An excess of males: skewed sex ratios in bat flies (Diptera: Streblidae)

Ectoparasitic insects often exhibit female-biased sex ratios, a pattern usually explained by greater female longevity and the likelihood that smaller, more active males will disperse or be groomed off the host. Theory predicts that unbalanced sex ratios should favor males when resources are abundant and predictable, and when males are the dispersing sex. Sex ratios of streblid bat flies were evaluated based on a large biodiversity survey in Venezuela–more than 25,000 bats representing 130 species were searched for flies, yielding more than 36,500 bat flies of 116 species. These samples allowed us to analyze sex ratios in 112 bat fly metapopulations. Our results indicate that fully one-third of these metapopulations were significantly male-biased. Traditional explanations for sex-ratio bias, such as sampling effects, unequal longevity between the sexes, and differential dispersal capability are refuted for bat flies in favor of an alternative hypothesis—selective host grooming. Because host grooming is the principal cause of mortality for these slow-reproducing parasites, and because females are larger than males and gravid for a significant portion of their adult life, host grooming activity is more likely to kill or remove females than males. Incomplete understanding of population dynamics, such as mating behavior, dispersal, and reproductive success, cloud applications of male-biased sex ratios in bat flies to support or refute theoretical predictions. Population studies of mating competition and sex-related dispersal dynamics of this male-dominated group should yield important insights into sex ratio theory.