Does phenotypic integration constrain sexual size dimorphism in eastern lubber grasshoppers (Romalea microptera)

Abstract

Conspicuous sex differences in size, shape, color and behavior are pervasive throughout both the animal and plant kingdoms. Although previous work has shown that sexual dimorphism can evolve purely as the result of either natural (intersexual resource competition) or sexual selection (fecundity selection, male-male combat), a handful of studies have also shown that these mechanisms need not be mutually exclusive and may even act synergistically to enhance dimorphisms. More recent studies have focused on the factors that act to constrain the evolution of sexual dimorphism. For example, a high genetic covariance among traits between the sexes is thought to retard adaptive sexual differentiation or place an upper limit on the degree of intersexual divergence. However, high genetic correlations themselves are often the result of selection for functional coherence amongst traits (integration), and thus divergent selection between the sexes could theoretically drive sexual dimorphism in the genetic variance-covariance matrix (G). To address this issue, we consider the phenotypic variance-covariance matrix (P), within the context of the evolution of sexual dimorphism in the eastern lubber grasshopper (Romalea microptera). We show that: 1) adult female lubbers have relatively wider heads than adult males; 2) most morphological traits show significant integration with one another, but males show a higher degree of integration than females; 3) P in lubbers is not strongly correlated across the sexes. These results suggest that phenotypic integration does not constrain the evolution of sexual dimorphism in R. microptera. Hence, sex-related traits can evolve separately from other traits.