Costly melanin ornaments: the importance of taxon?

Abstract

Summary 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.

Not simply red: Colouration of red wood ant Formica rufa (Hymenoptera: Formicidae) is polymorphic, modular and size-dependent

“…In insects, melanin-based ornaments can accurately reveal individual quality because of the immunomodulatory and anti-oxidative action of the pigments (McGraw, 2005). However, this is species-specifi c (Stoehr, 2006), as species vary in the degree of genotype-by-environment interactions in colour pattern origin, and genetically pre-lugubris (Zettersted, 1840), Formica polyctena (Forster, 1850), Formica pratensis (Retzius, 1783) and Formica rufa (Yarrow, 1955) all of which are ecological dominants in terrestrial ecosystems (Punttila & Kilpeläinen, 2009). Although closely related, they have different life history strategies and social organisations (Rosengren et al, 1993).…”

“…The role and signifi cance of colouration traits differ between species, even closely related species (Stoehr, 2006). Therefore, in this study we focused on precise descriptions of the colouration of the workers of the red wood ant, Formica rufa (Linnaeus, 1761); in particular the morphs, shape and size of the dark melanized patterns and the degree of melanization of the head and thorax.…”

Nanostructural self-assembly of iridescent feather barbules through depletion attraction of melanosomes during keratinization

“…hotly debated [62,63], but, if so, the large amount of melanin needed for self-assembly, or pleiotropic effects of melanin expression on other metabolic pathways [64][65][66], may indeed create an energetic cost. Also, keratinization is likely associated with the speed at which the feather grows [34,38], potentially explaining previously reported associations between colour and growth bar width [22,28].…”

The peppered moth and industrial melanism: evolution of a natural selection case study

“…Their synthesis is involved not only in melanin production but also immune defence and sclerotin production, so that competitive trade-offs are possible (Windig, 1999;Stoehr, 2006). Pre-adult fitness differences between melanic and non-melanic morphs are reported for the noctuid species Mythimna separata (Jiang et al, 2007).…”