Sexual selection and the geography of Plasmodium infection in Savannah sparrows (Passerculus sandwichensis)

According to Hamilton and Zuk's hypothesis of parasite-mediated sexual selection, host-parasite coevolution maintains variation in male genetic quality and allows for strong intersexual selection in species with high rates of infection. In birds, most interspecific tests of this hypothesis relate the prevalence of blood parasites to some measure of the intensity of sexual selection. Such tests often rely on limited sampling of single populations to estimate species-wide infection rates, and many tests are thus vulnerable to intraspecific (geographic) variation in the evolutionary ecology of disease. Here, we used molecular techniques to examine variation in the prevalence of Plasmodium spp. across 14 populations of Savannah sparrows, Passerculus sandwichensis, in eastern North America. Plasmodium could not be detected in any of 68 island birds, but 34 of 119 (29%) mainland males, and 7 of 43 (16%) mainland females were infected. Among mainland birds, infection was common in southern populations but rare in New Brunswick, Canada. Overall, the prevalence of Plasmodium ranged from 0 to 60% across populations, although only 17.8% of birds were infected in the pooled (species-wide) sample. The extent of this geographic variation suggests that limited sampling of single populations is unlikely to yield accurate estimates of species-wide infection rates. However, among mainland Savannah sparrows, the prevalence of malaria correlated strongly with average male size and the degree of sexual size dimorphism. We speculate that either sexual selection leads to male-biased infection or, conversely, that high rates of infection promote the evolution of strong intersexual selection.