Moves like a marsupial, climbs like a primate

Koalas really are built for life in the treetops.

Australia’s cuddly koala seems to have evolved tree clambering abilities that rival those of the apes and monkeys that never made it to the continent.

Technically a marsupial – bearing its young in a pouch like kangaroos and wombats – Phascolarctos cinerus has relatively long limbs with two thumb-like digits on powerful grasping hands and feet. And it is more comfortable in the trees than on the ground.

Christofer Clemente, from the University of the Sunshine Coast, and colleagues were therefore curious to know whether it uses the same movement strategies as tree-dwelling primates.

As described in the Journal of Experimental Biology, the researchers ran experiments at Queensland Zoo’s Wildlife HQ, filming six koalas in their semi-natural enclosures using several high-speed cameras synchronised to give a 3D reconstruction of their movements.

This took a few months, because koalas spend a lot of time sleeping. They also moved straight to the nearest tree when placed on the ground – even attempting to climb up the team’s tripods.

Eventually, the researchers gained around 40 minutes of footage from more than 200 videos, from which they analysed 198 movements on the ground, ascending and descending and moving along a narrow horizontal beam.

They found that, unlike other mammals such as cats and dogs that use two legs from the same side to support each other, koalas use a gait that allows greater stability from diagonally opposite leg pairs when climbing.

“As far as we can tell, they are the closest thing we have to primates in Australia,” says Clemente, “likely because of the similar gripping structures on the hand. They do this more so than other tree species like possums, from which they share a common ancestor.”

Unlike primates, though, their speed was more consistent with their sleepy demeanour, only reaching 2.5 kilometres per hour – although they were surprisingly agile.

“We occasionally saw leaps of over one metre from branch to branch, or a koala moving along a branch while hanging underneath just using their forelimbs,” says Clemente.

On the ground their movements were more like marsupials, walking then bounding like rabbits. They achieved speeds of up to 10 kilometres per hour – relatively slow for their body mass, but possibly slower than speeds they are capable of, the authors suggest.