Sexual signal reliability in male zebra finches: food intake explains the impact of immune activation on carotenoid-based coloration

Theory predicts that traits evolving as animal signals must transmit reliable information about the bearer’s quality. Two hypotheses are often invoked to explain how carotenoid-based colorations function as reliable individual quality signals. The first hypothesis suggests that carotenoid allocation trade-offs between signaling and survival-related functions maintain signal reliability. Only high-quality individuals could resolve these trade-offs without incurring a fitness loss. Such signals are plastic, and the signaler can strategically manage their expression level to increase fitness returns. The second hypothesis posits that certain carotenoid-based signals are strongly constrained in their expression, not allowing strategic manipulations or cheating. We challenged captive male zebra finches ( Taeniopygia guttata ) with simulated infections by injecting lipopolysaccharides (LPS) derived from Escherichia coli and quantified food intake, carotenoid and vitamin levels in several tissues. Compared to controls, LPS-treated birds reduced the bill redness, a sexual signal. However, this reduction was largely explained by decreased food intake caused by LPS. Bill redness negatively correlated with body temperature and spleen mass but positively associated with blood, spleen and subcutaneous fat carotenoid levels. Overall, our findings suggest that carotenoid-based bill coloration predominantly reflects the recent physiological condition of individuals and resource allocation trade-offs rather than directly signaling individual quality.