Four-legged Australian kangaroos could be more accurately described as five-legged, researchers say, since they use their tails to help them move forward.
Uniquely among all animals, the tail of red kangaroos -- the largest of Australia's kangaroo species -- works as hard as the leg of a human walking at about the same forward speed, they say.
In a study published in the journal Biology Letters, the scientists speculate the use of the tail as an additional leg evolved to offer an efficient movement when they were grazing on all fours rather than hopping, their other unique and classic mode of movement.
A research team of Australian, U.S. and Canadian scientists found the kangaroo's tails give as much propelling strength as the front and hind legs together as they forage over the landscape.
"We went into this thinking the tail was primarily used like a strut, a balancing pole, or a one-legged milking stool," Rodger Kram of the University of Colorado, Boulder, says. "What we didn't expect to find was how much power the tails of the kangaroos were producing. It was pretty darn surprising."
The kangaroos have evolved a method of locomotion that involves moving their rear feet forward together while supporting their bodies with their front legs and their tail.
"They appear to be awkward and ungainly walkers when one watches them moseying around in their mobs looking for something to eat," says Kram. "But it turns out it is not really that awkward, just weird."
He compares a walking red kangaroo with a human skateboarder who keeps one foot atop the board while using his other foot -- in the kangaroo's case the tail -- to push back, increasing forward motion.
The kangaroo is the only animal that uses its tail in this way, essentially as an extra leg, the researchers said.
It's evolved a structure capable of handling the task, they say.
"Their tails have more than 20 vertebrae, taking on the role of our foot, calf, and thigh bones," says Maxwell Donelan from Canada's Simon Fraser University.
The scientists gathered data on the animals' walking style by observing five red kangaroos trained to walk forward while on a treadmill in order to reach a treat.
Sensors monitored vertical, forward and backward forces generated by the legs and tail of the animals.
"I'm envious of kangaroos. When they hop faster, they don't use energy at a faster rate. The have the ability to move faster and not get tired, the ultimate goal of a runner," says Kram, himself a competitive runner himself.
