The tale of a kangaroo's tail

July 2, 2014 •

Kangaroos may be nature’s best hoppers. But when they are grazing on all fours, which is most of the time, their tail becomes a powerful fifth leg, says a new study.

Involving researchers at the University of Colorado Boulder, Simon Fraser University in Burnaby, Canada, and the University of New South Wales in Sydney, Australia, the study illuminates the seemingly mundane task of foraging by red kangaroos. While such activity appears awkward, it turns out their tails provide as much propulsive force as their front and hind legs combined as they eat their way across the landscape.

“We found that when a kangaroo is walking, it uses its tail just like a leg,” said Associate Professor Maxwell Donelan of Simon Fraser University, corresponding author for the study. “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.” 

“We went into this thinking the tail was primarily used like a strut, a balancing pole, or a one-legged milking stool,” said Associate Professor Rodger Kram of CU-Boulder’s Department of Integrative Physiology, a study co-author. “What we didn’t expect to find was how much power the tails of the kangaroos were producing. It was pretty darn surprising.”

Red kangaroos are the largest of the kangaroo species in Australia. When grazing on grasses, they move both hind feet forward “paired limb” style while using their tails and front limbs together to support their bodies. “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.”

In human locomotion, the back foot acts as the gas pedal and the front foot acts as a brake, which is not especially efficient, said Kram. But he likens a walking kangaroo to a skateboarder who has one foot on the board and uses the other foot  -- in this case a tail -- to push backward off the pavement, increasing the forward motion.

A paper on the subject was published online today in Biology Letters. In addition to Kram and Donelan, the paper was co-authored by Postdoctoral Fellow Shawn O’Connor of Simon Fraser and Emeritus Professor Terence Dawson of the University of New South Wales. The study was funded by the Natural Sciences and Engineering Research Council of Canada, the Australian Research Council, and traveling fellowships from the International Society for Biomechanics and the Journal of Experimental Biology.

Donelan, a former graduate student under Kram, said no animal other than the kangaroo uses its tail like a leg.  “Their tails have more than 20 vertebrae, taking on the role of our foot, calf, and thigh bones.”

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