Squirrels show killer parkour moves by jumping from branch to branch

The University of California, Berkeley campus is home to three known wild populations of fox squirrels, which spend their days frolicking among tree branches and foraging for food. They also made a contribution to science, through a series of experiments conducted by researchers at UC-Berkeley to assess how squirrels determine whether or not to jump from a given tree branch to another.

In the process, the team caught several squirrels that used innovative, parkour-like movements to perform particularly delicate maneuvers, reorienting their bodies to push back a vertical surface to ensure a safe and smooth landing. . The team described their findings in a new article published in the journal Science.

Squirrels are masters of the art of navigating the treetops, leaping from branch to branch without falling. “As a model organism for understanding the biological limits of balance and agility, I would say squirrels are second to none,” said co-author Nathaniel Hunt, a former UC-Berkeley graduate student. who is currently studying biomechanics at the University of Nebraska. , Omaha. “If we try to understand how squirrels do this, then we can discover the general principles of high performance locomotion in the canopy and other complex terrain that apply to the movements of other animals and robots.”

The UC Berkeley team figured the key might be learning by trial and error and built an outdoor obstacle course in a small forested area on the west side of campus (Eucalyptus Grove / Grinnell Natural Area). “Squirrels in this population were reliably found in the study area on a daily basis,” the authors wrote in their supplemental material. They used a syringe to mark individual squirrels with black fur dye in different patterns to track them.

The experimental apparatus consisted of a ramp leading to launch platforms of varying flexibility and rigidity – essentially rods that simulated branches, magnetically attached to a vertical steel wall – and a landing perch in face (a stud wrapped in athletic tape), with varying distances between them. There was also a mug on the perch containing a peanut to motivate the squirrels to make the jumps between the launch pad and the perch.

The experiments were preceded by a training period in which small pieces of peanuts were placed on the ramp to the jumping platform to attract the hungry squirrels, with half a peanut in a cup at the end of the beam as a reward. Once the squirrels were comfortable taking the peanut out of the cup, the team began to gradually increase the gap between the end of the beam and the landing platform. Most squirrels have mastered this task in 30 minutes.

The first experiment was about decision making. The team used three “branches” of varying flexibility attached to the jumping platform: a birch rod, a plastic tube, and a plastic tube covering a brass rod, all wrapped in duct tape so that squirrels have constant traction. The variation in perch flexibility was crucial to the experiment, as it forced the participating squirrels to find a workable compromise between stability and jumping distance.

UC Berkeley researchers made things a bit more difficult for the squirrels in the second experiment, increasing both the range of flexibility or stiffness of the limbs and the minimum distance of the gap. They used a rigid beam as a witness.

Squirrels figured out how to change the biomechanics of their jumps quite quickly. They were a bit more careful when it came to jumping off more flexible dummy branches, and it took them a few tries to master it. “This behavioral flexibility that adapts to the mechanics and geometry of the jump and landing structures is important for accurately jumping through space to land on a small target,” Hunt said.

Hunt et al. observed several distinct landing maneuvers that the squirrels used to compensate for jumps that were too fast or too slow. For example, they would roll forward around the branch if they passed the landing platform when they jumped and grabbed the branch with their front legs and swayed under it if their jump fell a bit short, pulling themselves up to get the peanut. And sometimes the squirrels would perform the jumps just to land a direct landing on the perch.

Ultimately, the team found that the flexibility or stiffness of the landing pad was the most critical factor in whether the squirrels decided to jump, far more than the distance of the gap. And none of the squirrels have ever fallen, maybe because they have such sharp claws.

“They won’t always have their best performance, they just have to be good enough,” Hunt said. “They have redundancy. So if they miss, they don’t hit their center of mass right on the landing perch, they’re amazing to be able to hang on to it. They’ll swing underneath, they’ll swing. above the at the top. They just don’t fall. This combination of adaptive planning behaviors, learning control, and responsive stabilization maneuvers helps them move quickly through branches without falling. “

The researchers were surprised to find that the squirrels sometimes resorted to innovative parkour-like movements to bridge the gap and achieve that tasty peanut, reorienting their body mid-jump so they could push back the vertical wall, thus adjusting their speed for smoother movement. landing. To test how often the squirrels used this strategy, the team changed both the gap distance and the landing height. In total, ten squirrels completed a total of 324 parkour jump attempts. The team found that squirrels used this maneuver quite regularly for medium and long jumps, but never for shorter jumps. The height of the landing pad was not a relevant factor in these decisions.

“Just as movement in the real world requires flexibility and creativity, researchers who study natural locomotion must be as resourceful as their animal subjects,” wrote Karen Adolph (New York University) and Jesse Young (Northern Medical University). east of Ohio) from a support perspective. “The trick is to capture the movement in all its complexity while retaining sufficient experimental control and measurement fidelity. Hunt’s study et al. is a fine example. Their unexpected results shed light on what any owner knows: Squirrels are intelligent acrobats when navigating complex environments. “

The UC-Berkeley team will continue to study the intricacies of squirrel biomechanics and its connection to cognition in the hopes of one day building a robot with similar jumping abilities. “I see this as the next frontier: how are movement decisions shaped by our bodies? Said co-author Robert Full. “It’s much more difficult, because you also have to assess your surroundings. This is an important question of fundamental biology. Fortunately, we can now understand how to embody control and explain innovation by creating physical models, like the most agile intelligent robots ever built. “

Main article: DOI: Science, 2021. 10.1126 / science.abe5753 (About DOIs).

Related perspective: DOI: Science, 2021. 10.1126 / science.abe6733 (About DOIs).

List image by Judy Jinn, UC Berkeley