The challenge hypothesis in insects

The ‘challenge hypothesis’ provides a predictive framework for how the social environment influences within-species variation in hormone titers. High testosterone levels are beneficial during reproduction and competition, but they also impose costs because they may suppress traits like parental care and immunity. As a result, the challenge hypothesis predicts that individuals will change their testosterone levels to match the current social environment. Although the vast majority of work on the challenge hypothesis has focused on androgens in vertebrates, there is growing evidence that insect hormones, especially juvenile hormone (JH), may respond to social stimuli in ways that parallel androgens in vertebrates. Many insects rapidly upregulate JH titers during social competition with rivals. Some insects also modulate JH titers based on contest outcomes, with winners upregulating JH and losers downregulating JH. This review will integrate work on social modulation of hormone titers in vertebrates and insects. First, we provide background on insect hormones and describe the functional parallels between androgens and JH. Second, we review evidence that insects rapidly change JH titers in response to social competition. Finally, we highlight opportunities for future work on social modulation of hormones in insects. Overall, the challenge hypothesis provides a useful conceptual framework for hypothesis-driven research in insect endocrinology. Comparing vertebrates and insects provides insight into how selection has shaped patterns of hormone responsiveness as well as the generality of hypotheses originally developed for vertebrates.