Biomechanics is the study of how living things move through the world. In Hunt’s case, that means studying fox squirrels, Sciurus niger. He was the lead author of a paper in the journal Science that explored squirrel mechanics.
Hunt and his collaborators conducted three experiments. All involved convincing squirrels on the campus of the University of California at Berkeley — where he was a graduate student — to leap from Point A to Point B, while their movements were recorded on high-speed cameras. The distances ranged from two feet to five feet.
Hunt was interested in how different variables would affect the choices the squirrels made. In the first experiment, researchers varied the bendiness of the squirrels’ launching perch. This bendiness — compliance, in biomechanics-speak — approximated the flexibility of a thin branch.
Jumping from near the sturdy, trunk end of the branch would be more stable, but mean more of a distance to cover. Jumping from the far end of the branch would reduce the gap, but the bendy tip would be less stable.
In the second experiment, the launching perch was even more compliant — bendy — along its entire length. In the third experiment, the gap was widened and a wall was placed parallel to the direction of travel.
The early leaps were not always elegant. A perfect leap involved landing on the “branch” with the front paws, followed quickly by the back paws. At first, some squirrels swung over the rigid landing rod, others under. But none fell — “That was kind of surprising,” said Hunt — and after about five jumps they all had it mastered, even when launching themselves from the tippy end of the bendiest launchpad.
Not that they relished that. They squirrels preferred leaping from something steadier but farther from the target than from something bendier but closer.
Most impressive was how the squirrels used the wall. They would rotate their bodies, align their feet with the wall, then bounce off the wall before landing on the perch, a move Hunt likened to parkour.
“They tended to use it in almost all situations,” Hunt said. “They actually used it to modify their velocity as they were coming into the landing.”
If they were coming in too slowly, they would bounce off the wall to pick up speed. If they were coming in hot, they would bounce off the wall to scrub some speed.
Why are squirrels good subjects for biomechanical study?
“They have a couple different capabilities that make them very intriguing,” Hunt said. “One is their athleticism, their ability to generate these big forces with their hind limbs, propelling themselves in the air in very controlled ways, to jump and move on the target, and move quickly through a challenging environment.”
Their brains are good at rapid problem-solving.
Said Hunt: “One of the big questions we’re trying to learn more about is how squirrels are able to chain together multiple types of movements. When they have to run up the trunk of
a tree and jump off a branch to another trunk, all of these things are very complex and require the squirrel to move through different body positions.”
A squirrel on the run through the treetops is never completely stable, Hunt said. It has to commit to movement. If it freezes, gravity will take over.
“As long as they’re moving, they’re dynamically stable,” Hunt said. “They’re able to keep moving without falling.”
Hunt continues to study squirrels, with an eye toward imbuing the mechanics with the essence of the bio.
“If we want highly mobile autonomous robots, then they need to cover all kinds of complex terrain,” he said. “That seems to require different types of movement: jumping, bounding, galloping.”
In other words: You think those dog robots are cool? Just imagine a robot modeled on a squirrel.
I asked Hunt how he convinced the squirrels to participate in his research.
“They were highly trainable with peanuts,” he said. “After they learned what they needed to do they would do it over and over again, until, an hour later, they were done eating and would just wander away from us.”
How a belly full of peanuts affects a squirrel’s agility is perhaps a subject for another time.
Tomorrow: Squirrel Week comes to a close with a visit to Franklin Square.