The challenge hypothesis across taxa: social modulation of hormone titres in vertebrates and insects

Many animals adapt to rapidly changing social environments by using social information to modulate their hormone titres. Modulation of hormone titres, as proposed by the challenge hypothesis, provides a mechanism by which individuals can match their behaviour to their current social environment and thereby avoid costs associated with prolonged high hormone titres. Thus far, most work on social responsiveness of hormone titres has focused on androgens in vertebrates. However, there is mounting evidence that insect hormones, especially juvenile hormone, may respond to social stimuli in ways that parallel androgens in vertebrates. This review will integrate work on social modulation of hormone titres in vertebrates and insects. First we review how the hormone-mediated trade-off between fecundity and life span may be a key selective force favouring socially responsive hormone titres in both systems. Then we review theoretical and empirical work in vertebrates and insects that address how factors such as social instability, dominance rank and mating system influence hormone titre responsiveness. These studies illustrate that (1) juvenile hormone (JH) is responsive to social stimuli in a range of insect taxa and (2) JH responses match key predictions of the challenge hypothesis. We conclude that there are strong similarities in endocrine responsiveness across vertebrate and insect taxa. The challenge hypothesis provides a useful conceptual framework for hypothesis-driven research in insect endocrinology. In addition, exploring areas of convergence and divergence across vertebrates and insects may help clarify how selection has shaped patterns of endocrine responsiveness.