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The '''estrous cycle''' (also '''oestrous cycle'''; originally derived from [[Latin]] '''oestrus''') comprises the recurring [[physiology|physiologic]] changes that are induced by [[sexual reproduction|reproductive]] [[hormone]]s in most [[mammalian]] [[placenta]]l [[female]]s. 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 [[blood]]y [[vagina]]l discharge, often mistaken for [[menstruation]].
==Differences from the menstrual cycle==
Mammals share the same reproductive system, including the regulatory [[hypothalamus|hypothalamic]] system that releases [[gonadotropin releasing hormone]] in pulses, the [[pituitary]] that secretes [[follicle stimulating hormone]] and [[luteinizing hormone]], and the [[sex hormone]]s including [[estrogen]]s 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 estrous phase of their cycle (see below for an explanation of the different [[Estrous cycle#The four phases of the Estrous Cycle|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 [[ovulation|ovulate]]. Humans, unlike some other species, do not have any obvious external signs to signal receptivity at ovulation ([[concealed ovulation]]). Research has shown however, that women tend to have more sexual thoughts and are most prone to sexual activity right before ovulation.
--><ref>{{cite journal | author= Susan B. Bullivant, Sarah A. Sellergren, Kathleen Stern, et al | title= Women's sexual experience during the menstrual cycle: identification of the sexual phase by noninvasive measurement of luteinizing hormone | journal=Journal of Sex Research | year=February 2004 | volume=41 | issue=1 | pages=82-93 (in online article, see pp.14-15,18-22) | url= | id=PMID 15216427}}</ref>
==Etymology and nomenclature==
Estrus is derived via Latin ''oestrus'' ([[frenzy]], [[gadfly]]), from Greek οιστρος (gadfly, breeze, sting, mad impulse). Specifically, this refers to the gadfly that [[Hera]] sent to torment [[Io (mythology)|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<ref> of the [[suitor]]s in [[Odyssey]] book 22</ref>. [[Plato]] also uses it to refer to an irrational drive<ref>Plato, Laws, 854b</ref> and to describe the [[soul]] "driven and drawn by the gadfly of desire"<ref>Plato, The Republic</ref>. 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 [[Spawning|spawn]]<ref>Herodotus Histories ch.93.1</ref>.
The earliest use in English is of "frenzied passion". In 1900 it was first used to describe "rut in animals, heat".<ref name= Freeman_1994/><ref>{{cite journal|author=W Heape|title=The 'sexual season' of mammals and the relation of the 'pro-oestrum' to menstruation'|journal=Q J Micr Sci|year=1900|volume=44|pages=1:70}}</ref>
In [[British English]], the spelling is oestrus or œstrus. 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]]) an animal may be in estrus when it is in that particular part of the estrous cycle. Estrum is sometmes used as a synonym for estrus.
==The four phases of the estrous cycle==
One or several [[ovarian follicle|follicle]]s 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'''," or "'''on heat'''" in British English). Under regulation by [[Gonadotropin|gonadotropic hormone]]s, [[ovarian follicle]]s 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 behavior|lordosis reflex]], in which the animal spontaneously elevates her hindquarters.
In some species, the [[vulva]]e 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.
During this phase, the signs of estrogen stimulation subside and the [[corpus luteum]] starts to form. The [[uterus|uterine]] lining is under the influence of [[progesterone]] and becomes secretory. 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 [[false pregnancy|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 [[hypothalamus|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 [[cat]]s may undergo an estrous cycle of 3 to 7 weeks. Domestication can affect estrous cycles due to changes in the environment.
==Frequency==<!-- This section is linked from [[Lion]] -->
Some species, such as [[cat]]s, [[cow]]s and [[pig]]s, 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]], [[goat]]s, [[deer]], [[fox]]es, [[Red Deer|elk]]&mdash;are sexually active in [[fall]] or [[winter]].
* Long-day breeders, such as [[horse]]s and [[hamster]]s, are sexually active in [[spring (season)|spring]] and [[summer]].
Species that go into heat twice per year, such as most [[dog]]s, are '''diestrous'''.
'''Monoestrous''' species, such as [[bear]]s, [[fox]]es, and [[wolf|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 [[rabbit]]s, do not have an estrous cycle and are able to conceive at almost any arbitrary moment.
==Specific species==
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 early 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, 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.
''For more information, see the article on [[Horse reproduction]].''
A [[mare (horse)|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 typically have rapid cycle times of 4 to 5 days. Although they ovulate spontaneously, they do not develop a fully functioning corpus luteum. They require coital stimulation to produce a fully functional corpus luteum. 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.<ref>{{cite journal |author=McCracken JA, Custer EE, Lamsa JC |title=Luteolysis: a neuroendocrine-mediated event |journal=Physiol. Rev. |volume=79 |issue=2 |pages=263-323 |year=1999 |pmid=10221982 |doi=}}</ref> The events of the cycle are strongly influenced by lighting periodicity.<ref name=Freeman_1994>{{cite book|author=Marc E Freeman|chapter=The Neuroendocrine control of the ovarian cycle of the rat|title=The Physiology of Reproduction|volume=2|edition=Second edition|year=1994|publisher=Raven Press|editor=E Knobil and JD Neill}}</ref>
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. <ref>{{cite book|author=Yoshinaga, K| chapter=Gonadotrophin-induced hormone secretion and structural changes in the ovary during the nonpregnant reproductive cycle| title=Hanbook of Physiology | volume = Endocrinology II, Part 1 | year = 1973}}</ref>
Estrus frequencies of some other mammals:
* [[Sheep|Ewe]] - 17 days
* Bovine - 21 days
* Goat - 21 days
* [[Pig|Sow]] - 21 days
* [[Elephant]] - 16 weeks
* [[Prairie dog]]
==See also==
* [[Fertility]]
* [[Mating system]]
* [[Reproductive cycle]]
* [[Whitten effect]]
==External links==
*[ Systematic overview]
*[ Etymology]
*[ Cat estrous cycle]
*[ Horse estrous cycle]
{{Reproductive physiology}}
[[cs:Estrální cyklus]]
{{enWP|Estrous cycle}}

Latest revision as of 00:13, May 24, 2013

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