WATCHING A SQUIRREL dart through a forest canopy will make any gymnast green with envy. Squirrels sprint along fences; skitter up, down, and around tree trunks; and pull off awe-inspiring feats of agility. What makes these small, fluffy mammals such masters of acrobatics? A mixture of morphology, motor skills, and sheer chutzpah. Squirrel anatomy is specialized for treetop antics. Their hyperflexible back ankles can rotate a full 180 degrees. This allows squirrels to take full advantage of their curving claws, explains Nathaniel Hunt, who studies biomechanics at the University of Nebraska Omaha. By reversing their ankles, squirrels can dig their back claws into a tree's bark even while descending headfirst. It's a rare talent. House cats and other species in the genus Fe//s, for example, can't twist their ankles all the way around-which is why cats can climb up trees but have trouble getting down, leaving squirrels squealing with mirth. A squirrel's back legs aren't just flexible; they're also strong. "They have this kind of big musculature around their hind limbs that really lets them propel themselves," Hunt says. He routinely sees fox squirrels (Sciurus niger) leap more than six feet. The average squirrel is a little under a foot long (without the tail). "That's over six times the length of their body."
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