>In virtually all mammals the oestrus cycle is a vital and efficient system of regulating sexual activity by causing the females to become receptive at certain fixed intervals.
>At other times sexual activity is, for the purposes of natural selection, a waste of time and energy and semen.
>Oestrus provides a clear signal – olfactory, visual or behavioural – of the time when the female will welcome and co-operate with sexual initiatives by the male. In many species the signal is the trigger of desire in the male. It thus promotes harmony between the sexes by maximising the chances that all sexual encounters are reciprocal, amicable and mutually rewarding.
>Since oestrus does not normally occur during pregnancy and lactation, it ensures that the female can concentrate on her maternal role at the proper time without being distracted either by her own desires or by male importunity.
>Among larger animals native to temperate zones, the cycle is an annual one, and sexual activity is so timed that all the young are born at a time of the year when food will be plentiful.
>In regions where seasonal variations in the food supply are less extreme, the cycle can be shorter; within a single breeding population, the timing of oestrus and childbirth in different females is staggered. In the African apes the average length of the cycle is 31–32 days in the gorillas and 35–36 days in the chimpanzee, as compared to 29 days in humans.
