Altriciality and the Evolution of Toe Orientation in Birds

Abstract

Specialized morphologies of bird feet have evolved several times independently as different groups have become zygodactyl, semi-zygodactyl, heterodactyl, pamprodactyl or syndactyl. Birds have also convergently evolved similar modes of development, in a spectrum that goes from precocial to altricial. Using the new context provided by recent molecular phylogenies, we compared the evolution of foot morphology and modes of development among extant avian families. Variations in the arrangement of toes with respect to the anisodactyl ancestral condition have occurred only in altricial groups. Those groups represent four independent events of super-altriciality and many independent transformations of toe arrangements (at least four zygodactyl, three semi-zygodactyl, one heterodactyl, one pamprodactyl group, and several syndactyl). We propose that delayed skeletal maturation due to altriciality facilitates the epigenetic influence of embryonic muscular activity over developing toes, allowing for repeated evolution of innovations in their morphology.