Pseudo-homeosis in avian feet

Abstract

Galis et al.'s analysis of the evolutionary constraints on vertebrate autopodal (manus–foot) development [1] includes questionable assertions of homeotic changes of digit identity (changes of identity of one digit into another) in the avian foot. The first example concerns the totipalmate condition that defines pelecaniform birds wherein all four pedal digits comprise a common foot web, with digit I being more forwardly directed than is typical of most birds where it is directed posteriorly. The authors state: ‘The fourth toe has moved forward’, but, in fact, it is the first toe. The other example is the foot of piciform birds, wherein both digits I and IV are directed posteriorly to produce a zygodactylous (yoke-toed) condition. Such a foot is well adapted for perching (puffbirds, etc.), climbing (woodpeckers), and running (roadrunners, etc.). The authors’ state: ‘In all cases, the extra opposed digit has been obtained by changing the form of the second digit into that of the first, opposable, digit’. In fact, in all zygodactylous avian feet, it is merely that digit IV has been repositioned. In the heterodactylous foot of trogons, a similar yoke-toed foot involves reversing digit II. There has never been any question concerning the identity of the digits in any of the above cases [2]: they correspond to their appropriate trochleae, connected with fidelity by their appropriate ligaments. Interestingly, owls, ospreys, touracos and mousebirds have a temporary or facultatively zygodactylous foot in which the fourth toe can be held either anteriorly or posteriorly during perching [2]. Galis et al. [1] offer no arguments to support their implication of homeotic changes in the evolution of the avian foot.