Red fish, blue fish:: trade-offs between pigmentation and immunity in Betta splendens

Carotenoid pigments are responsible for many examples of sexually attractive red, orange, and yellow coloration in animals and play an important role in antioxidant and immune defenses. Because vertebrates cannot synthesize carotenoids, limited dietary availability may impose a trade-off between maintaining ornamental coloration and health. We used an experimental approach to test the carotenoid trade-off hypothesis in the fighting fish Betta splendens, by examining whether carotenoid allocation strategies differ among conspecifics that exhibit a gradient of body coloration from blue to red. We found that male redness is underlain by carotenoids and that females preferred to associate with red males over blue males, suggesting a sexually-selected advantage to being red. Moreover, we found strong experimental support for the carotenoid trade-off hypothesis, as individuals that varied in color did not appear to allocate carotenoids equally to both immune response and coloration. Redder fish given supplemental carotenoids increased in both immune response (to a phytohemagglutination challenge) and redness compared with controls. In contrast, bluer fish given supplemental carotenoids did not become more red but instead benefited immunologically more so than either control or redder supplemented fish. These results enhance our understanding of the evolution and plasticity of carotenoid mobilization and utilization pathways in animals.