A new study revealed how kangaroos use their tail as a "fifth leg" when grazing on all fours.

The study shows the kangaroo's tail provides the majority of the propulsive force as their front legs allow them to forage the landscape, the University of Colorado at Boulder reported.

"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 kangaroo species in Australia. When they graze they move their hind feet forward while using their tail and front limbs at the same time to propel themselves.

"They appear to be awkward and ungainly walkers when one watches them moseying around in their mobs looking for something to eat," Kram said.

The researcher compared a walking kangaroo to a skateboarder who has one foot on the board and uses the other foot to move themselves forward. No other animal then the kangaroo uses their tail like a leg in this way. Their tails have over 20 vertebrae which eliminates the need for features like a foot or thigh bone.

The researchers coaxed a group of kangaroos to hop and walk on a motorized treadmill while measuring the energy cost of locomotion at varying speeds. The team found a kangaroo can increase its metabolism up to 50 times through this movement.

"Kangaroos are really special mammals," said Emeritus Professor Terence Dawson of the University of New South Wales. "Work over the past half century has turned the notion that they belong to an inefficient, primitive group of mammals totally on its head."

The study was conducted by researchers from the University of Colorado Boulder, Simon Fraser University in Burnaby, Canada, and the University of New South Wales in Sydney, Australia. A paper on the subject was published online today in the journal Biology Letters.

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