Reappraisal of the interspecific prediction of parasite-mediated sexual selection: Opportunity knocks

Abstract

Parasite-mediated sexual selection (PMSS) has been hypothesized to occur when individuals choose mates on the basis of parasite-indicative secondary sexual traits in order to acquire “good-genes” for parasite resistance, protection from parasite transmission, or healthy mates for assistance in parental care. Interspecific studies of PMSS test the prediction that parasite load and the extent of sexual selection, or “showiness”, are positively correlated across host species. The assumption inherent in this prediction is that larger parasite loads cause more intense sexual selection because larger loads have a greater effect on the mean fitness of host populations. This assumption is invalid on theoretical grounds: selection is not governed by mean fitness, but by variance in relative fitness, or the “opportunity for selection”. In this paper we model the potential influence of parasites on host fitness and examine the relationship of two measures of parasite load, prevalence and intensity, to the opportunity for parasite-mediated selection (I). Our results indicate that prevalence and intensity covary with I in some cases, but not others, depending on the precise effect of the parasite on host fitness and how this effect varies across host species. On the basis of these results, the interspecific prediction of PMSS is not necessarily warranted. The relationship of parasites to sexual selection, and hence the predicted relationship of parasites to showiness, depends on the nature of host-parasite interactions.