Kangaroos, best known for their pouches, use their tails as a kind of fifth leg, according to a new study. This helps the large marsupials balance. Remarkably, the tail can provide as much forward propulsion for the animal as all four legs combined.

University of Colorado (CU) Boulder biologists studied the species, along with researchers from the University of New South Wales in Sydney, Australia.

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

Red kangaroos are the largest of all species of the distinctive animal on the continent. The males can grow to be over five feet long, with a tail measuring an additional four feet or more. Females are significantly smaller, with bodies around three feet long, plus tails around 30 inches in length.

As these animals move, they rest their weight on their tail and two front limbs, as they bring hind legs forward. This may look awkward, but the method works well for the animals. Humans use a pull-and-brake system during bipedal locomotion, as the back foot acts as a brake, adding friction to accept the thrust of the other leg, moving forward.

Investigators believe that kangaroos are unique in their ability to use a tail in such a significant way to aid forward motion.

"Their tails have more than 20 vertebrae, taking on the role of our foot, calf, and thigh bones," Maxwell Donelan of Simon Fraser University explained to reporters.

This discovery had its start back in 1973, when Terence Dawson, then a visiting professor at Harvard University, studied the animals with professor Richard Taylor. The team placed kangaroos on treadmills, studying how the animals propelled themselves. They found metabolism of the animals can increase up to 50 times during physical exertion.

When competing for females, male red kangaroos will grab one another by the shoulder, bring the rear of their bodies in the air, then kick their opponent in the stomach. Investigators believe the animals also use their tail to soften the effects of this push.

"Kangaroos are really special mammals. Work over the past half century has turned the notion that they belong to an inefficient, primitive group of mammals totally on its head," Dawson told the press.

Study of kangaroos and investigation of the role of tails in producing locomotion was detailed in the journal Biology Letters.