Sex-biased dispersal is independent of sex ratio in a semiaquatic insect

Abstract

Dispersal influences a variety of ecological and evolutionary dynamics including metapopulation persistence and local adaptation. Sex-biased dispersal evolves when the costs and benefits associated with dispersal differ between the sexes. These costs and benefits may be fixed, resulting in a consistent pattern of sex-biased dispersal within species whereby one sex always disperses more and/or further than the other. Alternatively, the costs and benefits may vary depending on the intensity of competition experienced by the two sexes. In this case, the direction of the sex bias may be plastic and depend on the sex ratio of the population. In the current study, we asked whether a semiaquatic, flight capable insect (Notonecta undulata) exhibits sex-biased dispersal and whether the strength of intrasexual competition experienced by males and females determines the direction of the sex bias. We conducted a mesocosm experiment in which we manipulated the population sex ratio and measured the probability of dispersal for males and females. We found that while both sexes dispersed, male dispersal rates were higher, and this pattern was independent of sex ratio. This suggests that fixed sex-specific dispersal costs and/or benefits are likely to be more important determinants of sex-biased dispersal in notonectids than population sex ratio. Dispersal is the process by which individuals move through space and cause gene flow and therefore is a major factor determining the distribution of individuals, populations, species, and alleles—a topic which is one of the core themes in ecology. Dispersers commonly differ from non-dispersers in a variety of phenotypes, including sex. Sex-biased dispersal may have important implications for the populations that send out and receive dispersers, because males and females have different impacts on populations. We explored whether sex-specific dispersal behavior in an insect (Notonecta undulata) changes depending on how intensely individuals have to compete for resources (e.g., food, mates) with individuals of the same sex. We found that males dispersed more, and this was true regardless of which sex experienced stronger intrasexual competition. This suggests that males experience lower costs and/or greater benefits from dispersing than do females.