Koalas aren’t primates, but they move like monkeys in trees

Koalas could be Australia’s take on monkeys and apes.

Pudgy, big-eared koalas are celebrity marsupials, nurturing teensy young in pouches as a kangaroo does. In koalas’ home trees, however, “we found they’re actually not moving like other marsupials,” says Christofer Clemente of University of the Sunshine Coast in Sippy Downs, Australia. “They’re moving more like primates.”

Australia doesn’t have trees full of primates. So koalas independently evolved a similar gait in treetops. Both tree-hugging lineages often navigate trunks and branches by coupling limbs diagonally — for instance moving the right hand and left foot forward while gripping with the other pair. As with many primates, this movement stabilizes koalas as they step, Clemente, an evolutionary biomechanist, and his colleagues report December 17 in the Journal of Experimental Biology.

Doing that requires a good grip, and koalas have double thumbs on their hands: two short digits separated by a gap from three longer ones. Hind limbs also have a thumblike appendage.

When koalas reluctantly descend to the ground, their top speed — about 10 kilometers per hour — comes in a common marsupial bounding motion with the hind legs landing together. Bounding across ground can be about four times faster than moving around in trees, but still leaves koalas vulnerable to predators, particularly dogs now common in Australia. In fact, this study of six captive animals grew out of worries about declining koala populations forced to traverse perilous ground as increasingly fierce fires destroy trees (SN: 7/2/18).