The biomechanics of tree frogs climbing curved surfaces: a gripping problem

Abstract

The adhesive mechanisms of climbing animals have become an important research topic because of their biomimetic implications. We examined the climbing abilities of hylid tree frogs on vertical cylinders of differing diameter and surface roughness to investigate the relative roles of adduction forces (gripping) and adhesion. Tree frogs adhere using their toe pads and subarticular tubercles, the adhesive joint being fluid-filled. Our hypothesis was that on an effectively flat surface (adduction forces on the largest 120 mm diameter cylinder were insufficient to allow climbing), adhesion would effectively be the only means by which tree frogs could climb, but on the 44 and 13 mm diameter cylinders, frogs could additionally utilise adduction forces by gripping the cylinder either with their limbs outstretched or by grasping around the cylinder with their digits, respectively. The frogs' performance would also depend on whether the surfaces were smooth (easy to adhere to) or rough (relatively non-adhesive). Our findings showed that climbing performance was highest on the narrowest smooth cylinder. Frogs climbed faster, frequently using a 'walking trot' gait rather than the 'lateral sequence walk' used on other cylinders. Using an optical technique to visualise substrate contact during climbing on smooth surfaces, we also observed an increasing engagement of the subarticular tubercles on the narrower cylinders. Finally, on the rough substrate, frogs were unable to climb the largest diameter cylinder, but were able to climb the narrowest one slowly. These results support our hypotheses and have relevance for the design of climbing robots.

A gyroscopic advantage: phylogenetic patterns of compensatory movements in frogs

“…We presumed a close relationship between head compensatory movements and the type of habitat and/or locomotion. A large body of empirical work shows that the habitat and locomotion type is closely linked to morphology and/or performance (Jorgensen and Reilly, 2013;Gomes et al, 2009;Manzano et al, 2017;Moen et al, 2013;Robovská-Havelková et al, 2014;Knight, 2018;Hill et al, 2018). However, our results did not support this hypothesis.…”

Gripping ease in southern green stink bugs Nezara viridula L. (Heteroptera: Pentatomidae): Coping with geometry, orientation and surface wettability of substrate

“…; Hill et al . ). Three‐segmented tarsi bear paired, curved claws, pulvilli and ventral setae with flattened, lanceolate terminals.…”

“…Nezara viridula showed a remarkable grasping performance on convex (curved) substrates. In addition to the characteristic ability of long, slender legs to reach distant terrain elements while walking and transiting on plant leaves, twigs or stems, grasping feet provide a firm grip and stable hold in discontinuous habitats in any position (Hildebrand & Goslow 2004;Schmidt 2010;Endlein et al 2013;Hill et al 2018). Threesegmented tarsi bear paired, curved claws, pulvilli and ventral setae with flattened, lanceolate terminals.…”

Tree frog attachment: mechanisms, challenges, and perspectives

“…The transmission of contact forces via skeletal elements suggests preferential directions of the contact force vector for whole limbs and single digits and thus anisotropic mechanisms of force generation. Gripping as a special case of force directionality is discussed elsewhere [ 39 , 61 , 72 , 73 ].…”