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Sexual dichromatism and the immunocompetence handicap: an observational approach using Arctic charr

Secondary sexual characters may signal parasite resistance because the development of such traits can have costs to immune functioning. Here we show that expression of red coloration in the salmonid Arctic charr is a sexually dimorphic character that reveals information of potential importance in sexual selection. Intensity of the carotenoid dependent coloration is in both sexes negatively related to lymphocyte counts. A causal mechanism for this relationship could be reduced availability of carotenoids for immunological application created by increased carotenoid requirements for ornamental development. Alternatively, if more ornamented individuals also have lower intensities of infection, the observed relationship might be explained by healthy individuals being both bright and having low antigen stimulation of lymphocyte transformation and proliferation. Parasite intensities also show relationships with intensity of coloration, and this is used to evaluate the latter explanation.