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Estrous cycle
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The estrous cycle (from Latin oestrus 'frenzy', originally from Ancient Greek οἶστρος (oîstros) 'gadfly') is a set of recurring physiological changes induced by reproductive hormones in females of mammalian subclass Theria.[1] Estrous cycles start after sexual maturity in females and are interrupted by anestrous phases, otherwise known as "rest" phases, or by pregnancies. Typically, estrous cycles repeat until death. These cycles are widely variable in duration and frequency depending on the species.[2] Some animals may display bloody vaginal discharge, often mistaken for menstruation.[3] Many mammals used in commercial agriculture, such as cattle and sheep, may have their estrous cycles artificially controlled with hormonal medications for optimum productivity.[4][5] The male equivalent, seen primarily in ruminants, is called rut.[2]

Differences from the menstrual cycle

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Further information: Menstrual cycle § Cycles and phases

Mammals share the same reproductive system, including the regulatory hypothalamic system that produces gonadotropin-releasing hormone in pulses, the pituitary gland that secretes follicle-stimulating hormone and luteinizing hormone, and the ovary itself that releases sex hormones, including estrogens and progesterone.

However, animals that have estrous cycles resorb the endometrium if conception does not occur during that cycle. Mammals that have menstrual cycles shed the endometrium through menstruation instead.

Humans, elephant shrews, and a few other species have menstrual cycles rather than estrous cycles. Humans, 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). Some species of animals with estrous cycles have unmistakable outward displays of receptivity, ranging from engorged and colorful genitals to behavioral changes like mating calls.

Etymology and nomenclature

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Estrus is derived via Latin oestrus ('frenzy', 'gadfly'), from Greek 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, who had been won in her heifer form by Zeus.[citation needed] Euripides used oestrus to indicate 'frenzy', and to describe madness. Homer used the word to describe panic.[6] Plato also used it to refer to an irrational drive[7] and to describe the soul "driven and drawn by the gadfly of desire".[8] 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.[9]

The earliest use in English was with a meaning of 'frenzied passion'. In 1900, it was first used to describe 'rut in animals; heat'.[10][11]

In British English, the spelling is oestrus or (rarely) œstrus. In all English spellings, the noun ends in -us and the adjective in -ous. Thus in Modern International English, a mammal may be described as "in estrus" when it is in that particular part of the estrous-cycle.

Four phases

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"Estrus" redirects here. For other uses, see Estrus (disambiguation).

Overview of the mammalian estrous cycle

A four-phase terminology is used in reference to animals with estrous cycles.

Proestrus

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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 of 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 degenerates; the uterus and the vagina distend and fill with fluid, become contractile and secrete a sanguinous fluid; the vaginal epithelium proliferates and the vaginal cytology shows a large number of non-cornified nucleated epithelial cells. Variant terms for proestrus include pro-oestrus, proestrum, and pro-oestrum.

Estrus

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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 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. Estrus is commonly seen in the mammalian species, including some primates.

In some species, the vulva becomes swollen and reddened.[12] Ovulation may occur spontaneously in others. Especially among quadrupeds, a signal trait of estrus is the lordosis reflex, in which the animal spontaneously elevates her hindquarters.

Controlled internal drug release devices are used in livestock for the synchronization of estrus.

Metestrus or diestrus

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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 pseudopregnancy) terminates with the regression of the corpus luteum. The lining in the uterus is not shed, but is reorganized for the next cycle. Other spellings include metoestrus, metestrum, metoestrum, dioestrus, diestrum, and dioestrum.

Anestrus

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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. Chronic exposure to anabolic steroids may also induce a persistent anestrus due to negative feedback on the hypothalamus/pituitary/gonadal axis. Other spellings include anoestrus, anestrum, and anoestrum.

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.[13] For example, the mouse has a fertile postpartum estrus that occurs 14 to 24 hours following parturition.

Cycle-variability

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Estrous 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.[14] Domestication can affect estrous cycles due to changes in the environment. For most species, vaginal smear cytology may be used in order to identify estrous cycle phases and durations.[15]

Frequency

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Some species, such as cats, cows and domestic pigs, are polyestrous, meaning that they can go into heat several times per 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 are diestrous. Canines are diestrous.[citation needed]

Monestrous species, such as canids[16] and bears, have only one breeding season per 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, instead being induced to ovulate by the act of mating and are able to conceive at almost any arbitrary moment.

