Estrous cycle: Difference between revisions

{{ForAbout||the biological genus name of the gadfly|Oestrus (fly)|Satyr leader|Oestrus (mythology)}}

The '''estrous cycle''' ({{etymology|la|{{wiktlatWikt-lang|la|oestrus}}|frenzy}}, originally {{etymology|grc|''{{wiktgrcWikt-lang|grc|οἶστρος}}'' ({{grc-transl|οἶστρος}})|gadfly}}) is a set of recurring physiological changes induced by [[reproductive hormone]]s in [[female]]s of [[mammal]]ian subclass [[Theria]].<ref>Hill, M. A. (2021, April 6) Embryology ''Estrous Cycle''. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Estrous_Cycle</ref> Estrous cycles start after [[sexual maturity]] in females and are interrupted by anestrous phases, otherwise known as "rest" phases, or by [[Pregnancy (mammals)|pregnancies]]. Typically, estrous cycles repeat until death. These cycles are widely variable in duration and frequency depending on the species.<ref name=":3">Bronson, F. H., 1989. Mammalian Reproductive Biology. University of Chicago Press, Chicago, IL, USA.</ref> Some animals may display bloody vaginal discharge, often mistaken for [[Menstruation (mammal)|menstruation]].<ref>{{Cite web|last1=Llera|first1=Ryan|last2=Yuill|first2=Cheryl|date=2021|title=Estrous Cycles in Dogs|url=https://vcahospitals.com/know-your-pet/estrus-cycles-in-dogs|access-date=April 6, 2021|website=VCA Hospitals}}</ref> Many mammals used in commercial agriculture, such as cattle and sheep, may have their estrous cycles artificially controlled with hormonal medications for optimum productivity.<ref name=":22">{{Cite web|title=Synchronizing Estrus in Cattle - How does estrus synchronization work?|url=https://agrilifeextension.tamu.edu/library/ranching/synchronizing-estrus-in-cattle/|access-date=2021-03-22|website=Texas A&M AgriLife Extension Service|language=en-US}}</ref><ref name=":12">{{Cite journal|last=Medicine|first=Center for Veterinary|date=2021-03-05|title=The Cattle Estrous Cycle and FDA-Approved Animal Drugs to Control and Synchronize Estrus—A Resource for Producers|url=https://www.fda.gov/animal-veterinary/product-safety-information/cattle-estrous-cycle-and-fda-approved-animal-drugs-control-and-synchronize-estrus-resource-producers|journal=FDA|language=en}}</ref> The male equivalent, seen primarily in ruminants, is called [[Rut (mammalian reproduction)|rut]].<ref name=":3" />

However, species vary significantly in the detailed functioning. One difference is that animals that have estrous cycles [[Resorption|resorb]] the [[endometrium]] if conception does not occur during that cycle. Mammals that have menstrual cycles shed the endometrium through [[Menstruation (mammal)|menstruation]] instead. Another difference is [[Animal sexual behavior|sexual activity]]. In species with estrous cycles {{Citation needed|date=March 2023}}, females are generally only sexually active during the [[#Estrus|estrus phase]] of their 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, [[elephant shrew]]s, and a few other species have [[menstrual cycle]]s rather than estrous cycles. TheyHumans, unlike most other species, have [[concealed ovulation]], a lack of obvious external signs to signal estral receptivity at [[ovulation]] (i.e., the ability to become pregnant). AnimalsSome species of animals with estrous cycles often have unmistakable outward displays of receptivity, ranging from engorged and colorful [[genitals]] to behavioral changes like [[mating call]]s.

''Estrus'' is derived via Latin ''{{lang|la|oestrus}}'' ('[[Rage (emotion)|frenzy]]', '[[Botfly|gadfly]]'), from Greek {{lang|grc|οἶστρος}} ''oîstros'' (literally 'gadfly', more figuratively 'frenzy', 'madness', among other meanings like 'breeze'). Specifically, this refers to the gadfly in [[Ancient Greek mythology]] that [[Hera]] sent to torment [[Io (mythology)|Io]], who had been won in her [[Cow|heifer]] form by [[Zeus]].{{factcitation needed|date=October 2021}} [[Euripides]] used ''oestrus'' to indicate 'frenzy', and to describe madness. [[Homer]] used the word to describe panic.<ref>Panic of the [[suitor]]s in Homer, ''[[Odyssey]]'', book 22</ref> [[Plato]] also used it to refer to an irrational drive<ref>Plato, ''[[Laws (dialogue)|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 ''oîstros'' to describe the desire of fish to [[Spawn (biology)|spawn]].<ref>Herodotus, ''[[Histories (Herodotus)|Histories]]'', ch. 93.1</ref>

'''Estrus''' or '''oestrus''' refers to the phase when the female is sexually receptive ("'''in heat'''" in American English, or "'''on heat'''" in British English). Under regulation by [[Gonadotropin|gonadotropic hormone]]s, [[ovarian follicle]]s mature and estrogen secretions exert their biggest influence. The female then exhibits sexually receptive behavior,<ref name="Miller et al.">{{cite journal|first=Geoffrey|last=Miller|date=April 2007|title=Ovulatory cycle effects on tip earnings by lap dancers: Economic evidence for human estrus?|url=http://www.unm.edu/%7Egfmiller/cycle_effects_on_tips.pdf|journal=Evolution and Human Behavior|volume=28 |issue=6|pages=375–381|doi=10.1016/j.evolhumbehav.2007.06.002}}</ref> a situation that may be signaled by visible physiologic changes. Estrus is commonly seen in the mammalian species, including some primates.

In some species, the [[vulva]] becomes swollen and reddened.<ref>{{Cite web |last=Weir |first=Malcolm |title=Estrus and Mating in Dogs |url=https://vcahospitals.com/know-your-pet/breeding-for-pet-owners-estrus-and-mating-in-dogs |website=VCA Animal Hospital}}</ref> [[Ovulation]] may occur spontaneously in others. Especially among [[quadrupeds]], a signal trait of estrus is the [[Lordosis behavior|lordosis reflex]], in which the animal spontaneously elevates her hindquarters.