Oestrous cycle

The oestrous cycle (U.S: estrous cycle) is the reproductive system in most mammals.

Reproductive hormones cause the cycles to start after puberty in sexually mature females. They are interrupted by non-breeding phases or by pregnancies. Usually estrous cycles continue until death.

Differences from the menstrual cycle

Mammals share the same reproductive system, including the regulatory hypothalamic system that releases gonadotropin-releasing hormone in pulses, the pituitary that secretes follicle-stimulating hormone and luteinizing hormone, and the ovary itself releases sex hormones like estrogens and progesterone.

However, species vary in the details. One difference is that animals that have oestrous cycles reabsorb the endometrium if conception does not occur during that cycle. Animals that have menstrual cycles shed the endometrium through menstruation instead.

Another difference is sexual activity. In species with oestrous cycles, females are generally only sexually active during the oestrus phase of their cycle. This is referred to as being 'in heat'.

In contrast, females of species with menstrual cycles can be sexually active at any time in their cycle, even when they are not about to ovulate. Humans and some other primates, do not have any obvious external signs to signal when ovulation occurs (concealed ovulation). Recent research suggests that women tend to have more sexual thoughts and are more prone to sexual activity right before ovulation (estrus). Nevertheless, this is nothing as obvious as the frenzy which other mammals show during 'heat'.