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

Contest competition is the direct struggle for mates, resources, or both. Winning contests is thought to be determined by a competitor's fighting ability (resource-holding potential, RHP), the value of a contested resource to a competitor (resource value, RV), or both. Larger competitors' winning against smaller competitors is often explained by larger competitors having greater RHP. Older competitors' winning against younger competitors is often explained by older competitors valuing resources more highly (RV). In this study, we explored whether RHP explained the size effects and whether RV explained the age effects hypothesized 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 the sizes of winning and losing competitors did not predict the degree of agonism. When we attempted to manipulate RHP through feeding, we found that feeding increased the probability of contest success among old males, but not among young males, and did not affect agonism, regardless of the competitors' ages. We conclude that larger males win as a result of their greater RHPs. Old males were more likely to win contests against young males and were more agonistic than young males in age-matched contests. When we attempted to manipulate RV through mating experience, we found that previously mated males did not differ from unmated males in the probability of contest success. However, competing pairs that included at least one mated male were more agonistic than all-virgin pairs. We conclude that it remains unclear whether RV adequately explains why old males win contests against young males, and that further investigation is warranted.