Eigensurface analysis, ecology, and modelling of morphological adaptation in the falconiform humerus (Falconiformes: Aves)

Abstract

Studies of morphological adaptation aim to quantify the relationship between an organism's form and its ecology. In the past such studies have been hampered by an over-reliance on either qualitative observations or the collection of a few, marginally representative two-dimensional linear measurements as morphological descriptors. Recent advances in morphometric data acquisition and analysis techniques now provide a means of accurately and comprehensively quantifying the morphological variation inherent in complex three-dimensional (3D) surfaces across a specimen set. Using birds of prey as a model group, we examine how one of these new morphometric methods – eigensurface analysis – can be used to investigate adaptation. Virtual models of the humerus of 50 falconiform species from 29 genera were examined to identify the morphological correlates associated with different flight styles, habitats, and behaviours of these ecologically sensitive predators. Results indicate that strong, consistent, unique, and subtle modes of shape variation are associated with differences in flight speed (fast–slow), flight style (perch hunting, chasing, low flight, soaring flight, hovering), habitat (forested–open landscape) and migratory behaviour (long-distance migrants–sedentary species). In each case, visual modelling of between-groups transitions in morphology can be used to facilitate identification of features of the humerus, such as the size and extent of surface-shape variation within specific regions of muscle attachment, that are important predictors of function and lifestyle. Eigensurface analysis-based representations of 3D morphology, in combination with standard linear discrimination techniques and new shape modelling procedures, represent a way to statistically evaluate hypotheses of morphological adaptation. This approach can be used not only for bird taxa, but more broadly in studies of ecology and adaptation in many vertebrate, invertebrate, and plant species, in ways that cannot be duplicated either by visual observation or by the representation of form using linear measurements, 2D–3D landmark coordinate sets or sets of 2D–3D boundary outline semilandmarks. © 2012 The Trustees of the Natural History Museum. Zoological Journal of the Linnean Society© 2012 The Linnean Society of London, 165, 390–419.