The estrous cycle (also oestrus cycle; derived from Latin oestrus and originally from Greek οἶστρος meaning sexual desire) comprises the recurring physiologic changes that are induced by reproductive hormones in most mammalian therian females. Estrous cycles start after sexual maturity in females and are interrupted by anestrous phases or pregnancies. 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 that 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 have menstrual cycles rather than estrous cycles. They, unlike other species, were thought to not have any obvious external signs to signal estral receptivity at ovulation (concealed ovulation). Some 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 οἶστρος (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 œstrus. In all English spellings, the noun ends in "-us" and the adjective in "-ous". Thus in American English, a mammal (humans included) may be described as "in estrus" when it is in that particular part of the estrous cycle. Estrum is sometimes used as a synonym for estrus.
One or several follicles of the ovary start to grow. Their number is species specific. Typically this phase can last as little as one day or as long as three 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; the old corpus luteum gets degenerated; the uterus and the vagina get distended and filled with fluid, become contractile and secrete a sanguinous fluid; the vaginal epithelium proliferates and the vaginal smear shows a large number of non-cornified nucleated epithelial cells.
Estrus refers to the phase when the female is sexually receptive ("in heat"). Under regulation by gonadotropic hormones, ovarian follicles mature and estrogen secretions exert their biggest influence. The female then exhibits 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. Estrus is commonly seen in the mammalian species, including primates. It is thought that this increased sexual receptivity is a functional design in order for the female to obtain mates with superior genetic quality.
In some species, the labia are reddened. Ovulation may occur spontaneously in some species.
Metestrus or diestrus
This phase is characterized by the activity of the corpus luteum, which produces progesterone. The signs of estrogen stimulation subside and the corpus luteum starts to form. The uterine lining begins to appear. 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 is reorganized for the next cycle.
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 the hypothalamic pulse activity of the gonadotropin-releasing hormone. Anestrus is induced by time of year, pregnancy, lactation, significant illness, chronic energy deficit, and possibly age.
After completion (or abortion) of a pregnancy, some species have postpartum estrus, which is ovulation and corpus luteum production that occurs immediately following the birth of the young. For example, the mouse has a fertile postpartum estrus that occurs 14–24 hours following parturition.
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. Domestication can affect estrous cycles due to changes in the environment.
Some species, such as cats, cows and domestic pigs, are polyestrous, meaning that they can go into heat several times a 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.
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 aid survival during 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 (comparable with humans, who, however, have a menstrual cycle in place of an estrous cycle).
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.
The female cat in heat has an estrus of 14–21 days and is generally characterized as an induced ovulator, in that coitus induces ovulation. However, various incidents of spontaneous ovulation have been documented in the domestic cat and various non-domestic species. Without ovulation, she may enter interestrus 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.
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 (9 days), while the estrus may last 4–13 days, with a diestrus of 60 days followed by about 90–150 days of anestrus. Female dogs bleed during estrus, which usually lasts from 7–13 days, depending on the size and maturity of the dog. Ovulation occurs 24–48 hours after the luteinizing hormone peak, which is somewhere around the fourth day of estrus; therefore, this is the best time to begin breeding. Proestrus bleeding in dogs is common and is believed to be caused by diapedesis of red blood cells from the blood vessels due to the increase of the estradiol-17β hormone.
A mare may be 4 to 10 days in heat and about 14 days in diestrus. Thus a cycle may be short, e.g., 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 that activate the breeding cycle to be released. As it happens, this benefits 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.
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 lasting about 10 days. Mice and hamsters have similar behaviour. The events of the cycle are strongly influenced by lighting periodicity.
A set of follicles starts to develop near the end of proestrus and grows at a nearly constant rate until the beginning of the subsequent estrus when the growth rates accelerate eightfold. Ovulation occurs about 109 hours after the start of follicle growth.
Oestrogen peaks at about 11 am on the day of proestrus. Between then and midnight there is a surge in progesterone, luteinizing hormone and follicle-stimulating hormone, and ovulation occurs at about 4 am on the next, estrus day. The following day, metestrus, is called early diestrus or diestrus I by some authors. During this day the corpora lutea grow to a maximal volume, achieved within 24 hours of ovulation. They remain at that size for three 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.
Estrus frequencies of some other mammals: