Ventral Digit Structures II

The ventral microstructures on the digits of birds are comprised of reticulate scales. This epidermal structure presents as cornified papillae, rather than the typical flattened scutellate scales (scutes) found on the dorsal surfaces of the tarsus and toes. Reticulate scales are likely important structures, particularly for predatory species when capturing prey with their feet, serving as the initial contact interface between a bird and an object. These scales are found only on the plantar surface of the feet and digits, and as such these structures offer a unique avenue of enquiry when investigating the various physical adaptations possessed by raptors in the context of predatory function. In this study, we investigate papillae morphology, density and size across a range of both predatory (diurnal and nocturnal) taxa and non-predatory representatives (Passeriformes, Psittaciformes) to identify and quantify differences. These differences are then assessed from a functional viewpoint using prey size categories. The most common overall papillae shape, seen across most species, was cylindrical. However, papillae apex was more varied, with species that hunted small prey having mostly rounded or spherical papillae apex shape (e.g. falcons and owls), mostly flat and rectangular apex shape for species that hunted large prey (e.g. goshawks, large kites and eagles), to elliptical and flat in the Passeriformes.
Papillae densities varied between the digits, and across taxa, with goshawks having the highest maximum papillae density across the raptorial species studied. There was low phylogenetic signal, in these patterns and linear discriminant analyses indicated that papillae size clearly separated based on prey sizes, suggesting that the development of these structures could be influenced by diet.