Abstract
Although a negative covariance between parasite load and sexually selected trait expression is a requirement of few sexual selection models, such a covariance may be a general result of life-history allocation trade-offs. If both allocation to sexually selected traits and to somatic maintenance (immunocompetence) are condition dependent, then in populations where individuals vary in condition, a positive covariance between trait expression and immunocompetence, and thus a negative covariance between trait and parasite load, is expected. We test the prediction that parasite load is generally related to the expression of sexual dimorphism across two breeding seasons in a wild salamander population and show that males have higher trematode parasite loads for their body size than females and that a key sexually selected trait covaries negatively with parasite load in males. We found evidence of a weaker negative relationship between the analogous female trait and parasite infection. These results underscore that parasite infection may covary with expression of sexually selected traits, both within and among species, regardless of the model of sexual selection, and also suggest that the evolution of condition dependence in males may affect the evolution of female trait expression.
Highlights
Parasitism is ubiquitous in the wild and is often expected to play a key role in determining realized fitness
Regardless of the mechanism of sexual selection and even if individuals face a trade-off between allocation to immunocompetence and a sexually selected trait, a negative phenotypic covariance between parasite load and trait expression across males could be expected under the assumption that somatic maintenance for immunocompetence is itself condition dependent
Our results support the hypothesis that parasite infection may generally covary with the expression of sexual dimorphism and that the sexes may often differ in total parasite load; our results are consistent with a fairly large body of work suggesting males may often carry higher parasite loads than females
Summary
Parasitism is ubiquitous in the wild and is often expected to play a key role in determining realized fitness. Parasitism has been envisioned as playing a key role in a number of evolutionary problems, including the maintenance of female preference for male traits that do not appear to offer a direct fitness benefit to choosy females (Hamilton and Zuk 1982). Regardless of the mechanism of sexual selection and even if individuals face a trade-off between allocation to immunocompetence and a sexually selected trait, a negative phenotypic covariance between parasite load and trait expression across males could be expected under the assumption that somatic maintenance for immunocompetence is itself condition dependent. Even for traits where mating biases are determined by competition among males for mating opportunities, rather than actual female preference, a relationship between parasite load and trait expression may be expected if condition-dependent expression of the trait is favored; higher condition individuals may have both higher immunocompetence, lower parasite load and greater expression of the sexually selected trait
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