Kangaroos are known for being some of the best hoppers in the animal kingdom and that’s just on two legs. But when they are grazing for food on all fours, their tail becomes their most powerful asset, acting practically like a fifth leg, says a new study.
Researchers at the University of Colorado Boulder, Simon Fraser University in Burnaby, Canada and the University of New South Wales in Sydney, Australia, illuminates the seemingly mundane task of foraging by red kangaroos in a study. 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. Kangaroos are known for using their tail to balance when they’re hopping, but they are the only animals that use their tails like a leg.”
When humans walk, the back foot acts as the gas pedal and the front foot acts as a brake. This motion actually isn’t particularly efficient. In contrast, kangaroos are a bit like skateboarders; one foot is on the board while the other foot, in this case the tail, pushes backward off of the pavement and increases forward motion.
“We went into this thinking the tail was primarily used like a strut, a balancing pole or a one-legged milking stool,” said Rodger Kram, a study co-author. “What we didn’t expect to find was how much power the tails of kangaroos were producing. It was pretty darn surprising.”
A paper on the subject was published online 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.
The kangaroo tail also acts as a dynamic, springy counterbalance during hopping and boosts balance when male kangaroos grab each other by the chests or shoulders, then rear back and kick each other in the stomach in an attempt to assert dominance for the purpose of reproduction.
For the study the team videotaped five red kangaroos in Dawson’s Sydney lab that 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.
“I’m envious of kangaroos,” said Kram, a competitive master runner in the mile and 1,500 meters. “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.”
“Kangaroos are really special mammals,” added co-author Terence Dawson of the University of South Whales.