Tree frogs Polypedates Dennysi landing on horizontal perches: the effects of perch diameters.

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 to 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 frogs Polypedates Dennysi on horizontally placed perches, explicitly emphasizing the impact 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 favour bellies on slimmer targets but double front feet on large perches. Analysis of limb-trunk relationships revealed their adaptability to modifying postures, including body positions and limb orientations, for successful landing. The variations in the initial landing postures affect the succeeding landing procedures and, consequently, the dynamics. As the initial contact position was switched from front foot to hind foot, the stabilization time decreased first, reaching the minimum in middle belly landings, and then increased. The maximum vertical forces changed in an inverse trend, whereas the maximum fore-aft forces continuously increased as the initial contact position switched. As the perch diameter rose, the time expenses dropped, whereas the maximum impact increased. These findings not only added to our understanding of frog landings but also highlighted the necessity of considering perch diameters and landing styles when studying the biomechanics of arboreal locomotion.

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