Allometry of the Postnatal Cranial Ontogeny and Sexual Dimorphism in Otaria byronia (Otariidae)

Abstract

We studied the cranial postnatal ontogeny of Otaria byronia in order to detect sexual dimorphism in allometric terms, analyzing the rate of growth of functional variables linked to specific capacities as bite and head movements. We used 20 linear measurements to estimate allometric growth applying bivariate and multivariate analyses in females and males separately. Males were also analyzed in two partitioned subsets considering non-adult and adult stages, when the dimorphism is accentuated in order to reach optimal performance for intra-sexual competition. In the comparison of the employed techniques, we detected an empirical relationship between our multivariate results and the ordinary least square bivariate analysis. The quantitative analyses revealed different ontogenetic trajectories between non-adult and adult males in most variables, suggesting that the adult skull is not a scaled version of subadult skull. For instance, variables related with longitudinal dimensions decreased their allometric coefficients when the adult stage was reached, whereas those related with breadth or vertical dimensions increased their values. In adult males this could indicate that skull breadth and height are more important than longitudinal growth, relative to overall skull size. Conversely, inter-sexual comparisons showed that females and non-adult males shared similar ontogenetic growth trends, including more allometric trends than did males along their own ontogenetic trajectory. In general, adult males exhibited higher allometric coefficients than non-adult males in variables associated with bite and sexual behavior, whereas in comparison to females the latter showed higher coefficients values in these variables. Such patterns indicate a complex mode of growth in males beyond the growth extension, and are in partial agreement with changes previously reported for this and other species in the family Otariidae.