Mating rate and fitness in female bean weevils

Females of most animal taxa mate with several males during their lifespan. Yet our understanding of the ultimate causes of polyandry is incomplete. For example, it is not clear if and in what sense female mating rates are optimal. Most female insects are thought to maximize their fitness by mating at an intermediate rate, but it has been suggested that two alternative fitness peaks may be observed if multiple costs and benefits interact in determining the relationship between mating rate and fitness. We studied the relationship between female fitness and mating rate in the bean weevil, Callosobruchus maculatus (Coleoptera: Bruchidae), a species in which several distinct direct effects of mating to females have been reported. Our results show that female fitness, measured as lifetime offspring production, is lowest at an intermediate mating rate. We suggest that this pattern is the result of multiple direct benefits to mating (e.g., sperm replenishment and hydration/nutrition effects) in combination with significant direct costs to mating (e.g., injury from male genitalia). Females mating at low rates may efficiently minimize the costs of mating, whereas females mating at high rates instead may maximize the benefits of mating. If common, the existence of bimodal relationships between female mating rate and fitness may help explain the large intra- and interspecific variation in the degree of polyandry often seen in insects.