Differential effects of endoparasitism on the expression of carotenoid- and melanin-based ornamental coloration

TLDR

Sexual dimorphism in nature is driven by ornamental traits that convey distinct information, with carotenoids reflecting nutritional condition and melanin indicating social status. The study aimed to experimentally test how endoparasitic infection affects carotenoid‑ and melanin‑based ornamental coloration in male American goldfinches. The authors infected captive male American goldfinches with intestinal coccidians (Isospora sp.) and measured changes in carotenoid‑ and melanin‑based plumage and bill coloration. Parasitized males exhibited less saturated carotenoid‑based plumage and bill coloration, while melanin‑based black caps remained unchanged, supporting that carotenoid and melanin ornaments convey distinct information.

Abstract

The striking diversity of sexual dimorphisms in nature begs the question: Why are there so many signal types? One possibility is that ornamental traits convey different sets of information about the quality of the sender to the receiver. The colourful, pigmented feathers of male birds seem to meet the predictions of this hypothesis. Evidence suggests that carotenoid pigmentation reflects the nutritional condition of males during moult, whereas in many instances melanin pigmentation is a reliable indicator of social status. However, as of yet there have been no experimental tests to determine how these two ornament types respond to the same form of environmental stress. In this study, we tested the effect of endoparasitic infection by intestinal coccidians (Isospora sp.) on the expression of both carotenoid– and melanin–based ornamental coloration in captive male American goldfinches (Carduelis tristis). We found that the carotenoid–based plumage and bill coloration of parasitized males was less saturated than that developed by unparasitized males, but that the brightness and size of melanin–based black caps did not differ between the groups. These findings provide the most robust empirical support to date for the notion that carotenoid and melanin ornaments reveal different information to conspecifics.