Serpents in the Air

A little contortionist can go a long way.

Story by Adam Summers   ~  Illustrations by Patricia J. Wynne

The ophiophobe worries, somewhat irrationally, about snakes-whether they're slithering across the sidewalk, lurking in laundry hampers, or even appearing on television. If you, too, are burdened by such anxieties, you might just skip this month's "Biomechanics." There's plenty to engage you in the rest of the magazine, and what you'll undoubtedly retain from this column will be just one more item in the list of directions from which snakes can suddenly appear: from above.

Although as a group snakes appear singularly unsuited for aerial exploits, herpetologists (those intrepid biologists who specialize in reptiles and amphibians) have heard credible accounts of "flying" snakes for more than a hundred years. Only lately, however, has one investigator begun to exhaustively document the extent and mechanics of those animals' aeronautical talents.

A paradise tree snake flares its ribs and curves itself into an S as it glides through the air.

To most people, any airborne snake is a flying snake-why bother with fine distinctions when the very idea of an airborne snake is probably unnerving enough to contemplate in the first place? But to biologists, an animal is properly called a flier only if it can generate enough force to gain altitude in still air. A critter is a glider (but not a flier) if it can manage at least a foot of horizontal travel for every foot it falls. An animal that moves less in a horizontal direction than it does in the vertical is said to "parachute" (unless it jumps or falls). A passive aerialist, of course, might catch an updraft and so soar to a higher altitude, but the lack of actively generated upward force still technically disqualifies it as a flier.

Until recently herpetologists thought all flying snakes were parachutists, barely able to slow their descent, let alone to take active control of their own direction and altitude. After all, such control would seem to require a body part in the shape of an airfoil, and tubular snakes apparently lack the broad, thin surfaces that guide the descent of such better-known gliders as flying squirrels, colugos (flying lemurs), frogs, and lizards. But John J. "Jake" Socha, a biomechanist who recently received his doctorate from the University of Chicago, has, by reconstructing the three-dimensional flight path and mechanics of the snakes' glides, discovered that it doesn't necessarily take webbed legs-or limbs at all-for an animal to turn a fall into a long glide.

In fact, explaining what it takes for a snake to glide sounds a bit like an episode of Sesame Street: today's program is brought to you by the letters J, S, and C. Socha worked with the paradise tree snake, Chrysopelea paradisi, a native of Southeast Asia, on the grounds of the Singapore Zoological Gardens. To get the snakes to jump, he induced them to slither out on a perch more than thirty feet above the ground. When a flying snake prepares to jump, it dangles like the letter J from a branch. It then flings itself upward and away from the branch, only to begin falling at such a steep angle that few would call it anything but a plummet.

Yet after falling less than ten feet, the two-foot-long snake assumes an S shape and begins to undulate, much as if it were crawling across the ground, albeit more slowly and with more lateral movement. At the same time, the angle of its trajectory begins to flatten out, eventually decreasing to as little as 13 degrees. The snake-quite deft at avoiding obstacles-seems to swim through the air; in Socha's tests the snakes landed as far as sixty-nine feet from the thirty-foot-high launch point. In the shallowest moments of their glide (when their fall angle has decreased to its minimum), the snakes can travel nearly four times farther horizontally than they fall vertically, which easily surpasses the one-to-one benchmark of a gliding animal.

One of the most important factors in the snake's midair shift from free fall to glide is a dramatic increase in the width of the animal's body. Like most other snakes, a flying snake is roughly circular in cross section. But while a member of Chrysopelea is falling after launch, it flares its ribs so far outward that its belly becomes concave. With its body molded into a highly flattened C, the area of the snake's ventral silhouette-that is, its silhouette when seen from below-nearly doubles. It's as though the hood present on some cobras were extended along the entire length of the paradise tree snake's body.

The flattening of the snake essentially turns the animal into an airfoil: the increase in body width effectively halves the ratio of the snake's body weight to the area of its underside, a measure known as wing loading, and a crucial indicator of aerobatic talent. For example, the wing loading of a highly maneuverable bird such as the chimney swift is ten times smaller than that of the aeronautically challenged common loon. Wing loading in the paradise tree snake falls between those two extremes, but it's closer to that of the swift.

Experts in aerodynamics have also suggested that the snake's tight S-bends make its entire body act like a highly slotted wing. In airplanes, slotted wings have gaps that run along their entire length, from fuselage to wingtip; because of the way air flows through the gaps, such wings develop more lift at low speeds. Flaps along the trailing edge of airplane wings have the same effect. That principle is also at work in the spread between the feathers on the wing tips of the best low-speed gliders, such as vultures and hawks. The gaps between the bends of the S-shaped snake in flight could produce more lift than the snake would have if it shot, arrowlike, through the air. Any extra lift is crucial for maneuvering while gliding.

The advantages of gliding for a snake seem obvious: moving through the air from tree to tree bypasses a host of earth-bound predators, and a flying snake threatened by an arboreal animal can just launch itself out of the tree. But the paradise tree snake glides so expertly that it could, in principle, mount an airborne attack, either on a passing bird or on some more pedestrian prey that, like the ophiophobe, is expecting anything but a snake assault from above.

Adam Summers (asummers@uci.edu) is an assistant professor of ecology and evolutionary biology at the University of California, Irvine, and he once caused a snake to appear unexpectedly in a laundry hamper.