but, it was not cooperative and moved off into the vegetation. The Green Vine Snake (Oxybelis fulgidus) is quite spectacular, given its bright color and large size. Rodrigo R. Scartozzoni and colleagues from the Instituto Butantan in São Paulo examined 106 specimens of the Green Vine Snake from northern Brazil and found females matured at 0.9 m, were larger than males, had larger heads than males, and produced eggs throughout the dry season (April to December). Males matured at 0.71 m, and had a larger testicular volume from February through July, suggesting the sexes had a synchronized reproductive pattern. Also of interest was O. fulgidus’ diet. Iguanian lizards made up most of the snake’s prey, but the species will also take nestling birds, suggesting the snake will do more than hunt from ambush and does, in fact, actively forage.
                If one group of snakes can claim to be the expert at bridging gaps and moving from one twig to another, they are the blunt-headed treesnakes (Imantodes). Blunt-heads are often not considered vine snakes because of their short head and nocturnal habits. Their body weight to length ratio, however, is probably the lowest of any snake and they have exceptionally long vertebrae with a shelf running between each so that they can be locked. The scale above each vertebra is also enlarged, giving the body added rigidity as it extends off the branch into the air. Gustavo Aveiro-Lins and colleagues looked at the position of the internal organs of the Blunt-headed Treesnake (I. cenchoa) and found that they are shifted toward the posterior end of the body, causing the snake’s center of gravity to be at about 74% of the body length in both sexes. There are six species of blunt-heads in the Neotropics and, while poorly studied, they clearly represent one of the snake lineages most highly adapted for arboreal life.
                Eight species of Asian vine snakes are currently recognized in the genus Ahaetulla, a group distributed from India to the Philippines and throughout the Indonesian archipelago. Long narrow heads, eyes with oddly shaped horizontal pupils, a fovea, and facial grooves enhance the snake’s binocular vision and strongly suggest that these snakes have excellent eyesight. Lizards make up the diet of most species, but a few species have shifted their diet to fish, and hunt from branches overhanging the water. The Burmese Vine Snake (Ahaetulla fronticincta) hunts gobies from the branches of mangroves in river deltas. San Francisco’s Steinhardt Aquarium installed an exhibit with this species and gobies so that the public could watch this unusual predatory behavior. But there is another Asian vine snake that is also a fisher. The Oriental Whipsnake (Ahaetulla prasina) [Figure 12–4b] is a widely distributed species complex that has yet to be sorted out. They come in a variety of colors ranging from bright green to grey, as well as bright red and orange. Interestingly, all of these color morphs may be present in a single litter. A snake in this complex from Java has also been reported to fish from perches. Gernot Vogel maintained captive snakes for five years that ate nothing but fish, and he proposed that the movement of the fish attracts the snake. Members of the A. prasina complex are well known as visual lizard hunters, but this one has switched its diet to aquatic prey. What adds considerable interest to this story is the fish probably taken most often by these snakes in mangroves are gobies, fish known to climb out of the water onto the branches of mangroves, similar to lizards climbing on branches. It is not difficult to imagine a scenario for how these arboreal snakes evolved to feed on fish.

INSERT FIGURE 12-4.
Figure 12–4. Two vine snakes in different families. A.  The Brown Vine Snake, a dipsidids, (Oxybelis aeneus) from the Neotropics. B: The Asian Vine Snake, a colubrid, (Ahaetulla prasinus). These two snakes are in different lineages but look similar because they have similar life styles. Both are diurnal, arboreal, visually oriented, lizard-eating snakes.

                The Brown Vine Snake (Oxybelis aeneus) is a Neotropical species widespread throughout Central and South America. There is a single report of it catching fish. Thomas Hetherington observed it with a fish in Cocle Province, Panama. Upon collecting it, he found the snake was not wet, suggesting the snake may have taken the fish from a branch. The fish was identified as a killifish in the genus Rivulus (Poeciliidae). This is a species that could not have been climbing around on branches near the water. However, they do sometimes flip themselves out of shallow pools in an attempt to get to another body of water, so the snake may have found it in the leaf litter.
                There is yet another arboreal snake known to feed on fish, but it does not fit the strict description of a vine snake. It is slender and green and it has an unusual cone-like, scale-covered projection coming off its snout. The Rhino Ratsnake (Rhynchophis boulengeri) inhabits stream and lakeside environments in northern Vietnam and southern China. This species is now being bred in captivity and is reported to use the rostral cone to lure fish by dipping it into the water [Figure 12–5].

