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 Felis, 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."

This anatomical advantage is coupled with impressive learning skills. In a study published in Science in 2021, Hunt and his colleagues showed that squirrels leaping to reach a tempting peanut altered their takeoff points based on the branch they were on. On a stiff branch, the animals went farther out for a shorter jump. The squirrels leaped sooner from a bendy branch.

The squirrels in the study learned by trial and error. One attempt might be an overshoot, sending the squirrel swinging above and around the branch. Another might not be quite enough, leaving them hanging off by their front paws. The squirrels never fell, but these near misses taught them how to stick the landing in future attempts.

What squirrels definitely have is moxie. When Hunt and his colleagues began widening the distance between the branches and the reward, instead of leaping farther, the squirrels employed parkour moves, bouncing off a vertical surface between the branch and the peanut, using that surface as an assist. "It seems like squirrels just reach out and touch anything they can," Hunt says. "If there's a way to get one foot on something and get a tiny boost off that, they'll do it."