The scansorial foot of the woodpeckers, with comments on the evolution of perching and climbing feet in birds. American Museum novitates ; no. 1931

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New York, N.Y. : American Museum of Natural History
"The scansorial foot of the woodpeckers is not a zygodactyl foot, as commonly believed, but a quite different structure - the ectropodactyl foot. With the exception of the most generalized members of the Picinae, which retain the ancestral zygodactyl foot as the climbing foot, the toes of a climbing woodpecker are arranged as follows: toes two and three point forward, the fourth toe is thrust out to the lateral side at right angles to the fore toes, and the hallux usually lies beneath the distal end of the tarsometatarsus in a cramped position and is functionless. In the ivory-billed woodpecker, the hallux is long and functional and is directed laterally next to the fourth toe. The fourth toe is either directed forward or thrust out to the side. In all the climbing woodpeckers, the fore toes, together with the stiffened tail feathers which are propped against the tree trunk, serve to support the bird against the downward and inward component of gravity. The laterally directed fourth toes, and to a slight extent the fore toes, prevent the bird from being pulled away from the trunk by the outward component of gravity. Any toe pointing down the tree trunk would be functionless. Lastly, the evolution of the ectropodactyl foot from the zygodactyl foot is outlined. 2. The evolution of the perching- and climbing-foot types in birds is described. It is shown that the main requirement of a perching foot is a set of strong opposable toes. The anisodactyl, syndactyl, heterodactyl, and zygodactyl arrangements of the toes fill the requirements of a perching foot. On the other hand, the toes of a climbing foot must be arranged to oppose the pull of gravity and should bear strongly curved claws. The anisodactyl, syndactyl, pamprodactyl, and ectropodactyl foot types comply with the functional demands of a climbing foot. A dendrogram showing the foot types and their evolution is given. It is shown that each arrangement of the toes evolved in response to a particular function (i.e., anisodactyl foot evolved for perching), but once evolved it was also suitable for other functions (i.e., running or climbing). 3. A brief discussion of the principle of multiple pathways of adaptation or evolution is presented. It is shown that there may be several morphologically different answers to the same selection force and that the morphological differences between these adaptive answers are not the result of differences in function but are a result of phylogenetically different starting points. Furthermore, it is shown that one cannot conclude that different structures (i.e., arrangements of the toes) are non-adaptive just because their morphological differences are non-adaptive. Examples from the evolution of the perching- and climbing-foot types were chosen to illustrate these conclusions"--P. 42-43.
45 p. : ill. ; 24 cm.
Includes bibliographical references (p. 43-45).