Flyers and gliders

If you can't fly, gliding is the second most efficient way of moving around a rainforest, foraging for fruit - the preferred food of both primates and fruit bats. And more than one group of animals adapted to clambering around on the trunks of tall trees has made the evolutionary leap to gliding flight.

Several groups of Australian marsupials have independently evolved gliding flight. So did small, feathered cousins of the warm-blooded Cretaceous predator, Velociraptor, which took to the trees and evolved into birds, sharing the skies with flying reptiles like Pterodactyl and Pteranodon.

In modern times, several rainforest frogs, lizards and even a snake have variously co-opted webbed feet or flared rib cages for short, gliding flights.

Until quite recently, zoologists puzzled over the colugo's place in the grand scheme of mammalian evolution - the two extant species, Cynocephalus volans (Philippines) and (Malaysia), are the last representatives of a unique mammalian order, the Dermoptera.

The strange, bug-eyed, cat-sized gliders forage nocturnally, like fruit bats, but eat only leaves, buds and flowers, not fruit. Uniquely among gliding mammals, the leathery gliding membrane or patagium encloses the wrists and ankles.

In microbats, the patagium stretches between greatly elongated metacarpals, the long bones that end in the knuckles. Fruit bats have their patagium stretched across elongated phalanges. The modified second finger ends in a hook on the leading edge of the wing. With wings folded, fruit bats clamber around a tree with almost lemur-like agility, says Pettigrew.

This striking divergence in the structure of the wing is more likely the result of convergent evolution in the descendants of unrelated gliding ancestors, he says. And this is just one at least 50 anatomical, physiological and behavioural traits separating megabats and microbas.

In the case of megabats and their colugo cousins, the fossil record offers a likely common ancestor: the paramomyids, whose fossils occur in rocks of early Paleocene (60 million years ago) to early Oligocene (35 million years ago) age. Their unusually long limb bones hint that they were gliders.

The earliest megachiropteran fossils are also of early Oligocene age. The coincidence suggests paramomyid gliders exited stage left, just as fruit bats entered from stage right, according to Pettigrew.

A rainforest frugivore that evolved flapping flight would be far more efficient at foraging than one limited to gliding - so efficient, says Pettigrew, that the fruit-eating paramomyid ancestors of fruit bats went rapidly extinct in the unequal contest for their preferred fruit diet. But one paramomyid lineage survived, because it had taken to herbivory and avoided competition with the new-wave fruit bats: the colugos.