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.

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."