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A photo of red kangaroos hopping away from a watering hole.

Red kangaroos hop away from a watering hole in Australia's Sturt National Park.

Photograph by Theo Allofs, Corbis

Traci Watson

for National Geographic

Published July 1, 2014

How many legs does a kangaroo have? The correct answer, according to new research, is five. A study in this week's Biology Letters says that a walking kangaroo propels itself with its tail, essentially transforming the appendage into a fifth "leg."

The study found that the tail of a walking kangaroo works as hard as the leg of a comparably sized human strolling at the same speed. No other animal is known to employ its tail this way, and the study's authors speculate that use of the tail as a leg evolved to make the hop—a classic and unique kangaroo maneuver—more efficient.

Kangaroos can hop at 12 miles (20 kilometers) an hour over long distances; they cruise along far more economically than other animals run, says study co-author Terence Dawson, an emeritus professor at Australia's University of New South Wales. As it hops, the kangaroo's long tail whips up and down, helping the animal control the angle of its body.

But despite the hop's reputation as a trademark move, kangaroos actually spend more time in a shambling, hunched-over walk, moving at a leisurely 3.5 miles (6 kilometers) an hour or less as they graze and socialize.

Scientists have long known that an ambling kangaroo plants its tail on the ground to act as a crutch while its hind legs are off the ground, but until now, no one has tried to calculate the forces generated by this movement.

Watch a kangaroo use its tail as a fifth leg.

Spare "Leg" Comes in Handy

To find out more, the researchers coaxed red kangaroos to walk along a platform that measured the forces created by their limbs. A low ceiling above the platform prevented them from gearing up into a hop. The kangaroos, bred in captivity, "were easy to tame," Dawson says via email. "A bucket of feed pellets helped a lot."

The measurements showed that the tail, far from serving as a mere prop, acts like "a motor to lift and help accelerate the kangaroo's body," says study co-author Shawn O'Connor of Simon Fraser University in British Columbia, Canada. That role makes it a leg in all but name, O'Connor and his colleagues argue.

The propulsive force generated by the tail rivals that of the front and hind limbs combined, and the work done by the tail probably helps the animal save energy as it moves between patches of tasty plants.

It's an odd job for a body part that grasped branches back in the days when the kangaroo's ancestors lived in trees. Perhaps, the researchers speculate, a hard-working tail allowed the animal's front limbs to shrivel, reducing the animal's load as it hops.

The muscular tail is strong enough to support a kangaroo's entire body weight when a fighting male lifts his hind legs to kick his opponent, notes Michael Bennett of the University of Queensland in Australia, who was not associated with the research. So the finding that the tail pushes the kangaroo forward came as no surprise.

The study "confirms what I would've expected," says Harvard University's Andrew Biewener. "They are five-legged animals when they're using their tail."

"What is surprising is the extent to which the tail is propelling the body forward and the amount of force it's providing," says Kristian Carlson of South Africa's University of the Witwatersrand. "It's amazing what these kangaroos are doing."

Meichen Qian
Meichen Qian

amazing! Australia is quite an interesting country that full of lives!

John Riolfo
John Riolfo

Roos also make the most wonderful pets!

Brian Hancock
Brian Hancock

No sh!t, Sherlock. Amber is totally right. I can't believe that it has taken the scientific world world so long to notice this. This is as funny as the British Museum declaring that the Australian platypus specimen sent to them in 1799 was most likely a hoax.

Amber Kerr
Amber Kerr

I can't understand why this story has been so widely reported. Of course kangaroos use their tails as legs! It's quite obvious if you spend even a few minutes watching a kangaroo. So pretty much everyone in Australia is already familiar with this fact. Oh well... of all the topics in the headlines, I guess kangaroo physiology is probably the most innocuous...

pamela letstalkaboutcorsica
pamela letstalkaboutcorsica

interesting, never thought of it like that before, makes perfect sense too - those tails must be very strong

Tanmay Sharma
Tanmay Sharma

They are the best hopper animal one would love to see. That’s the beauty of this animal. They are so fast in speed.

Jane Reeves
Jane Reeves

@Amber Kerr Not everyone is Australian and it's quite interesting to learn about animals in other countries. What animals does Russia or the USA have that YOU don't know about ... maybe we can comment then about how well OUR natives know about our species of animal life.  Geesh!  Don't be hating on the fact that the world doesn't know everything.

Lorretta Rollinson
Lorretta Rollinson

@Amber Kerr wow , every Australian kid who has ever seen a kangaroo  knows this , yet it takes scientists to tell us . please ....maybe  some staff should have read a little more as kids or travelled 

Louise Platiel
Louise Platiel

@Jane Reeves That's just what I was thinking!   It certainly doesn't give a good impression of Australians that the few who have commented here have thus "DUH, I knew that" attitude.  I live in Canada and home school my son and we found this article VERY interesting.  Do Australians know about Grizzlies? Elk? Caribou? Maybe this isn't as fascinating to you all but don't discount what it might mean for others. 


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