Sexual size and shape dimorphism are consistent with predictions that both natural and sexual selection are driving the evolution of sexual dimorphism in Mormon crickets, Anabrus simplex.

Background: Selection can be a powerful force causing morphological adaptation in populations. We tested predictions about the role of both natural and sexual selection in shaping morphology in the Mormon cricket, Anabrus simplex, a species with two population types that differ in their ecological conditions. Solitary populations are characterized by low densities, non-migratory individuals, and typical mating roles (males compete for access to choosy females), whereas gregarious populations are characterized by high densities, migratory behaviour, reversed mating roles, and widespread cannibalism. We collected individuals from both solitary and gregarious populations - characterized by their behaviour and not morphology - and measured several morphological traits. We transformed these traits to shape variables by dividing each measurement by a geometric mean of several metric dimensions representing body size. We tested for population type and sex differences in size and shape variables, and we tested for population type differences in several sex-limited shape variables. We also used discriminant function analysis to test whether a previously enigmatic population, found to be genetically like gregarious populations, but exhibiting many aspects of solitary population behaviour, was morphologically more like solitary or gregarious populations. Our analysis was used to determine the minimum number of measurements needed to assign specimens to the correct population type.

Results: We found that gregarious populations were larger in body size than solitary populations and that females were larger than males in both population types. This sexual size dimorphism was more pronounced in solitary populations. Solitary and gregarious populations displayed several other shape differences as well as differences in the degree of sexual dimorphism in shape. The enigmatic population was unambiguously classified as morphologically more like gregarious populations, a finding which agrees with previous work showing genetic similarities with gregarious populations. Head width was consistently the best character to distinguish members of both populations.

Conclusions: Patterns of sexual size and shape dimorphism are consistent with predictions that both natural and sexual selection are driving the evolution of sexual dimorphism in Mormon crickets. Future work should measure the direction and shape of selection on both males and females in solitary and gregarious populations, focusing particular attention on head shape.