Phasic Effects of Hormones in the Avian Brain During Behavioural Development

Abstract

Steroid hormones have been implicated in the sexual differentiation of behaviour in both birds and mammals (Beach 1974; Goy and McEwen 1980; Adkins-Regan 1983). Studies of the developing rodent brain, particularly in the rat, suggest that androgens irreversibly “organize” mechanisms of male sexual behaviour by direct action on the brain during a “critical” perinatal period (MacLusky and Naftolin 1981). The neural substrate of sexual behaviour in the male is differentiated in two ways. First, androgens masculinize by enhancement of systems underlying male behaviour and, second, defeminize by suppression of behavioural systems underlying female behaviour (Goy and McEwen 1980). Whether these processes operate independently or as a result of the action of different hormones is still unknown, but evidence has accumulated to support the view that an oestrogen, oestradiol-17ß(E₂), formed from testosterone within the brain, is important for the sexual differentiation of male behaviour (Plapinger and McEwen 1978; Olsen 1979; Martini 1982). The mechanisms involved in sexual differentiation of avian behaviour appear at first sight to be the reverse of those in mammals. Oestrogens result in the differentiation (or demasculinization) of behaviour in female Japanese quail during early development, whereas the behaviour of the male develops without hormonal intervention (Adkins 1975; Hutchison RE 1978; Adkins-Regan 1983; Schumacher and Balthazart 1983). The generalization has been made that behavioural mechanisms of the heterogametic sex (female in birds, male in mammals) require the differentiating effects of steroids, the homogametic sex is “neutral” (Adkins 1975). However, this has been questioned with respect to birds (Konishi and Gumey 1982), because song is differentiated by early hormone action in the male of at least one avian species, the zebra finch (Poephila guttata).