When kangaroos move slowly, their muscular tails work as a fifth leg.
Despite their reputation for bouncing, kangaroos spend much of their day walking on all four legs while they munch on grass. To determine the tail's role in this gait, Max Donelan at the Simon Fraser University in Burnaby, Canada, and his colleagues videotaped five red kangaroos (Macropus rufus, pictured) as they ambled across a force-measuring platform. The kangaroos' tails propelled them forward and lifted them, providing an average of around one-quarter of the positive mechanical work involved in walking.
Per kilogram of body mass, the animal's tail provided as much useful work as a single leg for a walking human, the researchers note.
Biol. Lett. http://doi.org/thh (2014)