Fitness effects of parasite-mediated spatial heterogeneity within a swarm

The fitness consequences associated with the position an individual adopts within a dynamic group are not well understood. I investigated mate acquisition by male chironomid midges using a simple swarming model and empirically collected data on midge aerobatic ability. Previous work has suggested that the aerobatic ability of a male is an important predictor of his reproductive success, although there is contrary (and counterintuitive) evidence that infection with ectoparasitic mites increases a male's chance of mating, despite having negative effects on flight speed. The model used here suggests that a male's location within the swarm, brought about passively through interindividual differences in flight speed, may explain these contradictory results. Specifically, slower flying males (including those burdened with mites) adopted positions nearer the center of the swarm, whereas faster males tended to occupy the periphery. This in turn affected their access to females because any mechanism that brought females nearer the swarm's center before capture (including high female flight speed and selective pairing by either males or females) significantly increased the relative reproductive success of both larger and parasitized males, with the benefits of parasitism peaking at around 4 mites per host. There may be selective pressure on hosts and parasites to maintain this optimal mite density because both are likely to benefit from the relationship: hosts enjoy an increased reproductive success, whereas only through host copulation can mites transfer to a female midge and return to water (their next life-history stage) during host oviposition. Copyright 2006.