REGULAR ARTICLES / ARTICLES RÉGULIERS Whether to reproduce or not: immune suppression and costs of parasites during reproduction in the Arctic charr

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Abstract
Secondary sexual characters are assumed to be costly to develop, and the costs of parasite infections and immune suppression are currently an active area of research within sexual selection. We investigated differences in parasitic infections and immunological activity between reproductively active and inactive Arctic charr (Salvelinus alpinus). Spawning fish were found to have higher intensities of macroparasite infections than nonspawning or resting fish. The difference in intensity between spawning and resting fish was only observed in males, and can be explained by differences in exposure or susceptibility to parasites. However, there is indirect evidence that the difference in parasite intensities does not stem from varying exposure originating from group differences in consumption of carotenoid-containing intermediate hosts. We show, rather, that spawning males may be more susceptible, since they have a smaller spleen, which is an important lymphocyte-producing organ, than resting males. As these costs of spawning are found predominantly among males, they are unlikely to be the result of energetic investment in gamete production, as gamete production in general is thought to be more energetically demanding in females than in males. Rather, we suggest that the observed costs of reproduction result from immune suppression related to ornamental development and spermatogenesis.