The evolution of neuroendocrine mechanisms regulating sexual behaviour in female primates.

The common marmoset is a small New World primate that lives in extended family groups. Female marmosets show rhythmic changes in proceptivity during their 28-Day ovarian cycle, but fluctuations in sexual receptivity are relatively subtle. Receptivity persists even after ovariectomy and adrenalectomy in the female marmoset. In the intact female, increases in proceptivity at mid cycle are due to the activational effects of oestradiol. Treatment of the ovariectomized female with oestradiol-17beta stimulates proceptivity and this effect is blocked by thermal or excitotoxic (neuronal cell body specific) lesions in the anterior or medial hypothalamus. Implantation of oestradiol into the anterior hypothalamus (via guide cannulae) also activates proceptivity. Luteinizing hormone releasing hormone (LHRH) may also form part of the neuroendocrine mechanism controlling proceptivity in the intact female, given that exogenous LHRH stimulates proceptivity in ovariectomized, oestrogen-primed marmosets. These effects of oestradiol (and LHRH) on proceptive displays are much more pronounced than any effects involving sexual receptivity. Conversely, treatment of the ovariectomized female with progesterone, at doses sufficient to produce luteal phase concentrations of circulating hormone, causes a marked suppression of proceptivity, but only small decreases in sexual receptivity. These experiments on marmosets remain some of the very few studies to define how hormones influence the brain and sexual behaviour in female primates. They support the conclusion that sexual receptivity is not under rigid neuroendocrine control in female anthropoids, and that there is no peri-ovulatory period of oestrus, such as occurs in most non-primate mammals.