How does testosterone act to regulate a multifaceted adaptive response? Lessons from studies of the avian song system

In male songbirds, song functions to attract a mate or to defend a territory; it is therefore often produced in the context of reproduction. Testosterone of gonadal origin increases during the reproductive phase of the annual cycle and significantly enhances song production, as well as song development, via effects on song crystallisation. The neural control of birdsong production and learning is highly modular. We implanted testosterone or...