ANNUAL CHANGES IN THE SEXUAL POTENCY OF CAPTIVE MALE RHESUS MONKEYS

Like many non-primate mammals, several non-human primates have well-marked mating and birth seasons (Lancaster & Lee, 1965; Michael & Zumpe, 1971). In a relatively controlled laboratory environment, Michael & Keverne(1971) found that intact rhesus males paired with intact females, whose oviducts had been ligated to prevent pregnancy, showed a peak in ejaculatory activity in December followed by a decline between February and May. The timing of the long-term changes in potency of these laboratory males was, therefore, very similar to that occurring in the natural habitat in North India(Southwick et al., 1965; Michael & Wilson, 1975). This suggests the existence of an annual behavioural rhythm that is to some extent independent of major environmental variables. A clearcut annual rhythm in the plasma testosterone of rhesus males maintained in the laboratory has now been demonstrated(Plant et al., 1974), and this raises the question of whether or not the annual changes in plasma testosterone play any causative role in determining the annual behavioural rhythms. To clarify this matter, eight intact male rhesus monkeys (weighing 7\m=.\6 to 12\m=.\3 kg) were studied over a period of 13 months (December 1972 to December 1973) during 2183 tests carried out in Atlanta. Each male was paired with fou