Abstract
Conspicuous carotenoid ornamentation is considered a signal of individual “quality” and one of the most intensely studied traits found to co-vary with parasitism. Since it has been suggested that only “high quality” individuals have enough resources to express excessive sexual ornaments and resist parasites, current theory struggles to explain cases where the brightest individuals carry the most parasites. Surprisingly little emphasis has been put on the contrasting routes to fitness utilized by different parasite species inhabiting the same host. Using Arctic charr (Salvelinus alpinus) as model species, we hypothesized that skin redness and allocation of carotenoids between skin and muscle (redness ratio) will be positively and negatively associated with parasites using the fish as an intermediate and final host, respectively. Both pigment parameters were indeed positively associated with abundances of parasites awaiting trophic transmission (Diplostomum sp. and Diphyllobothrium spp.) and negatively associated with the abundance of adult Eubothrium salvelini tapeworms. These empirical data demonstrate that contrasting associations between carotenoid coloration and parasite intensities relates to the specific premises of different parasite species and life cycle stages.
Highlights
Animal coloration has spurred some of the most controversial research in evolutionary biology
All fish were infected with Diphyllobothrium spp. and Diplostomum sp. (Table S2)
Using wild-caught naturally infected Artic charr, we found that abundances of parasites awaiting trophic transmission to bird hosts associate positively with skin redness (Diplostomum sp.) as well as the ratio of redness in skin versus muscle (Diplostomum sp. and Diphyllobothrium spp.)
Summary
Animal coloration has spurred some of the most controversial research in evolutionary biology. Considering the dual use of carotenoids for host physiological responses against parasites and for sexual ornamentation (i.e. physiological trade-off theory), bright ornamentation can be taken to signal good health and parasite resistance. Under this physiological trade-off framework, cases where the brightest individuals are found to carry more parasites than their paler conspecifics are more difficult to reconcile. Theories explaining such positive associations are less straightforward, but include the “immunocompetence handicap-principle” suggesting that bright individuals signal that they are of high quality, despite carrying many parasites[1,7,12]. Studies investigating carotenoid-based pigmentation in parasitized animals largely focus on host quality and fitness and appear to neglect that parasites need to heed their own life cycle and fitness
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