Tree frogs (Polypedates dennysi) landing on horizontal perches: the effects of perch diameter

Secure landing is indispensable for both leaping animals and robotics. Tree frogs, renowned for their adhesive capabilities, can effectively jump across intricate 3D terrain and land safely. Compared with jumping, the mechanisms underlying their landing technique, particularly in arboreal environments, have remained largely unknown. In this study, we focused on the landing patterns of the tree frog Polypedates dennysi on horizontally placed perches, explicitly emphasizing the influence of perch diameters. Tree frogs demonstrated diverse landing postures, including the utilization of: (1) single front foot, (2) double front feet, (3) anterior bellies, (4) middle bellies, (5) posterior bellies, (6) single hind foot, or (5) double hind feet. Generally, tree frogs favoured bellies on slimmer targets but double front feet on large perches. Analysis of limb-trunk relationships revealed their adaptability to modify postures, including body positions and limb orientations, for successful landing. The variations in the initial landing postures affected the subsequent landing procedures and, consequently, the dynamics. As the initial contact position switched from front foot back to the hind foot, the stabilization time decreased at first, reaching a minimum in middle belly landings, and then increased again. The maximum vertical forces showed an inverse trend, whereas the maximum fore-aft forces continuously increased as the initial contact position switched. As the perch diameter increased, the time expended dropped, whereas the maximum impact force increased. These findings not only add to our understanding of frog landings but also highlight the necessity of considering perch diameters and landing styles when studying the biomechanics of arboreal locomotion.