Abstract
Structural colors result from the physical interaction of light with organic materials of differing refractive indexes organized at nanoscale dimensions to produce significant interference effects. Because color properties emerge from these finely organized nanostructures, the production of structural coloration could respond to environmental factors and be developmentally more plastic than expected, functioning as an indicator of individual quality. However, there are many unknown factors concerning the function and mechanisms regulating structural coloration, especially relative to social environment. We hypothesized that social environment, in the form of competitive settings, can influence the developmental pathways involving production of feather structural coloration. We experimentally assessed the impact of social environment upon body condition, molt and spectral properties of two types of structural color that compose the nuptial plumage in blue-black grassquits: black iridescent plumage and white underwing patches. We manipulated male social environment during nine months by keeping individuals in three treatments: (1) pairs; (2) all-male groups; and (3) male-female mixed groups. All morphological characters and spectral plumage measures varied significantly through time, but only acquisition of nuptial plumage coverage and nuptial plumage color were influenced by social environment. Compared with males in the paired treatment, those in treatments with multiple males molted into nuptial plumage faster and earlier, and their plumage was more UV-purple-shifted. Our results provide experimental evidence that social context strongly influences development and expression of structural plumage. These results emphasize the importance of long-term experimental studies to identify the phenotypic consequences of social dynamics relative to ornament expression.
Highlights
Integumentary color is widespread in animal taxa and functions within various contexts, ranging from protective effects, for example as countershading or disruptive coloration [1] to advertisement in the attraction of prospective mates [2]
We found a significant interaction effect of time and treatment on the percentage of blue-black plumage coverage, indicating that molt patterns were not uniform for the males in different social contexts (Table 2; Fig. 2A)
Males in both the all males and mixed treatments achieved their peak nuptial plumage coverage earlier and maintained an overall higher coverage compared with males in the paired treatment throughout most of the experiment
Summary
Integumentary color is widespread in animal taxa and functions within various contexts, ranging from protective effects, for example as countershading or disruptive coloration [1] to advertisement in the attraction of prospective mates [2]. Sexual signaling based upon plumage color, relative to pigmentary colors, has been especially emphasized. It has been suggested that carotenoid coloration represents a trade-off between immune function and sexual signaling [5], and could ensure the maintenance of honest male signaling in several taxa. Carotenoid-based ornaments are suggested to reflect stress or social rank in organisms as diverse as lizards [8] and fish [9]. Ornamental carotenoid-based coloration of feathers or integument in a diversity of animal taxa can indicate the health of the individual [10] or level of stress relative either to the social or physical environments [11,12,13]
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