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

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Cats

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The female cat in heat has an estrus of 14 to 21 days and is generally characterized as an induced ovulator, since coitus induces ovulation. However, various incidents of spontaneous ovulation have been documented in the domestic cat and various non-domestic species.[17] Without ovulation, she may enter interestrus, which is the combined stages of diestrus and anestrus, 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.[18]

Dogs

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Main article: Canine reproduction

A female dog is usually diestrous (goes into heat typically twice per year), although some breeds typically have one or three cycles per year. The proestrus is relatively long at 5 to 9 days, while the estrus may last 4 to 13 days, with a diestrus of 60 days followed by about 90 to 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 occurs 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.[19]

Horses

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Main article: Horse reproduction

A mare may be in heat for 4 to 10 days, followed by approximately 14 days in diestrus. Thus, a cycle may be short, totaling approximately 3 weeks.[20] Horses mate in spring and summer; autumn is a transition time, and anestrus occurs during 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

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Rats are polyestrous animals that typically have rapid cycle lengths of 4 to 5 days.[21] 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 behavior.[22] The events of the cycle are strongly influenced by lighting periodicity.[10]

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.

Estrogen 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. 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.[23]

Bison

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Buffalo have an estrous cycle of about 22 to 24 days. Buffalo are known for difficult estrus detection. This is one major reason for being less productive than cattle. During four phases of its estrous cycle, mean weight of corpus luteum has been found to be 1.23±0.22g (metestrus), 3.15±0.10g (early diestrus), 2.25±0.32g (late diestrus), and 1.89±0.31g (proestrus/estrus), respectively. The plasma progesterone concentration was 1.68±0.37, 4.29±0.22, 3.89±0.33, and 0.34±0.14 ng/ml while mean vascular density (mean number of vessels/10 microscopic fields at 400x) in corpus luteum was 6.33±0.99, 18.00±0.86, 11.50±0.76, and 2.83±0.60 during the metestrus, early diestrus, late diestrus and proestrus/estrus, respectively.[24]

Cattle

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Female cattle, also referred to as "heifers" in agriculture, will gradually enter standing estrus, or "standing heat," starting at puberty between 9 and 15 months of age. The cow estrous cycle typically lasts 21 days.[5] Standing estrus is a visual cue which signifies sexual receptivity for mounting by male cattle. This behavior lasts anywhere between 8 and 30 hours at a time.[25] Other behaviors of the female during standing estrus may change, including, but not limited to: nervousness, swollen vulva, or attempting to mount other animals.[25] While visual and behavioral cues are helpful to the male cattle, estrous stages cannot be determined by the human eye. Rather, the stage can be estimated from the appearance of the corpora lutea or follicle composition.[26][27]

Estrous control

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Further information: Estrous synchronization

Due to the widespread use of bovine animals in agriculture, cattle estrous cycles have been widely studied, and manipulated, in an effort to maximize profitability through reproductive management.[25] Much estrous control in cattle is for the purpose of synchronization, a practice or set of practices most often used by cattle farmers to control the timing and duration of estrus in large herds.[4]

There is variation between the available methods of cattle estrous synchronization. Treatment depends on herd size, specific goals for control, and budget.[4] Some of the FDA-approved drugs and devices used to mimic natural hormones of the estrous cycle include, but are not limited to, the following classes:

  • Gonadorelin: There are currently five available gonadorelin products that are FDA-Approved.[28] Usually, gonadorelin is used in conjunction with another estrous control drug (typically, prostaglandin).[28][29] This drug is used to mimic gonadotropin releasing hormone (GnRH) and may also be used to treat ovarian cysts.[5]
  • Prostaglandin: Mimics the prostaglandin F2-alpha hormone released when no pregnancy has occurred and regresses the corpus luteum.[5] This drug is used to achieve more consistent results in artificial insemination.[29]
  • Progestin: Used to suppress estrus and/or block ovulation.[5] Most commonly, it is administered via an intravaginal insert comparable to an IUD, which used in controlling menstrual periods.[30] It is also available as a medicated feed, but this method is not yet approved for cattle crop synchronization.[5]

There is variation between the available methods of cattle estrous synchronization. Treatment depends on herd size, specific goals for control, and budget.[4]

Bovine estrous cycles may also be impacted by other bodily functions such as oxytocin levels.[31] Additionally, heat stress has been linked to impairment of follicular development, especially impactful to the first-wave dominant follicle.[32] Future synchronization programs are planning to focus on the impact of heat stress on fertilization and embryonic death rates after artificial insemination.[33]

Additionally, work has been done regarding other mammalian females, such as in dogs, for estrous control; However, there are yet to be any approved medications outside of those commercially available.[34]

Others

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Estrus frequencies of some other notable mammals:

