Estrous cycle
The estrus cycle (also œstrus or estrous cycle) refers to the recurring physiologic changes that are induced by reproductive hormones in most mammalian placental females (humans and great apes are the only mammals who undergo a menstrual cycle instead). Estrus cycles start after puberty in sexually mature females and are interrupted by anestrous phases (see below: anestrus). Typically estrus cycles continue until death. Some animals may display bloody vaginal discharge, often mistaken for menstruation.
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 sex hormones including estrogens and progesterone. However, species vary significantly in the detailed functioning. One difference is that animals that have estrus 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 estrus 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 estrus 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 external signs to signal receptivity at ovulation.
It has been claimed by some observers of animal behaviour (zoophiles) that certain species with estrus cycles can and do learn to show interest in sexual activity at any time of their cycle. Since experiments or evidence to prove or disprove this theory (and the theory itself) are illegal in many parts of the world, it remains controversial.
Etymology and nomenclature
Estrus is derived from Latin oestrus (frenzy, gadfly), in turn from Greek oistros (gadfly, breeze, sting, mad impulse). Its adjectival form is estrous. The earliest use in English is of "frenzied passion". In 1890 it was first used to describe "rut in animals, heat". Estrus, also œstrus (BE) or heat, is itself part of the estrus cycle. Estrus and estrum are synonyms.
The four phases of the Estrus Cycle
Proestrus
During the proestrus one or several follicles of the ovary are starting to grow, their number being specific for the species. Typically this phase lasts 2-4 days. 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"). Under regulation by gonadotropic hormones ovarian follicles are maturing and estrogen secretions exert their maximal 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 vulva is 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 a reentry into the estrus phase 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 is under the influence of progesterone and becomes secretory. This phase typically is brief and may last 1-3-5 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 in long-day breeders and stimulate reproduction in short-day breeders. Melatonin is thought to act by regulating hypothalamic pulse activity of GnRH. Other than season, anestrus is induced by 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 estrus cycle of 3 to 7 weeks. Domestication can affect estrous cycles due to changes in the environment.
Frequency
Some species—such as cows and pigs—are polyestrous and can go into heat several times a year. Seasonally polyestrous animals have multiple 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, elk—are sexually active in fall.
- 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 dogs and cats—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.
Specific species
Cats
The cat in heat (queen) has an estrus of 3-16 days (average 7) 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. While polyestrous, cats experience a seasonal anestrus in autumn and early winter.
Dogs
A female dog is biestrous and goes into heat typically twice a year. The proestrus is relatively long 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.
Horses
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.
Others
Estrus frequency of some other mammals:
- Cow - 21 days
- Elephant - 16 weeks
- Goat - 21 days
- Rodents - 4–6 days
- Sow - 21 weeks