Immune activation affects chemical sexual ornaments of male Iberian wall lizards

Many animals use chemical signals in sexual selection, but it is not clear how these sexual traits might have evolved to signal honestly male condition. It is possible that there is a trade-off between maintaining the immune system and the elaboration of ornaments. We experimentally challenged the immune system of male Iberian wall lizards, Podarcis hispanica, with a bacterial antigen (lipopolysaccharide), without pathogenic effects, to explore whether the immune activation affected chemical ornaments. Immune activation resulted in decreased proportions of a major chemical in femoral secretions (cholesta-5,7-dien-3-ol = provitamin D(3)) known to be selected in scent of males by females and which active form (vitamin D) has a variety of important effects on immune system function. This result suggests the existence of a potential trade-off between physiological regulation of the immune system and the allocation of essential nutrients (vitamins) to sexual chemical ornaments in male lizards.     

Carotenoids,immune response and the expression of sexual ornaments in male greenfinches (<Emphasis Type="Italic">Carduelis chloris</Emphasis>)

Allocation trade-offs of carotenoids between their use in the immune system and production of sexual ornaments have been suggested as a proximate mechanism maintaining honesty of sexual signals. To test this idea, we experimentally examined whether carotenoid availability in the diet was related to variation in antibody response to novel antigens in male greenfinches (Carduelis chloris aurantiiventris), a species with extensive carotenoid-dependent plumage colouration. We also measured the cost of mounting a humoral response in terms of circulating carotenoids. Finally, we examined the relationship between plumage colour, immune response and circulating carotenoids. We found that males with carotenoid-supplemented diets showed stronger antibody response than non-supplemented birds. We also found that activation of the immune system significantly reduced circulating carotenoids (24.9% lower in immune-challenged birds than in control birds). Finally, intensity (chroma) of ventral plumage colouration of males, a character directly related to concentration of total carotenoids in feathers, was negatively correlated with the immune response and circulating carotenoids in winter. These results support the idea that carotenoids are a limiting resource and that males trade ornamental colouration against immune response.