Free-ranging squirrels perform stable, above-branch landings by balancing using leg force and nonprehensile foot torque

Abstract

For gap-crossing agility, arboreal animals require the ability to stabilize dynamic landings on branches. Despite lacking a prehensile grip, squirrels achieve stable landings using a palmar grasp. We investigated the landing dynamics of free-ranging fox squirrels (Sciurus niger) to uncover strategies for stable, above-branch landings. Using high-speed video and force-torque measurements in the sagittal plane, we quantified landing kinetics across gap distances. Squirrels rapidly managed >80% of the landing energy with their forelimbs. With larger gaps, peak leg force and foot torque increased. Alignment between forelimbs, velocity and force also increased, likely reducing joint moment. We tested control hypotheses based on an extensible pendulum model used in a physical, hopping robot named Salto. Squirrels stabilized off-target landings by modulating leg force and foot torque. To correct for undershooting, squirrels generated pull-up torques and reduced leg force. For overshooting, squirrels generated braking torques and increased leg force. Embodying control principles in leg and foot design can enable stable landings in sparse environments for animals and robots alike, even those lacking prehensile grasps.