The snake moved along the tree branch in the dappled light of Borneo's rain forest. Suddenly it dropped over the edge, just holding on by its tail, and then pushed off into the damp air. It changed shape as it began to drop, ribs spreading and body flattening as it swam through the air. This would be the worst nightmare of someone afraid of snakes, but to me it was a part of a wild dream come true.
Since I first went to Borneo in 1987, I have been curious about the island's extraordinary diversity of gliders. It's not that they are easily noticed. Most of them have cryptic coloring, are nocturnal, or are hidden in the high canopy. Over the years as a field biologist I had exciting, fleeting encounters with them: A giant flying squirrel as big as a T-shirt, glimpsed soaring at dusk. The flying gecko that glided into my jungle hut and landed on my back. Two paradise tree snakes on a branch a hundred feet up in a tree next to the one I was climbing. A flying lemur letting out an unearthly scream as it eluded an attacking hawk.
These gliding animals can't propel themselves through the air like birds or bats. They shift body weight subtly or adjust tails and limbs to steer a controlled flight path through the canopy labyrinth. More than 30 gliding species can be found in Borneo alone. Why is this island so rich in gliding species while other rain forests like the Amazon have none and African forests harbor just a few gliders? I believe it is because Borneo's forests, as well as others in Southeast Asia, are different in one important way: They are dominated by giant dipterocarp trees, which fruit infrequently and unpredictably and crowd out other trees. These conditions appear to make food sparser in Borneo than in other rain forests, forcing the forest animals to range more widely. What better way than to glide? It allows them to go from tree to tree without making the long trip down to the ground and back up again. This need for efficient canopy travel can explain why gliding has emerged in such a variety of animal groups, each evolving unique structures to glide with, from the exaggerated webbed feet of flying frogs to the skin membranes of the flying lemurs. Locating and observing these gliders in the wild was a challenge. I climbed 150-foot (45.7 meters) trees and trekked the jungle day and night. Only slowly, slowly, did the timeless forest reveal its secrets.