Same colors for different functions: implications for the evolution of carotenoid-based ornamentation

Sexual ornamentation is often assumed to be costly, allowing honest signaling of individual quality, and carotenoid-based colors were proposed to bear significant costs. If carotenoid-based colors are costly to produce, sexually-selected signals should use more concentrated carotenoid pigments and have more saturated color than non-sexual signals, where honesty-guaranteeing costs are not required. We tested this prediction comparing carotenoid-based colors across canaries, goldfinches and allies, because many of these species use yellow plumage as sexual ornamentation, but also have yellow rumps that appear to be non-sexual flash marks. Only in the breast, but not the rump, was there an asymmetric co-distribution of male and female color saturation, with males similarly or more saturated than females, indicating evolution of breast color by sexual selection. Yellow was not consistently more saturated in the breast than in the rump, and the co-distribution of rump and breast color saturation indicated that saturated rumps can persist irrespective of breast color. This challenges the assumption that carotenoid-based colors bear significant costs. The use of carotenoid coloration as sexual signals in this clade may instead be due to social costs, cost-free index mechanisms for signaling quality, and/or socially-monogamous species evolving low-cost signals to mostly discriminate against low-quality mates.