Jumping squirrels rotate to brake as they mind the gap Free

Since London Underground began cautioning passengers to ‘Mind the gap’ in 1968, similar warnings have proliferated from Hong Kong and Jakarta to Seattle and São Paolo. But the voids that human passengers clear when disembarking from trains are nothing compared to the distances that squirrels span when they leap from branch to branch. Yet squirrels lack a vital attribute that might seem essential for a life bounding through trees: a grasping hand. Even though squirrels can clutch a nut, they cannot clasp a branch as they hurtle by. Instead, the rodents land palm down on a branch as they reach out, somehow swinging their body to rest with perfect dignity. But if the span is too long or short, squirrels might require more manoeuvrability when they botch a landing. Intrigued by the forces that allow agile squirrels to land safely despite their lack of grasp, Sebastian Lee [University of California (UC), Berkeley, USA] and colleagues rigged a horizontal wooden pole with sensors to record the forces exerted by fox squirrels (Sciurus niger) as they landed leaps.

Instead of studying captive animals, the team decided to investigate the manoeuvres of wild squirrels, because they are more natural. Every few days over a 4 month period, Lee, with UC Berkeley undergraduates, Stanley Wang and Duyi (Tina) Kuang, wheeled the experimental rig – complete with cameras, lights and laptop – to a eucalyptus grove on the Berkeley campus. There, they patiently built a rapport with four of the resident squirrels until the animals could scamper up a ramp to a take-off perch before jumping across gaps, ranging from 50 to 100 cm, to the horizontal pole equipped with sensors as they recorded every detail of the leap. ‘Squirrels will do almost anything for a peanut!’, chuckles Lee.

Back in the lab, Lee, Wang and Kuang painstakingly analysed the squirrels’ postures and impact forces as they touched down, discovering that friction held the squirrels’ front feet in place as they landed, before they reached forward with their hind feet to plant them safely on the perch in a crouching posture. And even though the hind legs were essential for stability at the end of the manoeuvre, the forelimbs absorbed the majority of the impact within the first 65 ms, with the force of impact doubling from 2.1-times body weight during a 50 cm leap to 4.3-times body weight after a 100 cm leap. And when the squirrels misjudged the distance and undershot the landing, their bodies swung beneath the perch until they were able to get a grip with their hind legs. But when they misjudged and overshot, they toppled forward over the perch, delaying the squirrel as it recovered and leaving it vulnerable to lurking predators.

But how did the squirrels bring themselves safely to a stop on top of the perch after an undignified over- or undershoot? The team realised that in addition to relying on friction, when the animals were in danger of toppling forward they exerted a large force through the leg to extend it, which increased the inertia of their body, causing them to brake and slow to a stop. However, when the squirrels were at risk of swinging beneath the branch after undershooting, they reduced their inertia by making the leg more crouched, pulling their bodies into a tighter ball, which increased their swing speed to bring them back up onto the branch.

In short, leaping squirrels are able to finetune their impact forces when they under- or overshoot a landing to come to a dignified stop, despite being unable to get a grip when they mind the gap.