Kangaroos Walk with a Fifth 'Leg': The Powerful Tail of a Grazing Mammal

Kangaroos are known for their powerful tails and legs that allow them to leap quickly across the landscape. But when these animals are grazing on all fours, their tail isn't extraneous. Instead, researchers have found that it acts as a fifth leg.

Red kangaroos are the largest of the kangaroo species in Australia. Most of the time, they graze on grasses and move on all fours. Yet while grazing on grasses, they move both hind feet forward "paired limb" style while using their tails and front limbs together to support their bodies.

"We found that when a kangaroo is walking, it uses its tail just like a leg," said Maxwell Donelan of Simon Fraser University, in a news release. "They use it to support, propel and power their motion. In fact, they perform as much mechanical work with their tails as we do with one of our legs."

When humans walk, the back foot acts as the gas pedal and the front foot acts as a brake. This motion actually isn't particularly efficient. In contrast, kangaroos are a bit like skateboarders; one foot is on the board while the other foot, in this case the tail, pushes backward off of the pavement and increases forward motion.

"We went into this thinking the tail was primarily used like a strut, a balancing pole, or a one-legged milking stool," said Rodger Kram, a study co-author. "What we didn't expect to find was how much power the tails of kangaroos were producing. It was pretty darn surprising."

The scientists videotaped five red kangaroos that had been trained to walk forward on a force-measuring platform with Plexiglas sides. This allowed the researchers to measure the kangaroos' movements and see exactly what the function of the tail was.

"They appear to be awkward and ungainly walkers when one watches them moseying around in their mobs looking for something to eat," said Kram. "But it turns out it is not really that awkward, just weird."

The findings reveal a bit more about this mammal and show how the way it moves is more efficient than once thought. This, in turn, sheds some light on the process of evolution in these mammals.