Insight into the neuroendocrine basis of signal evolution: a case study in foot-flagging frogs.

A hallmark of sexual selection is the evolution of elaborate male sexual signals. Yet, how the physiology of an animal changes to support a new or modified signal is a question that has remained largely unanswered. Androgens are important in regulating male reproductive behavior, therefore, selection for particular signals may drive the evolution of increased androgenic sensitivity in the neuro-motor systems underlying their production. Studies of the neuroendocrine mechanisms of anuran sexual signaling provide evidence to support this idea. Here, we highlight two such cases: first, a large body of work in Xenopus frogs demonstrates that sexually dimorphic androgen receptor (AR) expression in the laryngeal nerves and muscles underlies sexually dimorphic vocal behavior, and second, our own work showing that the recent evolution of a hind limb signal (known as the "foot flag") in Staurois parvus is accompanied by a dramatic increase in androgenic sensitivity of the thigh muscles that control limb movement. Together, these examples illustrate that the evolutionary modification or gain of a sexual signal is linked with a novel pattern of AR expression in the tissues that support it. We suggest that such co-evolution of AR expression and sex-specific or species-specific signaling behavior exists across vertebrates.