Complex Song Development and Stress Hormone Levels in the Bengalese Finch

Abstract

Models of birdsong are useful in the study of developmental and evolutional mechanisms of vocal learning behaviour. Knowledge of these mechanisms is important for understanding the evolutionary adaptation strategies of organisms. The Bengalese finch sings a complex song believed to have evolved by domestication because the song of its wild ancestor, the white-backed munia, was very simple. There is evidence that the song complexity is used as an indicator of male quality during mate selection. Because songbirds learn their song as juveniles, development of the song system is affected by developmental stress. Therefore, song traits may reflect developmental conditions and that may be honest sexually selected signals. We compared the level of the stress hormone corticosterone between Bengalese finches and white-backed munias. Results showed that Bengalese finches had lower circulating corticosterone levels than white-backed munias. Relaxation from natural selection pressures and the addition of artificial selection due to domestication may have decreased the corticosterone levels and allowed for the evolution of the complex song in the Bengalese finch. Two subtypes of corticosterone receptors, the glucocorticoid receptor and mineralocorticoid receptor, showed expression in song nuclei of the Bengalese finch brain. Thus, these results suggest the possibility that song development is regulated by corticosterone via binding to the corticoid receptors. Song-nuclei-related expression of corticoid receptors might have had an increasing developmental condition-dependent influence on song traits. Thus, our results suggest that decreased stress hormone levels secondary to domestication might account for one reason why songs became complex in Bengalese finches.