INSERT FIGURE 12-5.
Figure 12–5. The Rhino Ratsnake (Rhynchophis boulengeri), an arboreal colubrid from China and Vietnam that eats fish.

Parachuting and Gliding Snakes
                Lying on my back looking skyward at the edge of the Physical Science building on Prince of Songkhla University’s campus in HatYai, Thailand, I was waiting for Jake Socha to chase an Ornate Treesnake (Chrysopelea ornata) off a branch three stories above me. My camera was ready, and a few students were standing around waiting to recapture the snake after its flight. The outline of the snake was difficult to distinguish from the branch protruding from the roof.  The snake was soon airborne and I managed expose a frame of film before the snake landed several meters from me [Figure 12–6]. Ornate tree snakes are relatively common in Southeast Asia, and they seem to do well in agricultural habitats that have trees. They also like to parachute.

INSERT FIGURE 12-6.
Figure 12–6. The Ornate Treesnake (Chrysopelea ornata) in flight. Southern Thailand.

Despite C. ornata’s reputation as a glider, Socha and Michael LaBarbera found Ornate Treesnakes to be relatively clumsy gliders when compared to the Paradise Treesnake (Chrysopelea paradisi) [Figure 12–7]. They examined the effect of size and behavior on the snake’s aerial performance and found smaller Paradise Treesnakes could glide farther than larger specimens and had lower sinking speeds. Because it has a lighter build, a less rounded venter, and possibly a different body posture during flight, the Paradise Treesnake was better at gliding. These researchers considered C. ornata a parachuter and C. paradisi a glider. A 1970 study reported launching a C. ornata from a height of 41 meters and having it travel 30 meters; Socha and LaBarbera predicted that had they launched a paradisi from the 41 meter height that it would have traveled 142 meters. The Paradise Treesnake has been reported to flatten its body while basking and if it does this while gliding, the posture may have a significant impact on the distance it glides.

INSERT FIGURE 12-7.
Figure 12–7. The Paradise Treesnake (Chrysopelea paradisi). Photo by Gernot Vogel.

                Bridging-gaps, getting from one branch to another, is a skill some snakes have mastered with modifications of muscles and bones. Bruce Jayne and Michael Riley examined the gap-bridging ability of the Brown Treesnake (Boiga irregularis). They experimented with snakes in the lab, videotaping their approach and the extension of their body to cross gaps. Jayne and Riley found that smaller snakes could span a greater percentage of their body length (64%) than could larger snakes. In most instances, the snakes simply crawled up to the gap and extended the head and body without stopping, kept the body rigid, and spanned the distance. But, 15% of the snakes tested did something different. These snakes held their neck at a 45º incline before rapidly lunging toward the perch on the other side of the gap; the lunging was reminiscent of a strike. Using this behavior, the snakes were able to cross wider gaps. When crossing a gap the snake has to cantilever its body so that it does not buckle from the torque of the suspended weight. Jayne and Riley suggested gap-bridging in the Brown Treesnakes may be at the limits of their ability to strain their muscles to prevent buckling. Additionally, they proposed an interesting hypothesis regarding the evolution of gliding, suggesting that striking may have been a precursor to the lunging behavior, and the lunging may be a precursor to launching off a perch for a flight. They used the analogy of human rock climbers performing dynamic moves to reach hand holds they could not otherwise attain.
                But why should a snake take to the air? Escaping predators is an obvious possibility, though, in doing so the risk of injury is high. Another possibility is it facilitates movement through a discontinuous canopy without the need to return to ground. Gliding and parachuting vertebrates occur in forests around the world, but the forests of Southeast Asia have the greatest number and greatest diversity of gliding species. Borneo is particularly rich in gliding tetrapods. Many squirrels, frogs, and squamates have evolved increased surface area to assist their flight and slow their descent. Louise Emmons and Allan Gentry suggested the high number of gliders, including the flying snakes, evolved in Southeast Asia because the forests have fewer lianas than other tropical forests. Lianas act to connect trees and form a structural network that can be used by small vertebrates to move around without going to the forest floor. The liana hypothesis was questioned by Robert Dudley and Phil DeVries. They suggested Southeast Asian forest trees are taller (some reaching 70 meters) than Neotropical and African forest trees, and that the increased height of the canopy with many emergent trees was the selection factor for more gliding Southeast Asia species. In a more recent paper, Dudley and colleagues reviewed gliding and parachuting behavior and suggest it may have resulted from intense predator pressure in the canopy.
                In support of the idea that gliding is a response to canopy predators, Jake Socha and Christian Sidor observed the Ornate Treesnake visually tracking airplanes on two occasions; on another occasion, a Paradise Treesnake visually tracked a bird. They proposed that these two species of Chrysopelea probably have excellent vision and possess a fovea, a structure known to occur in the Asian vine snakes (Ahaetulla) and a close relative of Chrysopelea. The fovea is the point of greatest visual acuity, most snakes don’t have this structure.

