A study in Biology Letters suggests that a walking kangaroo propels itself with its tail, essentially transforming the appendage into a fifth "leg." The study showed that the tail of a kangaroo, while walking, works as hard as the leg of a human who is walking at the same speed.
The study author's speculate that the use of the tail for walking was evolved to make the unique kangaroo maneuver more efficiently. No other animal is known to use its tail for walking.
Terence Dawon, study co-author and an emeritus professor at University of Wales, said, "Kangaroos can hop at 12 miles an hour over long distances; they cruise along far more economically than other animals run. As it hops, the kangaroo's long tail whips up and down, helping the animal control the angle of its body."
Scientists have never tried to find out the force generated by keeping the tail on the ground to act as an aid when its legs are off the ground. To measure the forces, the researchers cajoled the kangaroos to walk along a platform with a low ceiling, which prevented them from gearing up into a hop. The kangaroos were easy to tame and a bucket of feed pellets helped a lot, noted Dawson.
Study co-author, Shawn O'Connow of Simon Fraser University, Canada, explained that 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. The force generated by the tail equals that of the force frm the combination of the front and hind limbs. Moving between plants, the tail helps the animal save energy.
Views of other researchers
Michael Bennett of the University of Queensland: 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 who was not associated with the research. So the finding that the tail pushes the kangaroo forward came as no surprise.
Andrew Biewener of Harvard University: The study confirms what I would've expected. They are five-legged animals when they're using their tail."
Kristian Carlson of University of Witwatersrand, South Africa: What is surprising is the extent to which the tail is propelling the body forward and the amount of force it's providing. It's amazing what these kangaroos are doing.