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The estrous cycle (also oestrous 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 puberty in sexually mature 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 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 other species, were thought to 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 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.
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.
Four phases 
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 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 biggest influence. She then 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.
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 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 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 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 
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.
Some species, such as cats, cows and domestic pigs, are polyestrous and 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:
- Short-day breeders, such as sheep, goats, deer and elk are sexually active in fall or winter.
- Long-day breeders, such as horses, hamsters and ferrets are sexually active in spring and summer.
Species that go into heat twice per year, such as most dogs, are diestrous.
A few mammalian species, such as rabbits, do not have an estrous cycle and are able to conceive at almost any arbitrary moment. (Compare humans, which 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.
Specific species 
An article titled "Ovulatory cycle effects on tip earnings by lap dancers: economic evidence for human estrus?" by Geoffrey Miller, Joshua M. Tybur and Brent D. Jordan of the Department of Psychology, University of New Mexico, presented in the journal Evolution and Human Behavior 28 (2007) 375–381, suggests that human estrus still exists. The abstract reads: To see whether estrus was really "lost" during human evolution (as researchers often claim), we examined ovulatory cycle effects on tip earnings by professional lap dancers working in gentlemen's clubs. Eighteen dancers recorded their menstrual periods, work shifts, and tip earnings for 60 days on a study web site. A mixed-model analysis of 296 work shifts (representing about 5,300 lap dances) showed an interaction between cycle phase and hormonal contraception use. Normally cycling participants earned about US$335 per 5-hour shift during estrus, US$260 per shift during the luteal phase, and US$185 per shift during menstruation. By contrast, participants using contraceptive pills showed no estrous earnings peak. These results constitute the first direct economic evidence for the existence and importance of estrus in contemporary human females, in a real-world work setting. These results have clear implications for human evolution, sexuality, and economics.
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 incidence of spontaneous ovulation has been documented in the domestic cat and various non-domestic species (Pelican et al., 2006). Without ovulation, she may enter interestrus before reentering estrus. With the induction of ovulation, the female will become pregnant or undergo a non-pregnant luteal phase, also known as pseudopregnancy. Cats are polyestrous but experience a seasonal anestrus in autumn and late winter (Spindler and Wildt, 1999).
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. They bleed during estrus, which will usually last from 7–13 days, depending on the size and maturity of the dog. Ovulations occur 24–48 hours after the LH 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 RBCs 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, 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 that 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.
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 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 corpora 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.
Estrus frequencies of some other mammals:
- Ewe (sheep): 17 days
- Cow (cattle): 21 days
- Sow (pig): 21 days
- Doe (goat): 21 days
- Doe (rabbit): variable
- Jenny (donkey): 23 days (with large variability, between 13-31 days)
- Cow (elephant): 16 weeks
See also 
- Mating system
- Progesterone Releasing Intravaginal Device
- Reproductive cycle
- Rut (mammalian reproduction)
- Geoffrey Miller (April 2007). "Ovulatory cycle effects on Tip earnings by lap dancers: economic evidence for human estrus?". Evolution and Human Behaviour (28): 375–381.
- Susan B. Bullivant, Sarah A. Sellergren, Kathleen Stern et al. (February 2004). "Women's sexual experience during the menstrual cycle: identification of the sexual phase by noninvasive measurement of luteinizing hormone". Journal of Sex Research 41 (1): 82–93. doi:10.1080/00224490409552216. PMID 15216427.
- of the suitors in Odyssey book 22
- Plato, Laws, 854b
- Plato, The Republic
- Herodotus Histories ch.93.1
- Marc E Freeman (1994). "The Neuroendocrine control of the ovarian cycle of the rat". In E Knobil and JD Neill. The Physiology of Reproduction 2 (Second ed.). Raven Press.
- W Heape (1900). "The 'sexual season' of mammals and the relation of the 'pro-oestrum' to menstruation'". Q J Micr Sci 44: 1:70.
- Geoffrey Miller, Joshua M. Tybur, Brent D. Jordan (2007). Evolution and Human Behavior. Ovulatory cycle effects on tip earnings by lap dancers: economic evidence for human estrus? Evolution and Human Behavior. Volume 28, Issue 6, Pages 375–381.
- medilexicon.com > postpartum estrus citing: Stedman's Medical Dictionary. Copyright 2006
- Walter, I; G. Galabova, D. Dimov, M. Helmreich (february 2011). "The morphological basis of proestrus endometrial bleeding in canines". Theriogenology 75 (3): 411–420. Retrieved 2011-12-16.
- McCracken JA, Custer EE, Lamsa JC (1999). "Luteolysis: a neuroendocrine-mediated event". Physiol. Rev. 79 (2): 263–323. PMID 10221982.
- Yoshinaga, K (1973). "Gonadotrophin-induced hormone secretion and structural changes in the ovary during the nonpregnant reproductive cycle". Handbook of Physiology. Endocrinology II, Part 1.
Further reading 
- Spindler, R.E., Wildt, D.E. Circannual variations in intraovarian oocyte but not epididymal sperm quality in the domestic cat.Biology of Reproduction1999;61:188–194.
- Pelican, K., Wildt, D., Pukazhenthi, B., Howard, JG. Ovarian control for assisted reproduction in the domestic cat and wild felids. Theriogenology 2006;66: 37–48.