ABSTRACT: Abstract Choosing to mate with an infected partner has several potential fitness costs, including disease transmission and infection-induced reductions in fecundity and parental care. By instead choosing a mate with no, or few, parasites, animals avoid these costs and may also obtain resistance genes for offspring. Within a population, then, the quality of sexually selected ornaments on which mate choice is based should correlate negatively with the number of parasites with which a host is infected (“parasite load”). However, the hundreds of tests of this prediction yield positive, negative, or no correlation between parasite load and ornament quality. Here, we use phylogenetically controlled meta-analysis of 424 correlations from 142 studies on a wide range of host and parasite taxa to evaluate explanations for this ambiguity. We found that ornament quality is weakly negatively correlated with parasite load overall, but the relationship is more strongly negative among ornaments that can dynamically change in quality, such as behavioral displays and skin pigmentation, and thus can accurately reflect current parasite load. The relationship was also more strongly negative among parasites that can transmit during sex. Thus, the direct benefit of avoiding parasite transmission may be a key driver of parasite-mediated sexual selection. No other moderators, including methodological details and whether males exhibit parental care, explained the substantial heterogeneity in our data set. We hope to stimulate research that more inclusively considers the many and varied ways in which parasites, sexual selection, and epidemiology intersect. Choosing a mate with few or (ideally) no parasites has many potential benefits. Indeed, good health seems like such an important attribute in a potential mate that many researchers have suggested that exaggerated sexual ornaments have evolved specifically to signal absence of infection, or parasite resistance, in the bearer. A key prediction of these ideas is that, within a population, individuals with the biggest, brightest, most attractive ornaments have the fewest parasites. Here, we summarize decades of tests of this prediction across host and parasite taxa to find overall weak support. In fact, this pattern is only significantly supported when ornaments can change rapidly in response to mate condition or parasites can transmit during mating. We propose that these findings strongly indicate a need to incorporate epidemiology into an updated framework of parasite-mediated sexual selection.
