Estrous cycle

The estrous cycle (from Latin oestrus 'frenzy', originally from Ancient Greek οἶστρος (oîstros) 'gadfly') is a set of recurring physiological changes induced by reproductive hormones in females of mammalian subclass Theria. Estrous cycles start after sexual maturity in females and are interrupted by anestrous phases, otherwise known as "rest" phases, or by pregnancies. Typically, estrous cycles repeat until death. These cycles are widely variable in duration and frequency depending on the species. Some animals may display bloody vaginal discharge, often mistaken for menstruation. Many mammals used in commercial agriculture, such as cattle and sheep, may have their estrous cycles artificially controlled with hormonal medications for optimum productivity. The male equivalent, seen primarily in ruminants, is called rut.

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

However, animals that have estrous cycles resorb the endometrium if conception does not occur during that cycle. Mammals that have menstrual cycles shed the endometrium through menstruation instead.

Humans, elephant shrews, and a few other species have menstrual cycles rather than estrous cycles. Humans, unlike most other species, have concealed ovulation, a lack of obvious external signs to signal estral receptivity at ovulation (i.e., the ability to become pregnant). Some species of animals with estrous cycles have unmistakable outward displays of receptivity, ranging from engorged and colorful genitals to behavioral changes like mating calls.

Estrus is derived via Latin oestrus ('frenzy', 'gadfly'), from Greek οἶστρος oîstros (literally 'gadfly', more figuratively 'frenzy', 'madness', among other meanings like 'breeze'). Specifically, this refers to the gadfly in Ancient Greek mythology that Hera sent to torment Io, who had been won in her heifer form by Zeus.^{[citation needed]} Euripides used oestrus to indicate 'frenzy', and to describe madness. Homer used the word to describe panic. Plato also used it to refer to an irrational drive and to describe the soul "driven and drawn by the gadfly of desire". Somewhat more closely aligned to current meaning and usage of estrus, Herodotus (Histories, ch. 93.1) uses oîstros to describe the desire of fish to spawn.

The earliest use in English was with a meaning of 'frenzied passion'. In 1900, it was first used to describe 'rut in animals; heat'.

In British English, the spelling is oestrus or (rarely) œstrus. In all English spellings, the noun ends in -us and the adjective in -ous. Thus in Modern International English, a mammal may be described as "in estrus" when it is in that particular part of the estrous-cycle.

A four-phase terminology is used in reference to animals with estrous cycles.