The world's gliding mammals
As forest canopies thinned, some tree-dwellers evolved to glide between branches. Six of them live in Australia.
UP TO 15 MILLION years ago the Australian landmass was covered in lush Gondwanan rainforest. As giant birds stalked below, tree-dwelling mammals had no trouble leaping between the dense branches above.
As the continent drifted further north however, the steamy rainforests receded. Tree cover thinned, and the mammals that lived the canopy had to leap further and further. It was then, according to researcher Dr Stephen Jackson, that some Australian marsupials probably learnt to glide.
Now some mammals can cover up to 400m using flaps of skin on their sides attached to their front and back limbs – creating the aerodynamics needed to harness extra uplift, to get them across greater distances.
"Most gliding mammals have their feet relatively free," says Stephen. "But the colugos found in Malaysia and the Philippines have their feet almost completely encased in the membrane. They look almost like a flying disk."
Possums may be evolving to glide
Gliding has developed independently as a skill a number of times around the world he says. Most of the world's 60-odd gliding mammals are in fact found in rainforested south-east Asia, China, Borneo and India, where their development may have been different to Australia's gliders. In the current-day rainforests of Asia for example, existing gliders may have evolved to fly above the canopy, between emergent tall trees.
Australia, however, has six species of gliding marsupials, ranging from the 65cm-long mahogany glider to the world's smallest gliding mammal, the feathertail glider (around 15cm long). While most of the world's gliding mammals are species of flying squirrel, which are not found in Australia, we do lay claim to members of three of the six overarching groups; all of which are marsupials.
These are: the feather tailed glider, a close relative of the non-gliding feather-tailed possum in New Guinea; the greater glider, a relative of the ring-tailed possum; and the lesser gliders, which include small sugar gliders and yellow-bellied gliders. Two more marsupial gliders are found across the Torres Strait in New Guinea.
In just a few million years, we could add a few more to that list. Biologists believe some Australian marsupials may still be developing the ability to glide. Stephen points to the example of membranes found on Australia's non-gliding ring-tailed possum: "Just near where the tail is and on the sides of its flanks it has loose skin," he says. "It's possible that is the early stages of developing a gliding membrane."
How gliding mammals fly through the air
Today, humans pressures are contributing to the rapid thinning of tree cover and some help is required to move gliders over man-made barriers. Travel down the Hume Highway between Sydney and Melbourne and you may spot tall pine poles, erected as launching pads for gliders crossing this arterial route.
Although tall trees or gliding poles are an essential component for gliding species, they're surprisingly nimble, often changing direction mid-air by adjusting the angle of their membranes.
Stephen, for example, has watched hundreds of gliders as part of his research, and says their ability to steer around trees is remarkable. "I've actually seen full banking turns," he says. "A glider once took off, saw me and effectively did a u-turn and landed on the same tree."
The beautiful illustrations and captions with this story come from the book Gliding Mammals of the World by Stephen Jackson and Peter Schouten and published by the CSIRO.
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