Arboreal Vipers
                The Neotropics have numerous species of arboreal pitvipers. Perhaps the best known of these is Schlegel’s Eye-lash Viper (Bothriechis schlegelii), a highly polymorphic snake that feeds on small vertebrates from arboreal perches and is endemic to Central America [Figure 12–8a]. Geoffrey Sorrell studied this species’ diet and movement in Panama and found that it is most likely to move at night, from one ambush site to another. While this snake was actively foraging on two occasions at night, literature records and stomach contents collected by Sorrell suggested it feeds both day and night. During the day it preys on anoles and at night it eats frogs, geckos, and small mammals. About nine species of Bothriechis are known, and all of them are thought to be arboreal and most likely found in low vegetation. Color polymorphisms of neonates and adults are striking in these species, and they may be highly cryptic with complex patterns or they may be uniform yellows, blues, or greens.
The six species of toad-headed pitvipers (Bothriopsis) are also arboreal. The 37 species of lanceheads (Bothrops) are mostly ground-dwelling, but also show tendencies to climb. The Golden Lancehead (B. insularis) is well known for its arboreal habits (see Chapter 10). I once released a Terciopelo (Bothrops asper) at the site at which it was caught the night before. The snake moved directly to a small tree and climbed into the crown.
                Africa also has arboreal vipers; there are about 11 species of bush vipers in the genus Atheris [Figure 12–8B]. Overall, these are small, stout-bodied snakes with prehensile tails. They feed on frogs, lizards and rodents, and many are high altitude, hill-forest species. When disturbed, their defense display often includes rubbing their heavily keeled scales together to produce a hissing sound and threatening with an open mouth. Again, these snakes often exhibit color and pattern polymorphism as juveniles and sometimes it is retained into adulthood. While the adults are often cryptically colored others are yellow, blue, orange, or red.

INSERT FIGURE 12-8.
Figure 12–8. Two arboreal vipers. A: The Schlegel’s Eye-lash Viper (Bothriechis schlegelii) from Costa Rica. B: The Western Bush Viper (Atheris chlorechis) from West African rainforests.

                In Asia, there is a group of mostly green, arboreal pitvipers that were placed in the genus Trimeresurus. This genus has been recently divided into six genera (Cryptelytrops, Parias, Popeia, Peltopelor, Trimeresurus, and Viridovipera) that collectively contain about 40 species. The division of the genus Trimeresurus has been controversial and the new names are not accepted by everyone. Interestingly, these pitvipers show relatively little color polymorphism compared to the other arboreal snake clades discussed, the exception being the sexual dimorphism discussed in Chapter 9.
                So why are tropical arboreal snakes―, regardless of whether they are boas, pythons, vine snakes, ratsnakes, or vipers―, so often polymorphic? One theory suggests that color variation reduces predation by confusing the predator. Arboreal snakes are generally thought to experience heavier predation than species living on the ground or in the water.  If you are living in a tree, a predator can come from virtually any direction. Alan Bond has summarized the arguments for color and pattern polymorphism. Predators that depend on vision search for prey using a mental visual search image. By presenting the predator with two or more prey types, the predator is likely to focus on the one that is most abundant. Snakes that are not eaten survive and reproduce, and their young are also likely to be polymorphic. In some environments, more visible morphs are more easily located than others and they are removed from the population before they are able to reproduce.
                Between 1830 and 1835, the Audubon-Waterton controversy raged regarding the ability or willingness of rattlesnakes to climb. It began with John James Audubon’s plate 21, which illustrated a timber rattlesnake coiled in a tree attempting to feed on a mockingbird. The snake’s mouth is open and the fangs are shown as abnormally re-curved. Charles Waterton, British naturalist and traveler, wrote an article criticizing the painting and also Audubon’s description of rattlesnakes that included a comment about a rattlesnake swallowing a squirrel tail-first. Defenders and critics continued to comment on this controversy well into the 20th century. Lawrence Klauber has discussed this controversy extensively, and confirms that, yes, rattlesnakes do climb. However, Audubon may well have confused the rattlesnake with a ratsnake which is much more likely to feed on nestling birds or eggs.
                Herpetology journals are now full of anecdotal observations of Timer Rattlesnakes in vegetation, but perhaps the most detailed observations were made by Brad Coupe in Vinton County, Ohio. Coupe radio tracked 19 snakes and observed individual snakes between 0.5 and 5 m above the ground. In the summer of 1995, he located one female a total of 42 times between 5 June and 10 August. Twenty-five of these times (59.5%), the female was off the ground and, on one occasion, she was being courted by a male. It appears arboreal behavior is exhibited more often by individuals in the Vinton County population than in other populations studied to date. Like other characteristic behavior, even arboreal behavior can be polymorphic.

