Kangaroos can hop great distances. Some may even think those distances are unnatural for a four-legged creature. That's because they are, thanks to kangaroo tails.

Researchers studied red kangaroos, the largest kangaroos in Australia, and observed that they use their tails as a kind of fifth leg to propel themselves through their jumps. The kangaroo apparently produces an incredible amount of moving power with its tail, like a skateboarder pushing off the ground with his or her other leg to increase speed.

Researchers at the University of Colorado Boulder, Simon Fraser University in Burnaby, Canada and the University of New South Wales in Sydney, Australia published their observations in a paper released on July 1 in Biology Letters.

"We found that when a kangaroo is walking, it uses its tail just like a leg," says Maxwell Donelan, a corresponding author of the paper, in a press release. "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."

The researchers studied red kangaroos as they foraged and awkwardly used their tails as fifth legs to propel themselves forcefully across the grass. Their hind feet move forward simultaneously as their front feet and tails balance and support their weight. No other animal uses its tail like this, says Donelan.

"Their tails have more than 20 vertebrae, taking on the role of our foot, calf, and thigh bones."

The study used field observations as well as laboratory experiments. Professor Terence Dawson of the University of New South Wales observed a small group of kangaroos as they hopped on a large treadmill. They were trained to walk on a platform that measured forces from the legs and the tail. Dawson set the treadmills to various speeds and measured the energy produced by the propellant tails. Using their tails, kangaroos increased their metabolism to 50 times their normal rates.

The researchers also found that the tails are used as support. Male kangaroos often fight for reproductive dominance, springing back on their hind limbs and kicking each other in the stomach. Researchers saw that their tails are used as counterbalances when the kangaroos rear, helping support their weight in a fight. This evidence and the use of the tail in hopping show that the kangaroo, once thought to be an odd, primitive species, is actually quite adapted and clever.