Hopping across vast, arid landscapes at 20 kilometers an hour is a trademark kangaroo move. “Their walking, on the other hand, is as awkward as their hopping is graceful,” says Max Donelan from Simon Fraser University. “But underlying the walking is this entirely new use for a tail.” When they’re just walking (which is most of the time), kangaroos use their tails like a fifth leg, according to work published in Biology Letters this week. In a perfectly coordinated sequence with their front and hind legs, kangaroos plant their muscular tails on the ground to propel their motion. 

When grazing on grasses, red kangaroos (Macropus rufus) move both hind feet forward in a paired-limb style, while their tails and front limbs 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,” says Rodger Kram from University of Colorado-Boulder. “But it turns out it is not really that awkward, just weird.”

To examine the tail’s role in this unique “pentapedal” gait, Donelan, Kram, and an international team of researchers measured the forces that tails exert on the ground and calculated the mechanical power they generate. They videotaped five red roos trained to walk forward on a force-measuring platform with Plexiglas sides. Sensors measured vertical, backward, and forward forces from their legs and tails.

“We went into this thinking the tail was primarily used like a strut, a balancing pole, or a one-legged milking stool,” Kram says in a CU release. “What we didn’t expect to find was how much power the tails of the kangaroos were producing.” The tail is responsible for as much of the propulsive force as the front and hind legs combined. And the power generated by the tail is almost exclusively positive mechanical power. “They performed as much mechanical work with their tail as we do with one of our legs,” lead author Shawn O'Connor says in an SFU release. “The kangaroo's more than 20 tail vertebrae take on the roles of our feet, calf and thigh bones.” 

When we walk, the back foot acts as the gas pedal while the front foot acts like a brake -- which isn’t especially efficient. Kangaroos, meanwhile, have very short front legs that can’t be used to push off. “The timing and position of the tail, on the other hand, is perfect,” Donelan says. “So, we wondered if they are able to use their tail just like a leg to push off and power their walking.” Sure enough, a walking kangaroo is like skateboarder using one foot to push backward off the pavement, increasing the forward motion.

No other animal uses its tail like a leg. Additionally, when hopping, the tail acts like a springy counterbalance, and it boosts balance when males rear back to kick each other. Researchers think this tail evolved from a prehensile one when the possum-like kangaroo ancestor lived in trees. 

Images: Catharina Vendl, Fowlers Gap (top), Shawn O'Connor, Simon Fraser University (middle), Heather More, Simon Fraser University (bottom) via SFU Locomotion Lab