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