Tall trees may have sparked evolution of gliding

The gliding animals of south-east Asia have something in common with the Himalayas. Both owe their existence to the collision of India with Eurasia around 50 million years ago. In the wake of the continental clash, tall tropical trees spread from the subcontinent and began to dominate Asian rainforests, providing a perfect environment for the evolution of gliding.

South-east Asia's rainforests are famed for their exceptional variety of gliders. Geckos, Draco lizards, flying squirrels, colugos and frogs have all taken to the skies, suggesting the adaptation evolved several times in the region. Now a study by Matthew Heinicke at the University of Michigan at Dearborn and colleagues has found evidence to support a link between the adaptation and the forests' unusual vegetation, which is dominated by dipterocarps ? trees that grow unusually tall and typically mature to lack any branches on the lower 30 metres of their trunks.

"It makes more energy sense for a small animal to glide between trees than to climb all the way down one tree and then climb back up another," says Heinicke.

Heinicke and his colleagues looked at the evolutionary history of the animal groups that contain one or more gliding species. Their analysis suggests that gliding evolved independently eight times in the forests, and that six of those evolutionary events occurred between 20 million and 50 million years ago ? the time during which dipterocarp trees were first able to spread from India across southeast Asia.

A seventh group, Draco lizards, split earlier from its non-gliding relatives ? perhaps as early as 60 million years ago ? pre-dating the collision of India with Eurasia. These lizards evolved in India, however, and so may still have begun to glide in dipterocarp-dominated forests. The eighth group ? the colugos ? apparently split from non-gliding primates nearly 80 million years ago, making them an exception to the theory.

"Conceptually, it's really clever what they have done," says palaeontologist David Hone at the University of Bristol, UK.

The evolution of gliding by four-legged tree climbers was probably a key step on the path to flight for bats and pterosaurs, although none of the gliders in the modern dipterocarp forests are closely related to these active flyers.

Birds, though, may have taken another route to the skies, says Michael Habib of the University of Southern California in Los Angeles. They evolved from feathered dinosaurs like the four-winged Microraptor, which looks like it could have run bipedally on the ground, says Habib.

Journal reference: Biology Letters, DOI: 10.1098/rsbl.2012.0648

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