When kangaroos aren't hopping, they use their tail as an extra limb and essentially walk on five "legs," making them unique in the animal kingdom, a new study has found.

While kangaroos are known for their fast and efficient hopping, most of the time they are actually down on all fours, grazing, said Max Donelan, a Simon Fraser University physiologist who co-authored the study published Tuesday in the journal Biology Letters.

Other people who have studied kangaroos noticed that when they are in that pose, they appear to use their tail as a crutch — the tail moves with the front limbs as the hind limbs swing through, and the hind limbs take the animal's weight as it lifts is front legs and tail off the ground.

Kangaroo stride

Kangaroos move their two front limbs and their tail together, followed by their two hind limbs together as they walk. (Shawn O'Connor/Simon Fraser University)

But measurements by Donelan and his collaborators found that the tail is far more than a crutch — it provides more propulsive force and powers the kangaroo's walking movements more than the animal's hind and front legs put together.

"This is not a tail that is performing kind of like a leg," Donelan said. "It's doing an incredible job at it."

To his team's knowledge, no other animals use their tails as a leg.

Donelan said the kangaroo's adaptation of its tail in this way is "remarkable" because it is so anatomically different from a leg and originally evolved to help the kangaroo's ancestors swing on branches.

Kangaroo standing

Kangaroos are known for their hopping abilities but actually spend most of their time on all fours, grazing. ( Catharina Vendl/Fowlers Gap)

Donelan mostly studies how humans walk efficiently in order to figure out how people can regain that ability after an injury, such as a stroke. He first became interested in kangaroos after noticing that the way they timed their walking movements was similar to the way humans do.

In order to figure out just how kangaroos were using their tails as they walked, Donelan teamed up with Terence Dawson, a kangaroo expert at the University of New South Wales in Sydney, and University of Colorado, Boulder physiologist Rodger Kram.

They built a walkway with Plexiglas sides and a ceiling high enough to allow red kangaroos — large kangaroos that can grow to be up to 90 kilograms — to walk comfortably, but too low to let them hop. The walkway was tiled with plates similar to bathroom scales that can take very quick measurements of the forces exerted by each of the kangaroo's limbs as it walked.

Shawn O'Connor, a postdoctoral researcher working with Donelan, figured out how to analyze the data in order to calculate the forces being exerted by the kangaroos.

Donelan said the results of the study provide information not just about how kangaroos move but also some general principles about timing walking movements that apply to humans — his own area of expertise. 

While the study was conducted on red kangaroos, Donelan said he suspects that other large kangaroos such as grey kangaroos walk in the same way. Small kangaroos such as wallabies do not appear to use their tails to walk.