Why do older and larger males win contests in the parasitoid wasp Nasonia vitripennis?

Contest competition is the direct struggle for mates or mating-related resources. Winning contests is thought to be determined by a competitor’s fighting ability (resource-holding potential, RHP) and/or the value of a contested resource to a competitor (resource value, RV). Larger competitors’ winning over smaller competitors is often explained by larger competitors having greater RHP. Older competitors’ winning over younger competitors is often explained by older competitors “valuing” resources more greatly (RV). In this study, we examined whether RHP explained the size effects and whether RV explained the age effects predicted to occur in pairwise male-male contests in the parasitoid wasp, Nasonia vitripennis. We found that larger males won contests against smaller males, but that size did not predict agonism (number of physical interactions). When we attempted to manipulate RHP through feeding, we found that fed males with substantially increased masses and presumably increased energy reserves won contests against unfed males, but feeding did not predict agonism. Thus, larger males appear to win due to their greater RHP, but their winning is not mediated by agonism. Furthermore, we found that old males were more likely to win contests against young males, and that old males were much more agonistic than young males when paired against an age-matched competitor. When we attempted to manipulate RV through mating experience, we found that pairs including mated male(s) were more agonistic than all-virgin pairs, but that mated males were not more likely to win contests than unmated males. Hence, it was unclear whether RV was truly manipulated through mating experience, and further investigation is needed to adequately explain why old males win against young males.