When you see a squirrel jump to a branch, you might think (and I myself thought, up until just now) that they’re doing what birds and primates would do to stick the landing: just grabbing the branch and hanging on. But it turns out that squirrels, being squirrels, don’t actually have prehensile hands or feet, meaning that they can’t grasp things with any significant amount of strength. Instead, they manage to land on branches using a “palmar” grasp, which isn’t really a grasp at all, in the sense that there’s not much grabbing going on. It’s more accurate to say that the squirrel is mostly landing on its palms and then balancing, which is very impressive.
This kind of dynamic stability is a trait that squirrels share with one of our favorite robots: Salto. Salto is a jumper too, and it’s about as non-prehensile as it’s possible to get, having just one limb with basically no grip strength at all. The robot is great at bouncing around on the ground, but if it could move vertically, that’s an entire new mobility dimension that could lead to some potentially interesting applications, including environmental scouting, search and rescue, and disaster relief.
In a paper published today in Science Robotics, roboticists have now taught Salto to leap from one branch to another like squirrels do, using a low torque gripper and relying on its balancing skills instead.
Squirrel Landing Techniques in Robotics
While we’re going to be mostly talking about robots here (because that’s what we do), there’s an entire paper by many of the same robotics researchers that was published in late February in the Journal of Experimental Biology about how squirrels land on branches this way. While you’d think that the researchers might have found some domesticated squirrels for this, they actually spent about a month bribing wild squirrels on the UC Berkeley campus to bounce around some instrumented perches while high speed cameras were rolling.
Squirrels aim for perfectly balanced landings, which allow them to immediately jump again. They don’t always get it quite right, of course, and they’re excellent at recovering from branch landings where they go a little bit over or under where they want to be. The research showed how squirrels use their musculoskeletal system to adjust their body position, dynamically absorbing the impact of landing with their forelimbs and altering their mass distribution to turn near misses into successful perches.
It’s these kinds of skills that Salto really needs to be able to usefully make jumps in the real world. When everything goes exactly the way it’s supposed to, jumping and perching is easy, but that almost never happens and the squirrel research shows how important it is to be able to adapt when things go wonky. It’s not like the little robot has a lot of degrees of freedom to work with—it’s got just one leg, just one foot, a couple of thrusters, and that spinning component which, believe it or not, functions as a tail. And yet, Salto manages to (sometimes!) make it work.
Those balanced upright landings are super impressive, although we should mention that Salto only achieved that level of success with two out of 30 trials. It only actually fell off the perch five times, and the rest of the time, it did manage a landing but then didn’t quite balance and either overshot or undershot the branch. There are some mechanical reasons why this is particularly difficult for Salto—for example, having just one leg to use for both jumping and landing means that the robot’s leg has to be rotated mid-jump. This takes time, and causes Salto to jump more vertically than squirrels do, since squirrels jump with their back legs and land with their front legs.
Based on these tests, the researchers identified four key features for balanced landings that apply to robots (and squirrels):
- Power and accuracy are important!
- It’s easier to land a shallower jump with a more horizontal trajectory.
- Being able to squish down close to the branch helps with balancing.
- Responsive actuation is also important!
Of these, Salto is great at the first one, very much not great at the second one, and also not great at the third and fourth ones. So in some sense, it’s amazing that the roboticists have been able to get it to do this branch-to-branch jumping as well as they have. There’s plenty more to do, though. Squirrels aren’t the only arboreal jumpers out there, and there’s likely more to learn from other animals—Salto was originally inspired by the galago (also known as bush babies), although those are more difficult to find on the UC Berkeley campus. And while the researchers don’t mention it, the obvious extension to this work is to chain together multiple jumps, and eventually to combine branch jumping with the ground jumping and wall jumping that Salto can do already to really give those squirrels a jump for their nuts.
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