Within-population variation in the chemistry of life: the stoichiometry of sexual dimorphism in multiple dimensions

Abstract

In sexual species, phenotypic divergence between males and females, or sexual dimorphism, is often the source of the most staggering examples of phenotypic variation in nature. Theory suggests that exaggerated sexual traits should drive sex-specific nutritional demands. Advances in spectrometry enable rapid quantification of the elements that make up individuals and traits, which can be used to assess patterns of intraspecific variation and the contribution of nutritionally-demanding sexual traits to these patterns. We measured dimorphism in the whole body stoichiometry of Hyalella amphipods and examined whether nutritional demands of exaggerated sexual traits differ from those of similar traits not under sexual selection. We found striking sexual dimorphism in multivariate whole body elemental composition (i.e., the ionome), including elements important for organismal growth and performance. In males, the exaggerated, sexually-selected claw-like appendage (posterior gnathopod) differed significantly in mass-specific stoichiometry from a similarly sized and serially homologous non-sexual trait (fifth pereopod), indicating that there are fundamental differences in the construction of sexual traits in relation to similar traits that are not under sexual selection. While sexually selected traits do differ from non-sexual traits in their ionomes, we found that possessing an exaggerated trait does not change organismal stoichiometry, indicating that trait exaggeration may not be directly driving ionomic sexual dimorphism. Finally, we found that larger traits are not comparatively larger resource sinks for any element, suggesting that the possession of larger traits is not a function of greater allocation of resources. Together, we discovered substantial sexual dimorphism at the lowest level of organization, chemical elements. Such information illuminates predictions about dimorphisms in foraging behavior, nutritional physiology, and sex-specific selection on the underlying loci. High throughput, multidimensional data on sexual divergence in stoichiometric composition is a powerful tool in understanding the evolutionary ecology of sexual dimorphisms.