Arboreal Elapids
                This book opened with a fictionalized story about a mamba, probably the most feared and maligned snake on the African continent, if not in the world. Contrary to popular folklore, these snakes do not chase people on foot or on horseback. Four species of the genus Dendroaspis inhabit sub-Saharan Africa, and three of the species are arboreal. Of them, the Black Mamba (D. polylepis) is predominantly terrestrial, considered a savanna species, it may exceed 3 m in length. Young Black Mambas climb, but they become more terrestrial as adults. While these snakes are not aggressive, they are fast, agile, highly venomous and, at some locations, relatively abundant. Black Mambas can also be nervous when confronted, and they do not hesitate to defend themselves when cornered. The Black Mamba’s defense behavior is relatively well known. Usually it will gape its black-lined mouth, flatten its neck, and, of course, try to escape. However, Thomas Madsen and Beata Ujvari had one curl up on the ground put its head under a coil and raise its tail 20 to 30 cm in the air. It did this after it had been used for a photography session and was probably exhausted. The three arboreal species of mambas are green, all exceed 2 meters in length, and they feed upon birds and mammals.
The Common Green Mamba (Dendroaspis angusticeps) is deserving of its name because it is both green and, at some locations, quite abundant. Over a seven year period about 1,000 were collected in a 300 square mile area by C. J. P. Ionides. Population estimates of 200–300 per square kilometer have been suggested for coastal Kenya and southeast Tanzanian where the species inhabits agroecosystems and urbanized environments. Ionides reported finding them in mango and cashew trees, but, at other times, more commonly in secondary growth thickets. Most were collected between 0830 and mid day, reinforcing the idea that these are diurnal snakes that sleep at night coiled on branches. Jameson’s Mamba (D. jamesoni) is also green and distributed to the west of D. angusticeps. Godfrey Akani and colleagues found Jameson’s Mamba to be one of 11 snake species to use oil palm plantations. This mamba composed about 9.5% of the total snakes collected. The third species, the West African Green Mamba (D. viridis), is poorly known, but reportedly can be relatively common in some areas and appears to have some ability to survive urbanization.
                There are two species of tree cobras in the genus Pseudohaje that are endemic to the forests of West Africa. Godfrey C. Akani and colleagues collected data on the poorly known Goldie’s Tree Cobra (Pseudohaje goldii) and found it most common in primary forest, but found they also used plantations and suburban areas. Interestingly, 70% of their specimens were found within 25 m of water, and the stomach contents revealed it fed mostly on frogs and toads, though the authors also found fish. It is tempting to speculate that this arboreal elapid is eating aquatic prey from branches like some of the vine snakes.
                Australia has few truly arboreal elapid snakes, although several will climb trees. I once found a Golden Crowned Snake (Cacophis squamulosus) that had crawled out from under a piece of loose bark on a tree trunk during a nighttime rainstorm. It was probably searching for skinks sleeping on nearby vegetation and was about 2 meters above the ground. The three species broad-headed snakes (Hoplocephalus) are usually considered good climbers, but Stephen’s Banded Snake (H. stephensii) is the most arboreal using tree holes, loose bark, and rock crevice as refuges.
                Arboreal adaptations in snakes are found in many lineages, including clades that are mostly terrestrial, burrowing, or aquatic, reinforcing the point that snakes are indeed diverse. Tree-dwelling snakes are likely to be encountered by humans because they are in our line of sight. Given the ability of our brains to detect the serpent form, tree-dwelling snakes may be most disturbing to people because they are not crawling down a hole or swimming off into muddy water. They are above our heads or at face level where ancient arboreal primates would have encountered them.




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