Hormone-Mediated Behavioural Transitions: A Role for Brain Aromatase

Abstract

One facet of behavioural endocrinology that has received little attention is the relationship between hormones, environment and short-term changes in behaviour. The idea that an individual animal is subjected to a continuously changing spectrum of meaningful stimuli is, of course, not novel. Ethological studies have emphasized the importance of rapid changes in the environment affecting the individual which in turn influence the organization of behaviour on a short-term basis (Fentress 1978). There is also a wealth of new information indicating that hormones, particularly the sex hormones, show not only daily rhythms in concentration, but also rapid fluctuations which accompany social stimulation (O’Connell et al. 1981) or stressful events in the environment (Wingfield 1983). But do hormones have any precise role in mediating the effects of a rapidly changing environment on reproductive behaviour? The answer to this question requires an understanding of the mode of action of hormones on behaviour. Beach has pointed out (Beach 1974) that the action of hormones on the behaviour of an animal is to “increase or decrease the probability that a given response will occur under carefully specified conditions of external stimulation and previous history of the individual”. This makes the important point that sex hormones, at least in vertebrates, do not in themselves appear to initiate behaviour, but are required as a background to the effects of external stimulation. However, the complex problem remains of how hormones influence the probability of behaviour. In other words, what physiological mechanisms in the brain mediate the action of hormones and environmental effects on behaviour?