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

Secondary sexual characters are assumed to be costly to develop, and the co sts of parasite infections and immune suppression are currently an active a rea of research within sexual selection. We investigated differences in par asitic 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 re sting fish. The difference in intensity between spawning and resting fish w as only observed in males, and can be explained by differences in exposure or susceptibility to parasites. However, there is indirect evidence that th e difference in parasite intensities does not stem from varying exposure or iginating from group differences in consumption of carotenoid-containing in termediate hosts. We show, rather, that spawning males may be more suscepti ble, since they have a smaller spleen, which is an important lymphocyte-pro ducing organ, than resting males. As these costs of spawning are found pred ominantly among males, they are unlikely to be the result of energetic inve stment in gamete production, as gamete production in general is thought to be more energetically demanding in females than in males. Rather, we sugges t that the observed costs of reproduction result from immune suppression re lated to ornamental development and spermatogenesis.