Oestrus
The estrous cycle (also oestrous cycle; derived from Latin oestrus and originally from Greek ) comprises the recurring physiologic changes that are induced by reproductive hormones in most mammalian placental females.  Humans undergo a menstrual cycle instead. Estrous cycles start after puberty in sexually mature females and are interrupted by anestrous phases. Typically estrous cycles continue until death. Some animals may display bloody vaginal discharge, often mistaken for menstruation also called a "period".

# 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 including estrogens and progesterone. However, species vary significantly in the detailed functioning. One difference is that animals that have estrous 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 estrous cycles, females are generally only sexually active during the estrus phase of their cycle (see below for an explanation of the different phases in an estrous cycle). This is also 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, unlike some other species, do not have any obvious external signs to signal estral receptivity at ovulation (concealed ovulation). Recent research suggests, however, that women tend to have more sexual thoughts and are far more prone to sexual activity right before ovulation (estrus).  

# Etymology and nomenclature
Estrus is derived via Latin oestrus (frenzy, gadfly), from Greek  (gadfly, breeze, sting, mad impulse). Specifically, this refers to the gadfly 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 uses the word to describe panic. Plato also uses 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 oistros to describe the desire of fish to spawn.
The earliest use in English is of "frenzied passion". In 1900 it was first used to describe "rut in animals, heat".
In British English, the spelling is oestrus or strus. In all English spellings it has a '-us' ending when used as a noun and an '-ous' spelling when used as an adjective. Thus (in American English) a mammal (humans included) may be described as 'in estrus' when it is in that particular part of the estrous or menstrual cycle. Estrum is sometimes used as a synonym for estrus.

# The four phases of the estrous cycle

## Proestrus
One or several follicles of the ovary are starting to grow. Their number is specific for the species. Typically this phase can last as little as one day or as long as 3 weeks, depending on the species. Under the influence of estrogen the lining in the uterus (endometrium) starts to develop. Some animals may experience vaginal secretions that could be bloody. The female is not yet sexually receptive.

## Estrus
Estrus refers to the phase when the female is sexually receptive ("in heat," or "on heat" in British English). Under regulation by gonadotropic hormones, ovarian follicles are maturing and estrogen secretions exert their biggest influence. The animal exhibits a sexually receptive behavior, a situation that may be signaled by visible physiologic changes. A signal trait of estrus is the lordosis reflex, in which the animal spontaneously elevates her hindquarters.
In some species, the vulvae are reddened. Ovulation may occur spontaneously in some species (e.g. cow), while in others it is induced by copulation (e.g. cat). If there is no copulation in an induced ovulator, estrus may continue for many days, followed by 'interestrus,' and the estrus phase starts again until copulation and ovulation occur.

## Metestrus
During this phase, the signs of estrogen stimulation subside and the corpus luteum starts to form. The uterine lining begins to secrete small amounts of progesterone. This phase typically is brief and may last 1 to 5 days. In some animals bleeding may be noted due to declining estrogen levels.

## Diestrus
Diestrus is characterised by the activity of the corpus luteum that produces progesterone. In the absence of pregnancy the diestrus phase (also termed pseudo-pregnancy) terminates with the regression of the corpus luteum. The lining in the uterus is not shed, but will be reorganised for the next cycle.

# Anestrus
Anestrus refers to the phase when the sexual cycle rests. This is typically a seasonal event and controlled by light exposure through the pineal gland that releases melatonin. Melatonin may repress stimulation of reproduction in long-day breeders and stimulate reproduction in short-day breeders. Melatonin is thought to act by regulating hypothalamic pulse activity of GnRH. Anestrus is induced by time of year, pregnancy, lactation, significant illness, and possibly age.

# Cycle variability
Cycle variability differs among species, but typically cycles are 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. Domestication can affect estrous cycles due to changes in the environment.

# Frequency
Some species, such as cats, cows and pigs, are polyestrous and can go into heat several times a year. Seasonally polyestrous animals have more than one estrous cycles during a specific time of the year and can be divided into short-day and long-day breeders:
- Short-day breeders, such as sheep, goats, deer, foxes, elkare sexually active in fall or winter.
- Long-day breeders, such as horses and hamsters, are sexually active in spring and summer.
Species that go into heat twice per year, such as most dogs, are diestrous.
Monoestrous species, such as bears, foxes, and wolves, have only one breeding season a year, typically in spring to allow growth of the offspring during the warm season to survive the next winter.
A few mammalian species, such as rabbits, do not have an estrous cycle and are able to conceive at almost any arbitrary moment.

# Specific species

## Cats
The female cat in heat has an estrus of 14-21 days and is an induced ovulator. Without copulation she may enter interestrus before reentering estrus. With copulation and in the absence of pregnancy, cycles occur about every three weeks. Cats are polyestrous but experience a seasonal anestrus in autumn and late winter.

## Dogs
A female dog is diestrous and goes into heat typically twice every year, although some breeds typically have one or three cycles a year. The proestrus is relatively long at 5-7 days, while the estrus may last 4-13 days. With a diestrus of 7-10 days, a typical cycle lasts about 3 weeks followed by about 150 days of anestrus. They bleed during this time, which will usually last from 7-13 days, depending on the size and maturity of the dog.

## Horses
For more information, see the article on Horse reproduction.
A mare may be 4 to 10 days in heat and about 14 days in diestrus. Thus a cycle may be short, i.e. 3 weeks. Horses mate in spring and summer, autumn is a transition time, and anestrus rules the winter.
A feature of the fertility cycle of horses and other large herd animals is that it is usually affected by the seasons. The number of hours daily that light enters the eye of the animal affects the brain, which governs the release of certain precursors and hormones. When daylight hours are few, these animals "shut down," become anestrous, and do not become fertile. As the days grow longer, the longer periods of daylight cause the hormones which activate the breeding cycle to be released. As it happens, this has a sort of utility for these animals in that, given a gestation period of about eleven months, it prevents them from having young when the cold of winter would make their survival risky. This is why animals can reproduce during only certain times of the year.

## Rats
Rats typically have rapid cycle times of 4 to 5 days. Although they ovulate spontaneously, they do not develop a fully functioning corpus luteum unless they receive coital stimulation. Fertile mating leads to pregnancy in this way, but infertile mating leads to a state of pseudopregnancy which lasts about 10 days. Mice and hamsters have similar behaviour. The events of the cycle are strongly influenced by lighting periodicity.
A set of follicles start to develop near the end of proestrus and grow at a nearly constant rate until the beginning of the subsequent estrus when the growth rates accelerate eightfold. They then ovulate about 109 hours after starting growth. 
Oestrogen peaks at about 11am on the day of proestrus. Between then and midnight there is a surge in progesterone,   LH and FSH, and ovulation occurs at about 4am on the next, estrus day. The following day, metestrus,  is called early diestrus or diestrus I by some authors. During this day the corpus lutea grow to a maximal volume, achieved within 24 hours of ovulation. They remain at that size for 3 days, halve in size before the metestrus of the next cycle and then shrink abruptly before estrus of the cycle after that. Thus the ovaries of cycling rats contain three different sets of corpora lutea at different phases of development. 

## Others
Estrus frequencies of some other mammals:
- Ewe - 17 days
- Bovine        - 21 days
- Goat          - 21 days
- Sow   - 21 days
- Elephant  - 16 weeks