Reproductive biology of captive male cottontop tamarin monkeys as a function of social environment

The cottontop tamarin, Saguinus oedipus oedipus, is a cooperatively breeding monkey in which mature male and female offspring serve as helpers to assist in rearing younger siblings. Generally, only one female per social group reproduces; breeding restriction is mediated in postpubertal female offspring through low and acyclic levels of reproductive hormones. We investigated (1) reproductive activity of postpubertal male offspring, and (2) whether aggression towards male offspring and a cortisol-mediated stress response might restrict breeding of male offspring in the natal group. We examined sexual behaviour, olfactory communication and urinary hormone levels (testosterone, dihydrotestosterone, luteinizing hormone, cortisol) of the subject males while we manipulated their social environment from housing in natal groups to pairing with a novel female, and after the production of their own offspring. Mounting and erection rates of the male subjects were as high in the natal group as when paired with a novel female. However, most mounts in the natal group were directed towards other males, and complete copulation sequences did not occur with natal-group females. Social environment had no significant effect on olfactory investigation of breeding females. Although hormone levels increased significantly after the subjects were removed from the natal group, the elevation was transient; the hormone levels of subjects in their natal groups did not differ from the levels shown by the same males when successfully producing their own offspring. Male offspring received more contact aggression in the natal group than when paired with the novel female. However, most of the aggression was received from siblings rather than the breeding pair, and levels of cortisol did not correspond with levels of aggression. Thus, at both a behavioural and endocrine level, mature male offspring in captive natal groups were potentially fertile, but sexual activity with natal-group females appeared to be behaviourally restricted and directed instead towards group males. In wild cottontop tamarin groups, this reproductive potential may allow male helpers flexibility to respond to breeding opportunities. (C) 2001 The Association for the Study of Animal Behaviour.