Disparate Postnatal Ontogenies Do Not Add to the Shape Disparity of Infants

Integrating studies of ontogeny with analyses of disparity can reveal important and surprising insights into the origins of disparity and why it varies among groups. One such potentially surprising insight is that disparity could be constant over ontogeny even though species differ in both rates and timings of development and in their ontogenetic changes in shape. Several studies of both primates and rodents have concluded that disparity is generated prenatally although some have concluded that it arises postnatally. However, neither constancy nor an ontogenetic increase in disparity has been ever been rigorously documented for either primates or rodents. For a small sample of rodents, we show that species differ in their postnatal ontogenies but infants are neither more nor less disparate than adults and the major dimensions of disparity distinguishing the main clades also do not change. The constancy in both the level of disparity and its main dimensions does not result primarily from the subtlety of postnatal differences. Those differences are indeed subtle but the disparity in directions of ontogenetic shape change is nonetheless sufficient to increase shape disparity significantly. Disparity does not increase postnatally primarily because ontogenies are not strictly linear; disparity generated postnatally counteracts that produced earlier. What limits the progressive accumulation of disparity is the curvature of ontogenetic trajectories, a curvature presumably due to ontogenetic changes in the spatial distribution of rates of bone deposition and resorption.