Fancy aerodynamics explain why some snakes are good gliders

MANY animals have mastered the trick of gliding through the forest canopy. Flying frogs stretch the webbing between the toes of their enlarged feet. Some lizards spread elongated ribs covered in flaps of skin. And the colugo, a strange South-East Asian mammal, can travel as much as 70 metres (230 feet) between trees by spreading a membrane of skin that connects its limbs. Despite a lack of obvious body parts that can double up as a decent pair of wings, some snakes, too, can glide for remarkable distances.

To discover how snakes manage it Jake Socha of Virginia Tech and his colleagues conducted a series of test flights with paradise tree snakes, a mildly venomous variety found in parts of Asia. They launched the snakes from the top of a 15-metre tower and used four video cameras to construct 3-D images of the animals' trajectories. The results, just published in Bioinspiration and Biomimetics, show not only that flying snakes are surprisingly good aviators but also that they employ some complex aerodynamic tricks.

A paradise tree snake flattens its body into an aerofoil-like shape that can provide a degree of lift before it takes off from a branch by jumping upwards. It then falls at a steep angle and starts undulating from side to side. As it gains speed its glide path becomes shallower, allowing it to cover a greater distance. The researchers found some snakes could glide for 24 metres.

The 3-D analysis showed the snakes take up a staggered posture when gliding, with their bodies angled upwards at about 25° to the oncoming air and their heads slightly higher than their tails. Dr Socha says this orientation seems to provide the creatures with an aerodynamic advantage. As a result of it, the wake of air forced backwards by one segment of the snake's undulating body may provide extra lift to the segment behind in the sidewinding glide. Flying fish rely on a similar flow of accelerated air from their pectoral fins to provide lift to their smaller pelvic fins. Indeed, it is possible that the effect works both ways, and that rearward segments provide lift to those in front.

The snakes are also surprisingly manoeuverable and have been seen to turn in mid-air. To find out if this might be deliberate, Dr Socha limited their view from the tower so that they could see only straight ahead when they launched themselves. Once airborne, and thus able to observe trees to the side of their glide-paths, some of the snakes turned towards them.

Dr Socha, who studies the relationship between form and function in animals, was partly funded in his snake research by America's Defence Advanced Research Projects Agency, which said it was interested in the science but obviously wonders about military applications.

Dr Socha does not expect to see snakelike drones in the skies soon. However, the ability to slither through holes, to swim and to climb trees and rocks, as well as to glide between elevated features of the landscape, is an attractive mixture for designers of robotic vehicles. The staggered-wing effect, in which the front of the body helps to provide lift for the back, may also be of interest to aircraft engineers. In the long term, then, the world's battlefields may see machines that resemble some people's worst nightmares, slithering as they do towards the enemy both on the ground and through the air.