The scene, by night: a palm-size possum steps off a high branch in an Australian forest, spreads its arms and sails. National Geographic puts the mark at about 165 feet, half a soccer pitch in a single trip, for Australia’s sugar gliders. The scare quotes around “fly” on that fact sheet are doing quiet work. These animals do not fly. They glide, and the difference is the whole story.
The hardware is a membrane called a patagium, a thin sail of skin running from the fifth finger of each forepaw to the back ankles, with the bushy tail working as a rudder. Nothing flaps. The animal starts high and lands low, and everything between the two is controlled falling, steered by shifts in limb and body position.
How much control involved is no longer guesswork. In 2007, Kristin Bishop, then a biologist at Brown University, filmed four sugar gliders, bought at weaning through the pet trade and trained to glide to a pole, with paired high-speed cameras at Harvard’s Concord Field Station, then reconstructed their descents in three dimensions. The animals averaged 13.5 cm from nose to tail base and weighed 73.4 g.
Published in the Journal of Experimental Biology, the findings read like an aircraft test report. The gliders leaned on high angles of attack, averaging 44.15 degrees, well past the point where a conventional wing stalls, yet still produced lift coefficients near 1.48. Every trial but one showed slight downward acceleration together with forward acceleration, “indicating that the body weight was not fully supported by aerodynamic forces” in the paper’s words. Steering was active, not passive. “Movements of the limbs were strongly correlated with body rotations,” Bishop reported, and different animals used different limb moves to make the same turn.
Not a squirrel, and not a kite
Comparison did the evolutionary arguing. Under similar lab conditions, southern flying squirrels generated more lift and less drag, yet their glides came out only marginally shallower. The squirrels carry morphological specializations the marsupials lack, which may explain the edge. The kinship runs the other way: despite the squirrel-like silhouette, “sugar gliders are more closely related to other marsupials like kangaroos” than to the rodents they resemble, as National Geographic puts it.
The rest of the dossier: nocturnal and big-eyed, grey with a white belly and black head stripes, feeding on nectar, pollen and tree sap plus the odd beetle or spider, nesting in tree hollows in groups of up to 10 adults. Wild populations remain stable despite feral predators, bushfires and land clearing. Biologists keep returning to these animals because gliding is widely discussed in the literature as “a possible precursor to powered flight in vertebrates,” and bats, by one longstanding view, evolved from an arboreal gliding ancestor not so different from these. The gliders themselves seem unbothered by the theory. After each landing they climb, and then they jump again.
Sources
- Aerodynamic force generation, performance and control of body orientation during gliding in sugar gliders (Petaurus breviceps) | Journal of Experimental Biology | The Company of Biologists
- Sugar Glider Facts | National Geographic
