The skin of birds' feet: Morphological adaptations of the plantar surface

Abstract

The skin of the foot provides the interface between the bird and the substrate. The foot morphology involves the bone shape and the integument that is in contact with the substrate. The podotheca is a layer of keratinized epidermis forming scales that extends from the tarsometatarsus to the toe extremities. It varies in size, shape, amount of overlap and interacts with the degree of fusion of the toes (syndactyly). A study of toe shape and the podotheca provides insights on the adaptations of perching birds. Our analysis is based on micro-CT scans and scanning electron microscopy images of 21 species from 17 families, and includes examples with different orientations of the toes: zygodactyl (toes II and III forward), anisodactyl (toes II, III, and IV forward), and heterodactyl (toes III and IV forward). We show that in these three groups, the skin forms part of a perching adaptation that involves syndactyly to different degrees. However, syndactyly does not occur in Psittacidae that use their toes also for food manipulation. The syndactyly increases the sole surface and may reinforce adherence with the substrate. Scale shape and toe orientation are involved in functional adaptations to perch. Thus, both bone and skin features combine to form a pincer-like foot.

Exploring the Ecomorphology of Two Cretaceous Enantiornithines With Unique Pedal Morphology

“…Predominant among anisodactyl-syndactyl birds in which digits III and IV are joined through soft tissue, thus increasing the surface area of the lateral portion of the foot similar to the condition in YLSNHM01001, are the Alcedinidae (kingfishers) and Meropidae (bee-eaters), both groups that forage from frequently mobile perches that require increased grip (Backus et al, 2015). The deep interpad sulci like those observed in YLSNHM01001 are also found in some arboreal taxa including some syndactyl species (Höfling and Abourachid, 2020).…”

“…The avian foot represents the interface between the body and substrate and in some groups is highly involved in prey acquisition and object manipulation (Tsang and McDonald, 2018;Höfling and Abourachid, 2020). As such, the foot can be highly informative with regards to ecology.…”

“…However, in these groups fusion is minimal, typically with only the proximal portion of digits III and IV conjoined. Increasing the plantar surface of the foot increases friction with the substrate and thus improves grip during perching (Höfling and Abourachid, 2020). Predominant among anisodactyl-syndactyl birds in which digits III and IV are joined through soft tissue, thus increasing the surface area of the lateral portion of the foot similar to the condition in YLSNHM01001, are the Alcedinidae (kingfishers) and Meropidae (bee-eaters), both groups that forage from frequently mobile perches that require increased grip (Backus et al, 2015).…”

“…The soft tissue morphology F. prehendens is also inconsistent with interpretations regarding raptorial function. In general, the plantar pad papillae are far more exaggerated, an adaption for dispersing pressure, with a sharp texture that increases grip (Lovette and Fitzpatrick, 2016;Höfling and Abourachid, 2020). Furthermore, the interpad sulci are more weakly defined compared to F. prehendens (Höfling and Abourachid, 2020).…”

“…Both the skeletal structure and soft tissue morphology of the avian foot are functionally important and ecologically informative because these morphologies reveal how the bird interacts with its substrate (Höfling and Abourachid, 2020), and in some groups are highly involved in the acquisition and manipulation of food (Tsang and McDonald, 2018). The placement, shape, length, and robustness of extant bird pedal digits vary greatly, and this variation correlates with ecological habit (Tsang and McDonald, 2018;Gill et al, 2019).…”