Indicators of development as sexually selected traits: the developmental stress hypothesis in context

The developmental stress hypothesis proposes that the honesty of bird song as a signal may be maintained by costs incurred during development. That is, song complexity or other features of song may be honest indicators of male quality because they reflect an individual's developmental history. Over the last decade much evidence in support of this hypothesis has accrued. Experimentally imposing stressors on songbirds early in development results in impaired development of song and the brain regions controlling song. We review the evidence in support of the developmental stress hypothesis and indicate where information is still lacking. Further, we propose that the developmental stress hypothesis may be specific example of a general process whereby indicators of developmental stability become sexually selected traits. In this light, birdsong and fluctuating asymmetry may have evolved through similar evolutionary processes. Finally, we highlight that a wide range of physiological systems may be simultaneously affected by stress early in life, thus resulting in correlations between sexual ornamentation, such as song and other cognitive and physiological traits. Such traits may indicate different aspects of development. First, traits may signal how well an individual coped with stressors, and could predict indirect genetic benefits to a female. Second, traits may signal the quality of the developmental environment, and could predict direct benefits to a female via developmentally correlated traits. Copyright 2011, Oxford University Press.