What makes a leg a leg? What if a leg is a kangaroo tail?

In a recent study, scientists observed, recorded, and measured data for five red kangaroos that were trained to walk slowly on a force-measuring platform. Their conclusion from that data: Kangaroos essentially have five legs! When the animal is walking, its muscular tail is used to “support, propel and power their pentapedal gait just like a leg,” and exerts as much force as the four other legs combined.

In the archives: more from ScienceTake and more animal locomotion videos.