Basic biology is not just "for the birds": how avian studies have informed us about vertebrate reproduction

Avian reproductive physiology has been studied for centuries, largely because of the importance of birds as food animals. It is likely that the ubiquity and ease of access to domesticated chickens led to them being used in some of the first experiments on transplantation of endocrine structures-in this case, the testes. Since then, study of seasonal changes in reproductive physiology (photoperiodism) in different orders of bird species has led to advances in the understanding of endocrine regulation of reproductive physiology and behavior. These include mechanisms of adult neuroplasticity, sexual selection, sperm competition, stress physiology, and circadian physiology. Here, we focus mainly on the discovery in birds of a neuropeptide named gonadotropin-inhibitory hormone that mostly has inhibitory effects on reproduction. This hormone has since been shown to exist in all mammals studied to date, including humans (it is known as RFamide-related peptide in mammals). We discuss the history and implications of avian studies on gonadotropin-inhibitory hormone/RFamide-related peptide for human reproductive biology.