See also

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References

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Estrous cycle

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The estrous cycle is the recurring reproductive cycle in most female mammals, characterized by periodic changes in the ovaries and reproductive tract that prepare the body for , , and potential , with sexual receptivity limited to a brief period known as estrus. Unlike the in humans and some , it lacks significant shedding of the uterine lining, instead involving reabsorption of the if conception fails. The cycle begins at and is interrupted only by or anestrous periods, varying in length from 3 to 21 days across species, with larger mammals generally exhibiting longer cycles due to extended luteal phases. The estrous cycle is divided into four distinct phases: proestrus, estrus, metestrus, and diestrus, each driven by fluctuating hormone levels that coordinate follicular development, , and preparation of the reproductive tract. During proestrus, the from the previous cycle regresses, leading to declining progesterone levels, while (FSH) stimulates the growth of ovarian follicles, causing a rise in (estradiol) that proliferates the and prepares the female for . Estrus follows, marked by peak levels that trigger a (LH) surge from the , inducing and rendering the female sexually receptive, often accompanied by behavioral signs like mounting or vocalization and physiological changes such as vaginal cornification. In metestrus, the ruptured follicle transforms into a new under LH influence, initiating progesterone production, while if fertilization occurs, the ovum travels to the . Diestrus represents the , where the functions optimally, secreting high levels of progesterone that maintain the in a secretory state and inhibit further follicular development through negative feedback on (GnRH) from the . Hormonal regulation of the estrous cycle involves intricate feedback loops between the , , and ovaries, ensuring synchronized events. GnRH pulses from the stimulate the release of FSH and LH, which in turn drive ovarian steroidogenesis; exerts during proestrus to amplify the preovulatory LH surge, while progesterone provides during diestrus to suppress GnRH and gonadotropins, preventing premature . If does not occur, prostaglandins from the trigger luteolysis ( regression), dropping progesterone and restarting the cycle. This system contrasts with the menstrual cycle's more prolonged and overt bleeding due to endometrial necrosis from progesterone withdrawal, highlighting evolutionary adaptations in non-primate mammals for efficient reproduction without energy loss from .

Introduction and Fundamentals

Definition and Overview

The estrous cycle refers to the recurring physiological changes in the reproductive tract of non-primate mammals, induced by reproductive hormones, that lead to periods of (heat) and . This cycle represents the primary reproductive process in these species, distinguishing it from the observed in higher . The core purpose of the estrous cycle is to synchronize behavioral receptivity to with development and the preparation of the reproductive tract for potential . These coordinated changes ensure that occurs during a discrete window of , optimizing by aligning physiological readiness with opportunities for conception. The duration of the estrous cycle varies widely among non-primate mammalian , typically from 4-5 days in to about 21 days in ruminants and horses, contrasting with the more continuous estrus-like receptivity in . The cycle involves key anatomical structures, including the ovaries for follicle maturation and , the for endometrial changes, and the hypothalamus-pituitary axis for hormonal orchestration. It comprises distinct phases—proestrus, estrus, metestrus, and diestrus—that drive these reproductive events, with an optional anestrus period of reproductive quiescence.

Etymology and Nomenclature

The term "estrous cycle" derives its first component from the noun "estrus," which originates from the Latin oestrus meaning "" or "gadfly," itself borrowed from the oîstros (οἶστρος), denoting a gadfly, sting, breeze, or mad impulse that drives animals into a frenzied state, metaphorically applied to the intense sexual behavior observed in females during . The second component, "cycle," comes from the cyclus and cicle, ultimately from the Greek kyklos (κύκλος) meaning "circle" or "wheel," signifying a recurring series of events or operations. The nomenclature was formalized in the early 20th century by British zoologist Walter Heape, who in 1900 introduced the term "estrous cycle" (spelled "oestrous" in ) to describe the reproductive periodicity in non-primate mammals, explicitly distinguishing it from the human based on observable behavioral and physiological patterns. Heape's work emphasized the cycle's role in the "sexual season" of mammals, coining phase-specific terms such as proestrus (the preparatory period leading into heat), estrus (the peak of sexual receptivity), metestrus (the immediate post-heat subsidence), diestrus (a brief rest within the breeding season), and anestrus (the extended non-breeding rest period). In modern usage, "estrous" is the standard American English spelling, while "oestrous" persists in , reflecting orthographic conventions for words derived from Latin and Greek roots. This terminology contrasts with the "," which Heape and subsequent researchers reserved for exhibiting overt bleeding, whereas estrous cycles in other mammals are characterized by behavioral "" without such . Colloquially, the estrous cycle is often referred to as "," "rut," or "breeding season" in veterinary and agricultural contexts, terms that capture the female's limited window of and willingness.

