Landing on branches in the frog Trachycephalus resinifictrix (Anura: Hylidae)

Abstract

Frogs (Lissamphibia: Anura) are famous for their saltatory or hopping locomotion, which is related to numerous anatomical specialisations that are characteristic for the group. However, while the biomechanics of take-off in frogs have been studied in detail, much less is known on how frogs land after a jump. Besides terrestrial and aquatic species, several lineages of frogs adopted an arboreal lifestyle and especially the biomechanics of landing on challenging, small, and unpredictable substrates, such as leaves or branches, are virtually unknown. Here we studied the landing kinematics of the arboreal frog Trachycephalus resinifictrix (Hylidae) on a wooden stick that was used to mimic a small tree branch. We observed two different landing behaviours: (1) landing on the abdomen and (2) attachment with the toes of either the forelimb or the hindlimb. In the latter case, the frogs performed a cartwheel around the stick, while they were only attached by their adhesive toe pads. We estimated the forces that act on the toes during this behaviour to be up to fourteen times the body weight of the animals. This behaviour demonstrates the remarkable adhesive capabilities of the toe pads and the body control of the frogs.Electronic supplementary materialThe online version of this article (doi:10.1007/s00359-016-1069-0) contains supplementary material, which is available to authorized users.

When the going gets rough – studying the effect of surface roughness on the adhesive abilities of tree frogs

“…However, in spite of the pad’s low elastic modulus, large scale roughness usually has the opposite effect due to the pad’s inability to mould to the asperities, leading to air bubbles appearing beneath the pad surface. Despite these limitations, tree frogs are still able to generate large forces when landing on a horizontal wooden rod with just one or two toe pads following a jump [49]. They can also climb the narrow twigs and branches of their natural environment by combining adhesion/friction with the ability to grasp even very small twigs [50].…”

Nanoscale friction and adhesion of tree frog toe pads

“…Although the surface of each cell is macroscopically flat, microscopically it is covered by a dense array of nanopillars, called pegs by some early authors, though the name is inappropriate as the nanopillars, indeed the whole pad epithelium, has a low elastic modulus [15]. Although adhesion and friction forces measured from individual toe pads with a custom-built force transducer are in the region of 1-5mN/mm 2 [10,16], a recent study of the forces acting on the toes of tree frogs landing on a wooden stick could be as high as fourteen times the body weight of the frogs [17]. This equates to a force of ca 26 mN/mm 2 , based on a mean force of 0.55 N and a pad area (2 toes) of 21 mm 2 (or as high as 57 mN/mm 2 if the frog lands using only a single toe).…”

Force‐transmitting structures in the digital pads of the tree frog Hyla cinerea: a functional interpretation

“…This is well above the maximum load of 1.27 N measured for single digital pads of Trachycephalus resinifictrix (Bijma et al. ). Considering peak shear stresses of up to 70 kPa ( Rhacophorus dennysi ; Endlein et al.…”

“…For example, we estimate that the ventral collagen layer can withstand a tensile load of 2.0-6.5 N before material failure, based on the measured transverse cross-sectional area and assuming a tensile strength of 100 MPa (Biewener, 2008). This is well above the maximum load of 1.27 N measured for single digital pads of Trachycephalus resinifictrix (Bijma et al 2016). Considering peak shear stresses of up to 70 kPa (Rhacophorus dennysi; Endlein et al 2017) and 140 kPa (Litoria caerulea; Crawford et al 2016) withstood by the epidermal surface, and the high tensile strength of the collagen layer, we argue that the digital pads are adapted primarily towards the generation and transmission of frictional rather than adhesive forces.…”