Spiky tails stick the landing

Gliding through the air is one thing – but landing safely is another. That is a daily challenge for gliding animals such as Pel's scaly-tailed squirrel (Anomalurus pelii), a little-known glider from the forests of West Africa. This large rodent resides high above the forest floor and navigates the complexity of the canopy terrain by either climbing or gliding from tree to tree. When African flying squirrels such as Pel's scaly-tailed squirrel glide, they leap from a branch and spread their arms and legs, which are connected by a flap of skin that works like a parachute, before landing on another tree – a maneuver with life-or-death consequences if insufficient contact is made between squirrel and tree. Many gliding animals rely on the rough bark of trees to dig their sharp claws into for stable landings; however, Pel's scaly-tailed squirrels land on smooth Iroko trees without nooks and crannies in the bark. What sets African flying squirrels apart – beyond their choice of landing site – is that they are the only animals with a row of tiny, hard scales running along the underside of the tail. This led scientists to wonder whether these scales were the key to successful landings on smooth-barked trees.To find out, Andrew Schultz from the Max Planck Institute for Intelligent Systems, Germany, along with colleagues from the State Museum of Natural History, Germany, and the Empa Swiss Federal Laboratories for Materials Science and Technology, speculated that the scales might act like a set of natural cleats, keeping the tail from sliding once the squirrel lands, giving the animal an extra point of support. To test this idea, the team first 3D-scanned a Pel's scaly-tailed squirrel museum specimen to reconstruct the shape and patterns of the squirrel's front and hind claws, and scaly-tail structure. They then 3D printed these parts and mounted them on a simple cross-shaped frame – like a stick-figure version of a flying squirrel – with a long bar for the body and tail, the front perpendicular bar holding the front claws and the rear perpendicular bar holding the hind claws. One version of this stick model included the scaly tail, while the other had a smooth tail.To see how much the tail scales helped with grip, the researchers placed both models on platforms topped with sandpaper ranging in roughness from smoother than the Iroko tree to rougher than it. They then gradually tipped up the front end to increase the incline, recording the angle at which the models slipped off. The model with the scaly tail stayed on steeper platforms than the smooth-tailed version, gaining the biggest advantage on the sandpaper that most closely matched the smoothness of the Iroko tree bark. The scaly tail let the model cling at angles roughly a fifth steeper before slipping, showing that the scales gave a real grip boost.Next, the researchers tested whether the size of the tail scales affected grip. They printed tails ranging in size from 0.5 times the size of a full grown Pel's scaly-tailed squirrel up to 1.5 times the natural size and mounted them on a flat plate. Using the same slip test on a sandpaper closest in smoothness to the Iroko trees, they found that the life-sized version held on better than the larger or smaller versions of the tail at the steepest angles. However, overall grip did not differ much between the different sizes tested.In short, the team showed that these tail scales act like built-in brakes, helping the squirrels hold their position during tricky landings. It is a clever solution for life in the trees – one that not only helps these gliders stick the landing but also could inspire new ways for robots to grip and move through complex environments.