Ontogeny of a sexual dimorphism in tiger salamanders
Sexual dimorphism results when the sexes differ in the degree to which trait elaboration confers a reproductive or survival advantage. Trait size dimorphism is often reported in terms of allometry, typically using adults of varying ages (static allometry). A static allometric analysis of tail length in breeding tiger salamanders ( Ambystoma tigrinum (Green, 1825)) revealed that tail length is a positive allometric trait in both sexes, as well as a sexually dimorphic trait. Although static analyses are common in the literature, ontogenetic allometric analyses in which individuals are measured through time are preferred because they provide insight into the heterochronic process underlying trait divergence between the sexes and which sex is diverging from its earlier growth trajectory. I reared 91 individuals from the zygote stage to sexual maturity. An ontogenetic analysis revealed that tail length was isometric in larvae and young metamorphs of both sexes; however, tail length became allometric in males but not in females prior to sexual maturation. I also present static allometric analyses and show how conclusions differ from those of ontogenetic analyses. Lastly, I discuss how sex differences in selection gradients, as well as resource allocation costs, might influence differences between the sexes in the duration and rate of trait growth.