Sexual signal exaggeration affects physiological state in male barn swallows

A prevailing view in sexual selection theory is that costly physiological processes underlie the development, maintenance and expression of sexual signals, and that the costs of these signals enforce their honesty [1,2]. However, this unidirectional view of how physiology governs signal expression is narrow, because many of the putative physiological underpinnings of signals, such as health status, are themselves dynamic [3]. As such, we predicted that physiological parameters should be affected by sexual signal expression. We therefore manipulated a known sexual signal - plumage coloration - in male barn swallows (Hirundo rustica erythrogaster) and measured circulating androgen levels and body mass before and after the manipulation. We found that androgen concentrations increased in color-enhanced males, but decreased in control males, as expected due to typical seasonal androgen declines [4,5]. Color-enhanced males also lost body mass, whereas control males gained weight between successive captures one week apart. These results indicate the existence of feedbacks between an individual's morphological signals and physiology - a finding that is not currently explained by honest signaling theory.