Red kangaroo’s tail acts like a fifth leg

When a red kangaroo moves fast, it bounds across the Australian landscape on its powerful hind legs. But when the animal moves more slowly, grazing for food, it uses both its front and hind limbs, with its tail on the ground behind. Scientists thought the tail’s main job might simply be for support.

Not so, report researchers July 2 in Biology Letters. At slow speeds, a kangaroo’s tail acts like a fifth leg, providing as much forward propulsion as the other four limbs combined.

Shawn M. O’Connor of Simon Fraser University in Burnaby, Canada, and colleagues began by training four adult female and one juvenile male red kangaroos to walk over a force-measuring platform. A low ceiling above the platform meant the animals had to walk, not bounce, through. The scientists recorded video of the kangaroos slowly walking using all five available limbs and calculated the force of each limb through the animal’s gait.

“We discovered that kangaroos use their tail as a very capable leg when moving pentapedally,” the researchers write. As the animals move forward, they alternate placing their weight on their hind and fore legs. When balanced on the front limbs, the kangaroo supports the weight of its body with its tail, which also propels the roo forward.

This is somewhat surprising for a limb that has articulated bones more like that of a prehensile tail than a leg — a leftover from the kangaroo’s arboreal ancestors. But the rest of a red kangaroo’s tail isn’t built for simply hanging around. The tail has powerful muscles that are much larger than those found in the fore limbs. And the cells in the tail are dense with mitochondria, which should supply the tail with a large capacity for aerobic activity.

A red kangaroo’s fore limbs are much smaller than the both the tail and the roo’s hind legs. And that’s the key to the animal’s pentapedal gait. Pentapedal locomotion, the researchers say, is necessary for an animal that has tiny arms. “Without a tail to widen the fore-aft base of support, the kangaroo would face a severe propensity to fall backwards when the hind legs are lifted and only the front legs remain on the ground,” they note.

Smaller tree kangaroos, which have more similarly sized front and hind legs, don’t use their tails in the same way. They don’t have to. But if a red kangaroo wants to avoid an embarrassing pratfall at slow speed, it needs to make use of that fifth limb.