The gait of kangaroos is a marvel. As they hop, they seem to float 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. Donelan 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 reported 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.

Plant that ate South creeps up

As the climate warms, the vine that ate the South is starting to gnaw at parts of the North, too. Kudzu, a three-leafed weed first planted in the United States more than 100 years ago for the beauty of its purple blossoms, has been spotted in every county in Georgia, Alabama and North Carolina. It chokes young trees, brings down power lines and infests abandoned homes. Now the plant, which can grow as fast as a foot per day, is creeping northward, wrapping itself around smokestacks in Ohio, overwhelming Illinois back yards and even jumping Lake Erie to establish a beachhead in Ontario, Canada.

The plant costs U.S. property owners about $50 million a year in eradication, said the Nature Conservancy. Other estimates are 10 times higher. Agronomists fear what the Department of Agriculture's Lewis Ziska calls the star of "a bad 1950s science-fiction plant movie" will continue to expand, carrying a disease devastating to soybeans.

File photo from 2004 shows a jogger running past Kudzu vines that overwhelm trees and the underbrush along the Potomac River on the Capital Crescent Trail in Washington. Kudzu, a three-leafed weed first planted in the United States more than 100 years ago for the beauty of its purple blossoms, has spread beyond the South, wrapping itself around smokestacks in Ohio, overwhelming Illinois backyards and even jumping Lake Erie to establish a beachhead in Ontario, Canada. Illustrates KUDZU (category a) by Megan Durisin (c) 2014, Bloomberg News. Moving Sunday, July 27, 2014 (MUST CREDIT: Washington Post photo by Lucian Perkins).

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