ONTOGENY OF INTEGRATED SKULL GROWTH IN THE COTTON RAT SIGMODON FULVIVENTER

Because development is epigenetic, diverse aspects of morphology are integrated during ontogeny. Using the method of thin-plate splines, and the decomposition of these splines by their principal warps, we examine the ontogeny of integrated features of skull growth of the cotton rat, Sigmodon fulviventer as observed in landmark locations in the ventral view. Postnatal growth of the skull in Sigmodon is not adequately described by the familiar contrast between relatively rapid facial elongation and slow, precocial growth of the cranial base. No developmental units corresponding to "facial skull" and "cranial base" emerge from analysis of geometric shape change. Rather, skull growth is both more integrated and more complex, exhibiting both skull-wide integration and locally individualized regions. Like skull shape, integration has an ontogeny; different regions of the skull can be partitioned into developmentally individualized parts in different ways at different ages. The effective count of individualized parts decreases substantially before weaning occurs, suggesting that the integration required by the functionally demanding activity of chewing gradually develops before the functional transition occurs. Our description of skull growth and integration does not depend upon arbitrary a priori choices about what to measure; rather, we base our decomposition of the whole into parts upon results of the data analysis. Our approach complicates the study of heterochrony, but, because it expresses the spatiotemporal organization of ontogeny, it enables the study of heterotopy.