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.[1] 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.[2] Some animals may display bloody vaginal discharge, often mistaken for menstruation.[3] Many mammals used in commercial agriculture, such as cattle and sheep, may have their estrous cycles artificially controlled with hormonal medications for optimum productivity.[4][5] The male equivalent, seen primarily in ruminants, is called rut.[2]
For the biological genus name of the gadfly, see Oestrus (fly).Etymology and nomenclature[edit]
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. Euripides used oestrus to indicate 'frenzy', and to describe madness. Homer used the word to describe panic.[6] Plato also used it to refer to an irrational drive[7] and to describe the soul "driven and drawn by the gadfly of desire".[8] 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.[9]
The earliest use in English was with a meaning of 'frenzied passion'. In 1900, it was first used to describe 'rut in animals; heat'.[10][11]
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.
Cycle-variability[edit]
Estrous cycle variability differs among species, but cycles are typically more frequent in smaller animals. Even within species significant variability can be observed, thus cats may undergo an estrous cycle of 3 to 7 weeks.[15] Domestication can affect estrous cycles due to changes in the environment. For most species, vaginal smear cytology may be used in order to identify estrous cycle phases and durations.[16]
Some species, such as cats, cows and domestic pigs, are polyestrous, meaning that they can go into heat several times per year. Seasonally polyestrous animals or seasonal breeders have more than one estrous cycle during a specific time of the year and can be divided into short-day and long-day breeders:
Species that go into heat twice per year are diestrous. Canines are diestrous.
Monestrous species, such as canids[17] and bears, have only one breeding season per year, typically in spring to allow growth of the offspring during the warm season to aid survival during the next winter.
A few mammalian species, such as rabbits, do not have an estrous cycle, instead being induced to ovulate by the act of mating and are able to conceive at almost any arbitrary moment.
Generally speaking, the timing of estrus is coordinated with seasonal availability of food and other circumstances such as migration, predation etc., the goal being to maximize the offspring's chances of survival. Some species are able to modify their estral timing in response to external conditions.
Specific species[edit]
Cats[edit]
The female cat in heat has an estrus of 14 to 21 days and is generally characterized as an induced ovulator, since coitus induces ovulation. However, various incidents of spontaneous ovulation have been documented in the domestic cat and various non-domestic species.[18] Without ovulation, she may enter interestrus, which is the combined stages of diestrus and anestrus, before reentering estrus. With the induction of ovulation, the female becomes pregnant or undergoes a non-pregnant luteal phase, also known as pseudopregnancy. Cats are polyestrous but experience a seasonal anestrus in autumn and late winter.[19]