Vladislava Urazova has nothing on fox squirrels. Sure, this year’s Russian gold medalist executed mind-bending twirls and flips on a balance beam only slightly wider than a credit card at the summer Olympics. But at least her beam didn’t bend—or break. Now, scientists are getting a better sense of how squirrels and other tree-dwelling creatures leap and bound through their death-defying habitats—and still stick the landing.
It might seem obvious that these agile animals are relying on more than just instinct, but a new study shows their prowess like never before, says Jesse Young, an organismal biologist at Northeast Ohio Medical University who was not involved with the work.
To systematically investigate squirrels’ leaping skills, integrative biomechanists at the University of California, Berkeley, ventured to the edge of campus, where fox squirrels (Sciurius niger) are plentiful. Nathaniel Hunt, now at the University of Nebraska, Omaha, and colleagues set up an outdoor laboratory where the centerpiece was a steel sheet suspended 1.5 meters off the ground. Mounted to it was a flexible horizontal pole and a landing perch, separated by an adjustable gap. The goal was to get squirrels to leap from the pole to the platform, thus replicating their leaps and bounds in nature.
To lure the fuzzy-tailed acrobats to their obstacle course, the scientists put out peanuts and waited. Once the rodents realized there were treats, they readily learned to climb a ramp to the pole and jump to the platform for another peanut. The researchers used poles of varying flexibility to simulate tree branches of different diameters and stiffness. They also adjusted the distance from the free end of the pole to the landing platform—and filmed their experiments using high-speed video.
In their first experiment, the researchers used three poles: a birch rod, a hollow plastic tube, and a plastic tube with a brass rod inside. After multiple jumps from each pole, the squirrels were challenged with a much thinner, floppier steel beam. Researchers also nearly doubled the size of the gap between the pole and the platform, making it about four squirrel body lengths—quite a long way for a squirrel.
In their final experiment, the scientists tested the limits of the squirrels’ agility by varying both the distance to the landing perch and its height. As the challenge got more difficult, the squirrels adjusted their movements, changing where—and how—they took off and landed. Before the trials, scientists had thought the squirrels’ main goal would be to minimize the distance of the gap they had to cross. Instead, the more bendy the “branch,” the sooner the squirrels jumped—even though they had a longer way to go before landing, Hunt’s team reports today in Science (see video, above).
“If that squirrel misses, it’s potentially a bad scene,” says Jake Socha, a biomechanist at Virginia Polytechnic Institute and State University, who was not involved with the work. “So, how they choose what they do and how they execute it is really a life and death decision.” Fortunately, the squirrels in these experiments never fell.
Scientists have long debated how much nonprimates “think” about their actions—and how much they can learn from previous experience. The new work demonstrates that squirrels do consider properties of branches and adjust what they do accordingly, Socha says. He also notes that over the course of several jumps, the squirrels adjusted their launch to ensure a more perfect landing.
That’s not all. When the gap was wider than the squirrels expected—and if it looked like they might miss their landing—they often maneuvered their bodies to extend their leaps, rolling, twisting, and even bouncing off the steel sheet like “free-runners,” or parkour athletes. “This implies that the squirrels have an impressive level of control both during and after leaping,” says Christian Brown, a graduate student working on tree-dwelling salamanders at the University of Southern Florida.
Lindsey Reader, a comparative biologist at the University of Utah who was not involved with the work, praises the study for duplicating what the squirrels might encounter in their daily life. “They illustrated the range of conditions that the squirrel can deal with,” she says, noting that such on-the-fly adjustments help squirrels deftly get through complex environments, such as treetops.
“With the Olympics, it’s taken for granted that the more you do, the better you get at it, but it’s not been clear what the capacity for [learning] in animals is,” says Christofer Clemente, a comparative biomechanist at the University of the Sunshine Coast who was also not involved with the work. “This shows they can change.”