Functional interpretation of the force-transmitting structures in a tree frog's toe pad

The morphology of tree frogs’ toe pads is adapted towards attachment, allowing these animals to move through their arboreal habitat. Whereas the superficial pad morphology has been studied extensively in previous research, little is known about the internal pad morphology and its functional relevance in attachment. We present the results of a 3-D characterisation and functional analysis of the internal toe pad morphology in the tree frog Hyla cinerea, combining histology, immunohistochemistry, synchrotron micro-computer-tomography, and finite element analysis. The morphology of force-transmitting pad structures, in particular of a collagen layer connecting the ventral epidermis with the medial phalanx, agrees in several aspects with the geometry of a pad model that is topologically optimised towards shear loading. This suggests that tree frogs’ toe pads are primarily adapted towards the transmission of shear loads, as experienced by the frogs while hanging on a vertical substrate. A collagenous septum running from the distal tip of the distal phalanx to the ventral cutis may play a role in peeling and detachment. Furthermore, we show that the adhesive toe pads of tree frogs contain smooth muscle fibre bundles with yet unknown functionality. Overall, our study contributes to the functional understanding of tree frog attachment, hence offering novel perspectives on the ecology, phylogeny, and evolution of anurans, as well as the design of tree-frog-inspired adhesives for technological applications.