Suppression of oestrogen-induced LH surges by social subordination in talapoin monkeys

IT is axiomatic that the social structure of a group of primates has a pervasive effect on each individual's behaviour. Thus, many monkeys form dominance hierarchies, based on the successful outcome of aggressive interactions, and the more subordinate animals show characteristic behavioural traits 1,2 . In particular, the social hierarchy has marked effects on reproductive behaviour, especially in talapoin monkeys, where sexual interactions tend to be the prerogative of the dominant males 3 . Subordinate male rhesus monkeys have limited access to sexually attractive females and produce fewer offspring 4 , and in the few species that have been carefully examined, subordinate females, although apparently receiving sexual attention, are, nevertheless, less fertile than would be expected 5–7 . Recently, it has become clear that the dominance hierarchy affects not only behaviour but also hormone levels. Testosterone is higher in dominant males 8,9 , cortisol and prolactin are raised in more subordinate animals 10 . We report here evidence that subordination can prevent the luteinising hormone (LH) surge which is normally induced by giving oestrogen to female monkeys, thus offering an explanation for reduced fertility and suggesting that this can be induced by behavioural interactions.