Kangaroos may be nature's best hoppers, but when they are grazing on all fours their tail becomes a powerful fifth leg, according to a new study.
Kangaroo tails provide as much propulsive force as their front and hind legs combined as they eat their way across the landscape, researchers have found.
Researchers at the University of Colorado Boulder, Simon Fraser University in Burnaby, Canada, and the University of New South Wales in Sydney, Australia, studied foraging by red kangaroos.
"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," Donelan said.
"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," Kram said.
For the study the team videotaped five red kangaroos in the lab of Emeritus Professor Terence Dawson of the University of New South Wales.
They had been trained to walk forward on a force-measuring platform with Plexiglas sides.
The platform's sensors measured vertical, backward and forward forces from the legs and tails of the animals. The kangaroos had been taught that walking forward on the platform resulted in being rewarded with sweet treats, said Kram.
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.
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.
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," he said.
The research was published in the journal Biology Letters.
