THE ONTOGENY OF POSTCRANIAL INTEGRATION IN THE COMMON TERN, STERNA HIRUNDO

Empirical and theoretical work supports the concept that developmentally or functionally related characters tend to covary and hence evolve together. However, the dynamics of the developmental processes that establish within population covariance structures are not well understood. This paper focuses on a description of the relative growth of 16 postcranial skeletal elements during embryonic and posthatching development in the common tern, Sterna hirundo, and on a comparative analysis of ontogenetic and adult patterns of variation. Multivariate methods are used to assess the relative contributions of general growth and size-independent "shape" transformations during three major development phases. The observed patterns of integration generally correspond to functional units that tend to show greater relative growth during development immediately preceding when a unit will first be functionally required. Such age-dependent functional adaptations provide an opportunity for selection to act directly on ontogenies with consequences that will be age dependent. Size and shape contributions to adult covariances, which tend to be generated during different phases of development, appear to reflect ontogenetic integration patterns characteristic of periods when global constraints, such as overall size, are most relaxed, but more localized constraints, most likely linked to age-dependent functional requirements, are relatively intense.