Costly melanin ornaments: the importance of taxon?

1. Experimental work in vertebrates shows that different forms of coloration - particularly carotenoid-based vs melanin-based ornaments - are differentially sensitive to stressors such as diet manipulations and parasitic infections. Comparative work also shows that historical patterns of ornament evolution depend on the proximate mechanisms of colour production. 2. Based on the results from vertebrates, it has been argued that melanin-based ornaments may not be good indicators of quality because, unlike carotenoid-based ornaments, melanin pigments can be synthesized from amino acid precursors, and therefore melanin synthesis is unlikely to be costly. 3. However, there is evidence for costly melanin synthesis in insects, which use melanin in some ways that vertebrates do not. Differences across taxa in the costs of melanin synthesis challenge the generality of the hypotheses relevant to these ornaments. 4. In general, all pigment-based ornaments are a function of precursor intake and processing. Differences in costs of melanin synthesis across taxa may reflect differences in one or both of these parameters. 5. I urge communication between researchers working on related questions in disparate taxa (vertebrates and invertebrates). This should facilitate the discovery of the most fundamental aspects of colour ornament evolution.