Facial growth and the ontogeny of morphological variation within and between the primates Cebus apella and Cercocebus torquatus

Abstract

Differences in facial skeletal form arise in part through differences in growth and in part through differences in embryonic patterning and proportioning. This study examines post-natal facial growth, sexual dimorphism and adult differences within and between two primate species: Cebus apella and Cercocebus torquatus. The species were chosen because they are phylogenetically distant yet present roughly similar adult morphologies. The comparison of ontogenetic, sexual and phylogenetic differences between these species illuminates the relationship between pre-natal proportioning, growth, sexual dimorphism and evolutionary change in the facial skeleton.

The quantitative analytical methods used in this study are taken from the geometric morphometric toolkit. Additionally, facial growth is examined through scanning electron microscope studies of bone growth remodelling. The main findings are that adult sexual dimorphism differs markedly between the studied species. In both species, however, it arises through extension in males relative to females of a common size–shape growth trajectory plus late divergence of growth between the sexes. This similarity contrasts with the findings that growth remodelling in the facial skeleton and size-related shape changes in the face of each species differ. Thus, variations in facial form between species seem to be the result of pre-natally established differences in form plus post-natal growth divergence, whilst the different sexual dimorphisms of these species arise through the action of similar growth processes on divergent growth trajectories. The implications of these findings for the interpretation of adult variations in facial form and growth are considered.