Effects of carotenoid supplementation and oxidative challenges on physiological parameters and carotenoid-based coloration in an urbanization context

Worldwide urbanization continues to present new selection pressures on organisms. Carotenoid pigmentation of animals provides an ideal study system for identifying the source and significance of urban impacts because it is an environmentally derived trait and carotenoid molecules have widespread physiological, phenotypic, and fitness functions. Prior work indicates that in some bird species, urban individuals display less colorful carotenoid ornaments than rural birds. However, few studies have experimentally identified the causal factors that drive such a pattern of reduced “sexiness in the city”. We performed two common-garden experiments with house finches, in which we manipulated carotenoid access and exposure to oxidative stress to understand how urban and desert birds respond to these drivers of carotenoid utilization. Urban finches were less colorful than desert birds at capture, but we found no differences between urban and desert finches in how carotenoid provisioning or oxidative stress affected plumage coloration. The only notable site differences in our experiments were that (a) the oxidative challenge caused a larger mass loss in urban compared to desert birds (experiment 1), (b) urban birds circulated higher levels of carotenoids than desert birds after receiving the same diet for 4 months (experiment 2), suggesting that, compared to desert birds, urban finches can better assimilate carotenoids from food or do not deplete as many carotenoids for use in free-radical scavenging. Overall, our results fail to reveal key carotenoid-specific physiological differences in urban and desert finches, and instead implicate other ecophysiological factors that drive urban/desert differences in carotenoid ornamentation.