Long-term effects of testosterone injections on the social behaviour of male ducks and their physiological correlates

During three independent experiments, we demonstrated that repeated injections of testosterone propionate (TP) induce, after the termination of treatment, a strong inhibition of social displays in intact male domestic ducks. This behavioural inhibition may last for several months and appears relatively specific, as sexual behaviour was generally normal in the previously injected birds. This effect is probably not directly related to the neonatal differentiation of the brain, as it can be induced by injections given when the birds are already several months old. The inhibition of social displays is not paralleled by drastic changes in the plasma levels of pituitary—gonadal hormones (testosterone, follicle stimulating hormone and luteinizing hormone), and thus is not explained by the lack of endogenous testosterone. It is postulated that the behavioural inhibition results from a decreased sensitivity to testosterone of the neural mechanisms involved in the control of displays, and possible mechanisms explaining this decreased sensitivity at the biochemical level are briefly reviewed.