ON THE ECOLOGICAL BASIS OF INTERSPECIFIC HOMOPLASY IN CAROTENOID-BEARING SIGNALS

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Abstract

Evidence that similar color patterns occur in unrelated animals with different habits undermines the traditional view that homoplasy evolves through shared ecological selection pressures. Carotenoid pigments responsible for many yellow to red signals exhibit two related properties that could link ecology with appearance by nontraditional means. Ecologic homoplasy could arise through ecophenotypy because all animals must obtain carotenoids through their diet. Such homoplasy also could be hidden from view because increased carotenoid levels are more strongly encoded by decreased reflectance over ultraviolet (UV) wavelengths invisible to humans. To explore these possibilities, I examined apparent matches or mismatches between color and ecology among insectivorous (low carotenoid diet) and frugivorous (high carotenoid diet) bird species in relation to the typical yellow and black plumage pattern of insectivorous, UV-sensitive titmice (Paridae). Diagnostic features of reflectance spectra indicated that all yellow plumages resulted from carotenoids, black plumages from melanins, and olive green plumages from codeposition of both pigments. However, reflectance by carotenoid-bearing plumages correlated with diet independent of plumage pattern; compared to the insectivores, frugivores had reduced amounts of UV reflectance, and to a lesser extent, “red shifts” in longer-wavelength reflectance. Furthermore, an asymptotic decrease in amount of UV with increased redness implied that plumage reflectance of insectivorous species differed more over UV wavelengths, whereas that of frugivorous species differed more over longer wavelengths. I verified that dietary links to plumage reflectance resulted from greater amounts of plumage carotenoids in frugivores, presumably due to their carotenoid-rich diets. All of these ecological associations transcended post-mortem or post-breeding color change, and phylogeny. Thus, predictable associations between avian-visible plumage reflectance, pigmentation, and diet across evolutionary scales may arise directly (diet per se) or indirectly (honest signaling of diet) by ecophenotypy, although various genetic factors also may play a role.

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