Squirrels: Urban Sports Champions
A popular urban sport takes rapid eye-body coordination.
Squirrels do it flawlessly every time.
Parkour may be a new word to some. It’s an inner city sport that takes no equipment. It does require strength, agility, and eye-body coordination for making rapid decisions. Players leap from building to building, land on rails or parapets, sometimes twisting or somersaulting as they complete a route as fast as possible. They can bounce off of vertical walls to gain traction. They use whatever is available—stairs, rails, benches, ropes or cars—as props for their rapid-fire action. It’s the stuff of stuntmen in the movies, but is done in real time. Watch this sample video on YouTube to see how dangerous it is. When performed by a champion, it really is something to see! These rooftop tricks are the stuff of nightmares, especially when shot by point-of-view (POV) cameras the players are wearing. The police will never catch these guys!
It turns out that squirrels have been doing stunts like this ever since creation. We all know from watching squirrels in the trees that they have no fear of heights; they can tightrope-walk on telephone lines and leap from branch to branch with the greatest of aplomb. They can even leap from one flimsy twig to another, calculating whether it is safe to jump. They rarely make mistakes. One doesn’t usually find the ground littered with dead squirrels. Some of their amazing abilities were mentioned in our 29 July 2021 entry.
Measuring Squirrel Parkour
A team of four scientists at UC Berkeley investigated the daredevil stunts of squirrels. There were willing participants right in the eucalyptus trees on campus. This must have been a fun science project! The team was clearly stoked.
Every day, there are acrobatic extravaganzas going on above our heads. Squirrels navigate remarkably complex and unpredictable environments as they leap from branch to branch, and mistakes can be fatal. These feats require a complex combination of evolved biomechanical adaptations and learned behaviors. Hunt et al. characterized the integration of these features in a series of experiments with free-living fox squirrels (see the Perspective by Adolph and Young). They found that the squirrels’ remarkable and consistent success was due to a combination of learned impulse generation when assessing the balance between distance and branch flexibility and the addition of innovative leaps and landings in the face of increasingly difficult challenges.
Coaxing the squirrels with peanuts, the scientists built an obstacle course for them to observe how they calculate the risks and rewards of increasingly difficult leaps. Their work made the August 6 cover story of Science Magazine. Nathaniel Hunt and Robert Full, with two other colleagues, published a paper, “Acrobatic squirrels learn to leap and land on tree branches without falling.” A press release from UCB is titled: “Leaping squirrels! Parkour is one of their many feats of agility.” Watch the video in the article to see how they designed and filmed squirrels in the obstacle course.
Calculated Risks and Rapid-Fire Decisions
The team found that squirrels instinctively know their limits. When placed on a flexible, narrow strip like a squirrel diving board, the animal knew whether it was strong enough to hold it before takeoff. If not, the squirrel would back up and take a longer leap from a more secure spot. By varying the flexibility of the strip and the distance to the reward, they worked out how the squirrel acrobats decided their strategies. And if they overshoot or undershoot the target, they can grab the post with their claws and do giant swings forward or backward to get to the peanut cup.
“They’re not always going to have their best performance — they just have to be good enough,” he said. “They have redundancy. So, if they miss, they don’t hit their center of mass right on the landing perch, they’re amazing at being able to grab onto it. They’ll swing underneath, they’ll swing over the top. They just don’t fall.”
Like human parkour players, squirrels can even bounce off a vertical wall to get more traction. Most of the time, they land with pinpoint accuracy on the tiny platform, with all four feet fitting tightly together, as they quickly retrieve the nut. But the staged performances on camera are surely a small fraction of their daily lives, scampering vertically up tree trunks, leaping from branch to branch, navigating tight spots and thin twigs. Their sneaky parkour can be a nuisance at the picnic table, but they deserve our respect. Consider, too, that they don’t have the same degree of binocular vision humans do. How does the world look through squirrel eyes? Another amazing thought is that flying squirrels make super-long leaps with gliding ability. They must be able to see and calculate their ability to “stick” the landing over even longer distances.
Other Amazing Parkour Animals
Any animal that leaps and travels through the trees needs this combination of abilities: rapid-fire decision-making, balance, agility, good eyesight, and resourcefulness. Tarzan swinging on vines is no match for a gibbon acrobatically moving through the jungle grabbing branch after branch with little time for planning or correction. Some lizards live in trees, too. They skitter up and down trunks and from branch to branch with ease. There are even gliding snakes that can take off from high branches and make do with wherever they end up, whether on the ground or another branch. Can you think of other animals that have the requirements for parkour?
Commenting on the research in the same issue of Science, Karen E. Adolph and Jesse W. Young make another astute observation about the requirements for these amazing feats: animal bodies change over time! The animal has to make adjustments as it grows and proceeds through bodily changes:
Squirrels are not alone in the precision and creativity of their locomotion. Every animal must perceive and exploit affordances for locomotion under variable conditions. Bats and iguanas alter locomotor forces to compensate for increased body mass associated with feeding and pregnancy. Running guinea fowl adjust limb postures within a single step to maintain stability after an unexpected drop.
The Human Element
Back to human parkour stunts, Adolph and Young make some interesting observations about human babies:
Likewise, human infants gauge affordances with exquisite precision and invent new locomotor strategies on the fly (e.g., sliding down steep slopes or high drop-offs on their bottoms, backward feetfirst, or headfirst like Superman).
Part of this ability in babies is inborn, but some of it has to be learned.
Human infants can grow up to 2 cm in a single day. One week, babies are crawlers; the next, they are walkers—yesterday, objects on the coffee table were out of sight and beyond reach; today, they are accessible. Thus, learning occurs in the context of development, and the flux of body growth and motor-skill acquisition ensures that infants do not learn fixed solutions. Indeed, static solutions would be maladaptive in a continually changing ecosystem. Instead, infants “learn to learn.” They learn to detect information for affordances at each moment to determine which actions are possible with their current body and skills in a given environment.
How Did These Abilities Come About?
The Berkeley team really didn’t need Darwin. They inserted the e-word “evolution” at two points, it seems, out of habit or obligation. Earlier, we saw them speak of “evolved biomechanical adaptations” (what?). In the last sentence, they say,
The role of fast and accurate leaping in driving the evolution of biomechanical capabilities, learning-based decision-making, and innovation promises to reveal the mechanisms and origins of arboreal agility.
Oh come on. They say that more work will be required to “reveal the mechanisms and origins” of these abilities. One has to wonder if the peer reviewer(s) insisted that the authors insert the e-word, because it certainly did not add anything to understanding.
Here’s another difference between human and animal parkour. Squirrels and other animals move like they do for food or to escape predators. We don’t know if they do it for fun as well. Otters like to slide down snowbanks into the river, and will repeat the trick apparently for pleasure. Your dog can do tricks but expects a treat for it; other dogs don’t sit and watch and applaud or raise score cards. At dog and horse shows, it’s the people that keep score. Humans alone will do things like parkour that are not necessary for survival – in fact, survival seems the least of their concerns sometimes. The video collections do not show the guys in the hospital who failed to stick a landing.
There is something that is merely exhilarating about completing a difficult trick, exercising one’s strength and agility in a creative way and getting the congratulations of friends. The Olympics just ended in Tokyo; is that not the point? Achieving excellence in a skill is its own reward. The glory for these abilities, though, begins with the Creator who made these abilities possible. Much of the content on YouTube demonstrates that human beings have the most impressive set of physical and mental and creative abilities on earth.
Need more? Try this video, and see how the players use walls as part of their daredevil antics. Sweet dreams!