The relationship between blood parasites and ornamentation depends on the level of analysis in the common yellowthroat

摘要

The Hamilton-Zuk hypothesis predicts that ornament expression is a signal of the ability of individuals to resist parasite infection. Thus, across a population (i.e. between-individuals) more ornamented individuals should have lower levels of parasitism. Numerous studies have tested this prediction and the results are mixed. One reason for these conflicting results may be that many studies have examined this relationship at the between-individual level, which may be affected by confounding factors such as selective mortality. Using within-subject centering we examined the relationship between male ornamentation and avian blood parasites at both the between- and within-individual levels. These relationships focus on differences in genetically-based resistance to parasites and the trade-off in resource allocation between parasite resistance and ornament expression within an individual, respectively. We studied male common yellowthroats Geothlypis trichas, which have two plumage ornaments, a yellow, carotenoid-based bib (throat and chest) and a black, melanin-based facial mask. Surprisingly, within-individuals, an increase in parasitism between years was associated with an increase in mask size and, potentially, greater concentration of carotenoids in the yellow feathers. This suggests that males may be able to tolerate an increase in parasitism and still increase ornament expression. In contrast, ornamentation was not related to parasitism at the between-individual level. Thus, our study revealed relationships between ornaments and parasitism at the within-individual level that were not present at the between-individual level. Our results highlight the importance of examining both within- and between-individual relationships as correlations between variables, such as ornaments and parasites, may depend on the level of analysis (i.e. within- or between-individuals).