The signature gait of kangaroos is a marvel. As they hop, they seem to float along almost effortlessly on their large, springy hind feet, tail stretched out behind for balance.
A classic study showed that the faster they went, up to a point, the less energy they used.
But they also have a slower, walking motion that they use when they are feeding, moving only a few feet at a time to the next patch of grass. And for that, they depend on the tail not for balance or as a kind of crutch to lean on, but as a muscular, very important fifth leg.
J. Maxwell Donelan and Shawn M. O'Connor, both of Simon Fraser University in British Columbia, and colleagues studied kangaroo walking in the lab of Terence J. Dawson at the University of New South Wales in Australia, where they trained five red kangaroos to walk on a force-measuring device and made videos of them.
Dawson is the scientist who did the classic study on the efficiency of hopping by kangaroos.
Donelan, who has focused much of his work on humans and how they expend energy, said that "anybody who has ever seen a kangaroo walk knows that it uses its tail." But what interested him was how the animal used it.
It turned out, as he and his colleagues report in the journal Biology Letters, that the tail exerted as much force as the four other legs combined. It is as important in kangaroo walking as one of our two legs is in human walking. If a leg is defined by its function and contribution to movement, Donelan said, the kangaroo has five legs when it is walking.