Researchers at the University of Colorado at Boulder, Simon Fraser University and the University of New South Wales have discovered that kangaroos use their tail as a fifth leg when they are grazing on all fours. In fact, their tails provide as much propulsive force as their front and hind legs combined.

“We found that when a kangaroo is walking, it uses its tail just like a leg,” explained Associate Professor Maxwell Donelan of Simon Fraser University. “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.”

Red kangaroos are the biggest of the kangaroo species in Australia, When grazing on grasses, they move both hind feet forward “paired limb” style while utilizing their tails and front limbs together to support their bodies.

National Geographic notes that red kangaroos can reach speeds of more than 35 miles per hour. Their bounding gait also allows them to cover 25 feet in a single leap and to jump six feet high.

Associate Professor Rodger Kram of CU-Boulder’s Department of Integrative Physiology likens a walking kangaroo to a skateboarder who has one foot on the skateboard and utilizes the other foot to push backward off the pavement.

For the study the researchers videotaped five red kangaroos that had been trained to walk forward on a force-measuring platform with Plexiglas sides. The platform measured vertical, backward and forward forces from the legs and tails of the animals.

Kram calls the evolution of the kangaroo tail an “exaptation” — a shift in the function of a biological trait over time.

“I’m envious of kangaroos,” posited Kram. “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.”

The study’s findings are described in greater detail in the journal Biology Letters.