Ventral Surface Microanatomy of the Raptor Pes Is Most Strongly Linked to Prey Size and Diet Rather Than Phylogeny in Australian Species

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 functionally important structures, particularly for predatory species when capturing prey with their feet, as they serve as the initial contact interface between a bird and an object. These scales are found only on the plantar surface of 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 shape was more varied with species that hunted small prey having mostly rounded or spherical papillae apex (e.g., falcons and owls), a mostly flat and rectangular apex shape among species that hunted large prey (e.g., goshawks, large kites and eagles), through to elliptical and flat apex shapes 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.