Squirrels Use Gymnastics to Navigate Treetop Canopies

For the past two weeks, much of the world has watched, transfixed, as Olympic gymnasts flip, leap and vault for the gold. Under the glare of the spotlight, a lifetime of practice and physical mastery has come to fruition as these athletes perform superhuman tricks the average human could never fathom executing.

But outside the stadiums of the Tokyo 2020 Olympic Games, a different kind of acrobatic feat is performed every day. Both closer to the Olympics, in the forests of Japan, and in treetops all around the world, squirrels leap meters through the air to get from branch to branch. The stakes are different in this natural arena: the squirrels scurry around to find morsels of food, all the while trying to evade occasional airborne predators such as hawks. But the speed and ease with which they navigate the challenging and unpredictable canopy environment is “spectacular,” says University of California, Berkeley, biomechanics researcher Robert Full. The animals easily land leaps several times the length of their body. And we do not really know how they do it, Full says. “How do they know that they have the capability in their body to achieve those jumps?” he asks.

The coordination between a squirrel’s body and mind is not just a curiosity for human observers, Full explains. The well-executed moves could influence the design of smarter robots, incorporating some of the squirrels’ best physical traits: their flexible spine, grippy paws and grabby claws. And squirrels are not just daredevil acrobats; they are adept learners, too. “They have really good memories,” says Gregory Byrnes, who studies biomechanics at Siena College in Loudonville, N.Y. A squirrel you see scampering through the park has likely followed that same path before and mapped the route out in its brain, Byrnes says. And when the weather turns cold in the winter, the diligent rodents recall and retrieve the many caches of nuts they have stashed all over their territory. It is this squirreling away that differentiates them from other arboreal animals, such as some primates, says Nathaniel Hunt, who studies biomechanics at the University of Nebraska Omaha. Pair that learning ability with a responsive, flexible body, and you’ve got the makings of an extraordinary robot, he says.

Hunt and his colleagues wanted to evaluate three aspects of squirrels’ skills: their decision-making, learning and capacity for innovation. To capture the capabilities of free-ranging animals, the researchers decided to test these qualities in wild fox squirrels in an outdoor setup in the woods near U.C. Berkeley. First they had to get the wild squirrels to actually participate in experiments— not an easy task with such wily animals. “There were some squirrels that would get distracted and just decide, ‘It’s time to take a nap,’” Hunt says. Luckily, most of the subjects understood what to do after they had a few tries and received several reward peanuts. As any gardener with a bird feeder will tell you, “they’re very food-motivated,” Hunt says.

The researchers put the squirrels through an outdoor obstacle course and tracked their leaps with high-speed video. In one task, they coaxed the subjects to jump across gaps of different distances from branchlike wooden rods that behaved like diving boards. The ledges bent easily under the squirrels’ weight, depending on how far the animals stood from the “tree trunk.” This situation forced them to make a decision: Should they jump from the end of a branch to shorten the distance but risk an unstable bend when that branch is compliant? Or should they stay closer to the “trunk” when they launch but risk the longer jump? The squirrels were six times more sensitive to the bendiness of the branch than to the distance of the jump, the researchers report in a paper published today in Science. “They’re much more influenced by the amount the branch is going to flex rather than the distance they have to leap across,” Hunt says of the squirrels.

In their little brain, there might be an element of self-confidence at play: the squirrels are much surer of their own jumping ability than they are that the bending branch underneath them will not snap and fall, Hunt postulates. When the scientists altered the flexibility of the branch or the distance to leap, the squirrels quickly adjusted their technique to match the changes, learning within just a few tries to alter the velocity of their launch.

But the real fun started when the team placed the “branches” against a vertical wall to extend the leap distance even farther. Surprisingly, instead of merely jumping between ledges, the squirrels bounded and pushed off the wall in a wide array of parkourlike moves, which the researchers quantified and cataloged. The squirrels used a wall to establish an additional point of contact to make midair adjustments, either further propelling themselves forward or braking to slow down as they careened off the vertical surface, Hunt says. “One thing that really surprised me was the extent to which they would use any kind of structure they could reach,” he says. The scientists were not expecting the squirrels to use the wall to their advantage or to refine their parkour techniques over subsequent trails, Full explains. “They innovated right in front of our eyes!” he says. None of the squirrels fell in any of the trials. Some did not land on the target surface with perfect finesse: they might swing under or keel over the platform, for example. But the animals made the landing every time.

“This is really cool,” says Byrnes, who was not involved with the research. “It’s a great quantification of these things that animals do but which we don’t know how to describe.” Byrnes says he is curious to learn about the deeper mechanics of the leaps to see what the limbs are doing and how much energy a squirrel loses with different types of jumps. The work by Hunt and Full’s team is part of a larger, multi-institutional collaboration to study and apply the principles of squirrel locomotion. In addition to creating robots that can make similar moves, researchers are studying what exactly happens in a squirrel’s brain as it makes these split-second leaps and how the act of making these jumps develops over time as a baby squirrel grows up. Though the animals are ubiquitous, Full says, we are just starting to understand and appreciate how they scamper through the treetops. “They make great choices, and they’re very creative,” he adds. “They’re extraordinary.”



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