| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Wild Lives Burmese Vine Snakes
Pencil thin, delicate and about two feet long, a Burmese vine snake could easily pass for a particularly lovely branch. Whether draped across mangroves in their native Myanmar, or dangling over a tank stocked with fish in the Academy’s Steinhart Aquarium, the green and brown serpents nearly disappear into their surroundings. Only their sinuous forms and bulbous eyes give them away. And if you catch one coiled up like a giant spring, tongue taking aim, it’s about to pounce with deadly accuracy. These skinny snakes (Ahaetulla frontacincta) are unusual hunters. Biologists have found plenty of aquatic snakes that eat fish, and tree snakes that eat lizards. But they are still trying to work out how an arboreal snake developed a taste for fish. In May 2001 a team of Academy biologists and curators, including late herpetologist Joe Slowinski, traveled to Myanmar. They returned with 13 Burmese vine snakes—the only living specimens outside Myanmar. Rhonda Lucas, a San Francisco State University graduate student who helped collect the snakes, is studying their evolutionary history, behavior, diet, and physical capabilities. She’s already worked out how they make a living. “They are visually-oriented hunters,” says Rhonda. Their wide-set, raised eyes give them the exceptional depth perception they need to target prey. As they size up the fish below, vine snakes coil into a characteristic S and hover three to four inches above the water. They cling to branches by the tips of their tails, suspending themselves horizontally for almost the entire length of their bodies. Scientists hypothesize that, much like a hunter sighting down the barrel of a rifle, the snakes stretch out their tongues to aim at fish swimming below before attacking. When fish make the fatal mistake of coming too close to the surface, the snakes plunge their heart-shaped heads below the water and snap up their meals in an eyeblink. “Like a spring, the S shape gives them more push,” Lucas says. Waiting until fish stray to the surface means that, unlike aquatic snakes, they do not have to compensate for water’s distortion of distances. “They swallow the fish whole and head first. Most fish-eaters consume head first so as not to go against the dorsal fin,” Lucas says. Although Burmese vine snakes have venom glands, their poison isn’t toxic to humans. In fact, the snakes use their enlarged rear fangs, not their weak venom, to kill prey. The snakes’ hunting habits delight visitors. Each week, the snake tank is restocked with about 40 tiny, bug-eyed, spiny-finned fish called gobies and 20 goldfish. In the wild, Burmese vine snakes hunt during the day. But their green and brown skins blend in so well with their surroundings that the Academy team discovered they could only catch specimens at night. Luckily, Lucas says, “the scales on their bellies reflected our flashlights.” Burmese vine snakes are viviparous. New born snakes are a subtle shade of brown. As they grow into adults, some turn green. Some are even two-toned, with their backs a different color from their bellies. Lucas has noted that females tend to be brown while males are more likely to be green. Snake color was what drew Lucas and Slowinski to Myanmar in the first place. They hoped to study why the Burmese vine snake’s close relatives, the better-known Gunther’s whip snake (Ahaetulla prasina), comes in so many hues. Gray, vibrant orange, and yellow in addition to green and brown, these snakes also live in trees, but dine on lizards and have a distinctively-shaped scale on the upper cheek. “Joe and I had a lot of conversations about their polymorphism. We decided we had to go to their environment to glean patterns and trends,” says Lucas. But after spending a month in Mwe Hauk Village, Lucas became fascinated by the snakes camouflaged in the branches of mangroves growing alongside the salty, tidally-influenced Ayeyarwaddy River. “We would go out in dugout canoes. We noticed the vine snakes hanging above the water. We thought they were looking for lizards, but then why would they hang there?” says Lucas. Lucas says when she first threw out the idea that the snakes were hunting for fish. “Joe teased me, saying ‘Everyone knows vine snakes don’t eat fish.’” When they finally found snakes with stomach contents, their bellies were full of local Peri-ophthalmus goby fish. Gobies often jump out of the water to sit on the mudflats beneath the mangroves. So the researchers assumed the snakes were catching the gobies as they rested on land, much the way Gunther’s whip snakes catch lizards. The truth turned out to be far more interesting. As they honed their skills at analyzing snake stomach contents, the researchers started to find the remains of rice fish too. Unlike gobies, rice fish always stay in the water, suggesting that the vine snakes were catching their meals directly from the river. Lucas and Slowinski were the first to document the Burmese vine snake’s curious eating habits. Like much of Myanmar’s wildlife, vine snakes are extremely poorly known. Although Myanmar borders Thailand, China, Laos, Bangladesh and India, the country has become a scientific and political island. Years of war and political instability shut the Texas-sized nation’s borders. Little natural history research has been carried out there since the British left in 1937. So little is known about the fish-eating snake that scientists aren’t sure whether it’s rare or not. “I hypothesize that they need to be protected because they are not widely distributed,” Lucas says. She will assume the brackish Ayeyarwaddy is the snake’s only home until she sees evidence to the contrary. Lucas is now delving into the snake’s evolutionary background. By combining natural history observations with genetic analyses of their tissue, Lucas hopes to determine the Burmese vine snake’s position on the snake family tree. Her goals are to learn why the snakes became fish-eaters, and whether their fish-eating habits evolved before or after they came to live on the banks of the Ayeyarwaddy. Today, the slender pescatarians are pulling in crowds to Steinhart Aquarium’s swamp. Visitors can catch snakes in the act of swooping in on their fishy prey, and spot the fish-shaped bulges in their gullets. But until Lucas completes her studies, the snakes will remain an evolutionary mystery waiting to unfold. Celeste Biever is a freelance science writer based in Boston. |
