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 go, up to a point, the less energy they use.

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, where they trained five red kangaroos to walk on a force-measuring device and made videos of them.

Dr. Dawson is the scientist who did the classic study on the efficiency of hopping in kangaroos.

Dr. Donelan, who has focused much of his work on humans and how they expend energy, says he recognizes that his study may raise some eyebrows, particularly in the Southern Hemisphere.