Sexual Selection for Male Mobility in a Giant Insect with Female‐Biased Size Dimorphism

Abstract

Female-biased size dimorphism in which females are larger than males is prevalent in many animals, but the factors causing this pattern of dimorphism are still poorly understood. The agility hypothesis suggests that female-biased size dimorphism arises because smaller males are favoured in scramble competition for mates. Using radio telemetry, we assessed the agility hypothesis in the Cook Strait giant weta (Deinacrida rugosa), a species with strong female-biased size dimorphism, and tested the prediction that male traits promoting mobility (i.e. longer legs, smaller bodies) are useful in scramble competition for mates and thus promote reproductive success. Our predictions were supported: males with longer legs and smaller bodies exhibited greater mobility (daily linear displacement when not mating) and more mobile males had greater insemination success. No phenotypic traits predicted female mobility or insemination success. In species with female-biased size dimorphism, sexual selection on males is often considered to be weak compared to species in which males are large and/or possess weaponry. We found that male giant weta experience sexual selection intensities on par with males of a closely related harem-defending polygynous species, likely because of strong scramble competition with other males.