How costly is sex-biased dispersal?
Sex-biased dispersal, in which males and females of a species exhibit differing dispersal behavior, is extremely common throughout nature. It appears to be a byproduct of certain mating systems, and has also been theorized to be an efficient means of avoiding inbreeding depression.
But from a population ecology perspective, sex-biased dispersal seems inherently inefficient, costly, and disadvantageous. It results in mismatches in movement behavior, unbalanced local sex ratios, and mismatched spatial distributions across landscapes. It may become especially costly when one sex is disproportionately affected by disturbances. Given these costs then, why is sex-biased dispersal so common?
I developed an individual-based model, based on empirical studies of dragonflies (Insecta: Odonata), in which males and females disperse across a changing landscape. Females are largely philopatric, while males disperse more, following guassian dispersal kernels. The means of these kernels were varied for each sex. The length of each metapopulation’s persistence, overall sex ratios, and sex ratios within patches were then compared across models.
Females, the more philopatric sex, ultimately drove the success of each metapopulation. Higher female dispersal significantly increased the persistence time of each metapopulation. Males, the more dispersive sex, were generally inefficient if they dispersed too much, and so larger mean male dispersal distances significantly decreased the persistence time of their metapopulations, the opposite pattern from females. A two-way ANOVA confirmed these results, and also showed a significant interaction between male and female dispersal’s joint effects on population persistence.
Sex-biased dispersal clearly has significant costs, as populations with highly-dispersive females were able to persist far longer than populations with more philopatric females. Male dispersal had a smaller effect, but overly high male dispersal was generally inefficient for the population. Generally then, as the dispersal ranges of females and males became more similar, populations were more successful in the long-run.
Given these results, it must be true that these costs of sex-biased dispersal are typically overcome in nature, either by inherent advantages such as inbreeding avoidance, or as a byproduct of advantageous mating systems. But it remains somewhat puzzling that such a costly behavior should remain so prevalent and common throughout nature.