The Ontogeny of Sexual Dimorphism: Quantitative Models and a Case Study in Labrisomid Blennies (Teleostei: Paraclinus)

Abstract

Studies of sexual dimorphism commonly focus on male morphology, often viewing males as divergent both evolutionarily and ontogenetically. For example, in continuously grow- ing vertebrates such as lizards and fishes, it has been assumed that adult dimorphism arises from growth rate changes in male characters at the onset of maturity. However, there has been little explicit treatment of the ontogenetic origin of morphological sex differences. I present here a method for testing hypotheses of sex-specific allometric change, and offer an example of its application in four species of labrisomid blennies. I develop three hierarchical models to test for allometric divergence: (1) the null model, in which adult allometries of both sexes are simply an extension of the juvenile allometries (model A); (2) a model in which either adult males or females (but not both) diverge from the juvenile trajectory (model B); and (3) a model in which adults of both sexes diverge from the juvenile allometry (model C). Growth that fits either of the two latter models results in shape dimorphism. Although size dimorphism may occur in all three cases, it is not explicitly treated here. The models were tested with size-series of four species of fish (Paraclinus, Labrisomidae). To further explore overall shape similarity among males, females, and juveniles, a multivariate analysis of shape (independent of size) was applied. Analysis of 42 mensural characters showed that divergence from juvenile allometry occurred as frequently in female traits as in male traits. Multivariate analysis showed that in all four species, female shape diverges more from juvenile shape than does male shape. In addition, similarity among adult dimorphic character states often resulted from quite different growth patterns, suggesting that resemblance of adult characters among related species is insufficient evidence to conclude character homology. Interpretations of sexual dimorphism, currently emphasizing male morphology, may be biased and may underestimate the importance of selection on the female form as a cause of sexual dimorphism. (Allometry; homology; homoplasy; Labrisomidae; ontogeny; Paraclinus; sexual dimorphism; sexual selection.)