Most flying or gliding animals rely on a symmetrical pair of wing-like structures to generate lift, the force that opposes gravity and keeps the flier aloft. But `flying' tree snakes have no wings or any other obvious structures that might help them fly, explains Jake Socha, a biologist at Argonne National Laboratory, Illinois. Little was known about these enigmatic creatures until Socha, then a graduate student at the University of Chicago, set out for Singapore in search of the elusive paradise tree snake (p. 1817 and p. 1835). Aside from a desire to see up close the marvel of a flying snake in action, he was compelled by the question: How do they do it?
To answer this, Socha needed to describe the kinematics of snake flight. Luckily, with the help of local volunteers, catching paradise tree snakes wasn't too hard. But capturing their flights on videotape `was a nontrivial process,' says Socha. He coaxed the stubborn snakes to jump from 10 m high towers set up in the Singapore Zoological Gardens. Tony O'Dempsey, an expert on photogrammetric techniques, helped Socha extract 3-D flight information from the synchronized recordings of two video cameras. `Finding Tony was a stroke of luck', says Socha; the 3-D information was crucial in reconstructing the flying snakes' aerial trajectories, speed and body postures.
Socha found that paradise tree snakes are true gliders, and pretty good ones at that. Rather than simply parachuting to the ground, these cylindrical-bodied snakes somehow generate enough lift to carry them across a substantial horizontal distance. Socha suspected that the unusually dynamic flight behavior of the snakes might be responsible; while flying, the snakes create distinctive S-shaped aerial waves that travel from head to tail. To test this, he correlated the snakes' wave amplitude and frequency with flight performance variables. He found that snakes with the greatest combination of body length and wave amplitude (relative to body size) travelled fastest; but the largest snakes were not necessarily the ones making the biggest waves. While wave amplitude is important, Socha found that the frequency of these waves appears to play little or no role in producing aerodynamic forces during flight. So why do snakes undulate so frequently? Socha suggests that it may provide stability, keeping the snakes from spinning out of control.
Given that body size influences flight performance in other animals, Socha correlated snake body size with flight parameters and found that smaller snakes are generally better gliders. He also found that the paradise tree snake is a better glider than its larger, stockier cousin, the golden tree snake. But Socha noticed that paradise tree snakes tend to flatten their body more during flight, suggesting that body shape may also be a key factor in snake flight aerodynamics.
Socha has only begun to unravel the mysteries of snake flight. He is now working on physical and computational models to test more specifically how a snake's shape, size, and behavior influence flight dynamics. Ultimately, he hopes to understand how differences in morphology and flight performance might influence ecological differences between flying snake species. This will require more field studies, but luckily for Socha, `the thrill of watching snakes fly is something that never gets old.'
- © The Company of Biologists Limited 2005