Neuroendocrine mechanisms contributing to the coevolution of sociality and communication

•Social complexity often predicts signal complexity, but not universally.•Mechanisms linking social and signal complexity are poorly understood.•Steroids, monoamines, and nonapeptides coregulate social and communication behavior.•Neuroendocrine systems may be targets of selection in social and signal coevolution.•Electric fishes are a novel system for investigating signal and social diversity. Communication is inherently social, so signaling systems should evolve with social systems. The ‘social complexity hypothesis’ posits that social complexity necessitates communicative complexity and is generally supported in vocalizing mammals. This hypothesis, however, has seldom been tested outside the acoustic modality, and comparisons across studies are confounded by varying definitions of complexity. Moreover, proximate mechanisms underlying coevolution of sociality and communication remain largely unexamined. In this review, we argue that to uncover how sociality and communication coevolve, we need to examine variation in the neuroendocrine mechanisms that coregulate social behavior and signal production and perception. Specifically, we focus on steroid hormones, monoamines, and nonapeptides, which modulate both social behavior and sensorimotor circuits and are likely targets of selection during social evolution. Lastly, we highlight weakly electric fishes as an ideal system in which to comparatively address the proximate mechanisms underlying relationships between social and signal diversity in a novel modality.