Comparison to Menstrual Cycle

Key Differences

The estrous cycle, characteristic of most non-primate mammals, differs fundamentally from the observed in humans and some other in the handling of uterine tissue. In the estrous cycle, the undergoes resorption or reorganization if does not occur, without significant shedding or visible , conserving energy and minimizing physiological costs. In contrast, the involves the sloughing off of the endometrial lining, resulting in menstrual , which is a response to progesterone withdrawal in the absence of implantation. This absence of overt discharge in estrous species reflects an to maintain reproductive without the need for extensive tissue regeneration each cycle. Cycle continuity also sets the estrous and s apart. Estrous cycles can be polyestrous (either continuous year-round or multiple within defined breeding seasons), seasonally polyestrous, or monoestrous annually, with many aligning with environmental cues like photoperiod to concentrate reproductive efforts. For instance, many mammals exhibit multiple estrous cycles during a favorable period, followed by anestrus, whereas the operates continuously year-round, independent of seasons, allowing for more frequent reproductive opportunities. This seasonal patterning in estrous cycles optimizes for survival in variable environments. Behaviorally, the estrous cycle features pronounced sexual receptivity confined to the brief estrus phase, often marked by overt signs like vocalizations or postures that signal readiness for , tightly coupling behavior to . The menstrual cycle decouples these elements, with females potentially receptive throughout the cycle, not solely during fertile periods, which supports social bonding in groups. Evolutionarily, the estrous cycle represents an for with discrete breeding seasons, synchronizing with optimal conditions for viability, such as abundant or milder , thereby enhancing rates in non-primate mammals. Menstrual cycles, conversely, evolved in lineages favoring continuous cycling, possibly to facilitate selection and protection against suboptimal pregnancies through spontaneous . If conception fails, outcomes diverge markedly: the estrous cycle may result in quiet ovulation or pseudopregnancy, with the regressing without endometrial disruption, leading seamlessly to the next cycle or anestrus. In the , non-pregnancy triggers , clearing the for a new proliferative phase.

Physiological Similarities

The estrous and s share fundamental physiological mechanisms that underscore their evolutionary conservation across mammals, enabling through coordinated ovarian and uterine changes. Both cycles feature an ovarian component involving the sequential development of ovarian follicles, of a mature , and subsequent formation of the , which supports potential by secreting progesterone. In estrous cycles of like and dogs, follicles progress from primordial to antral stages under influence, culminating in triggered by a (LH) surge, followed by luteinization of granulosa and theca cells into the ; similarly, in the of primates including humans, (FSH) drives follicular maturation, with occurring mid-cycle and the forming post- to maintain the . These processes ensure the release of gametes at optimal times for fertilization. Hormonal regulation in both cycles is governed by the hypothalamus-pituitary-ovarian (HPO) axis, which integrates positive and negative feedback loops to orchestrate cyclicity. The releases (GnRH) in pulsatile fashion to stimulate pituitary secretion of FSH and LH, which in turn promote ovarian steroidogenesis; rising levels exert to induce the pre-ovulatory LH surge for , while from progesterone and inhibin suppresses gonadotropins during the to prevent premature follicle recruitment. This axis operates comparably in estrous mammals, such as rats and sheep, where feedback dynamics maintain short cycles, and in menstrual cycles, sustaining longer phases with similar steroid-mediated control. Uterine preparation for implantation exhibits parallel endometrial responses driven by ovarian hormones in both cycles. stimulates endometrial proliferation during the follicular/proestrus phase, thickening the lining through glandular and growth to create a receptive environment; from the then induces secretory changes for nutrient support if fertilization occurs. In estrous cycles, such as in dogs, this proliferation peaks pre-ovulation without overt , while in menstrual cycles, it prepares the endometrium for potential , highlighting conserved - synergy for implantation. The duration of the fertile window aligns closely relative to overall cycle length in both systems, with marking the peak fertility period typically 12-24 hours after the LH surge, and viable spermatozoa surviving up to several days prior. In a standard 28-day , occurs around day 14, yielding a 5-6 day fertile window; analogously, in the 4-5 day rodent estrous cycle, follows proestrus, confining fertility to estrus with similar relative timing for conception. This synchronization optimizes encounter across . At the molecular level, core genes encoding receptors for gonadotropins, such as FSH receptor (FSHR) and LH/choriogonadotropin receptor (LHCGR), demonstrate high conservation across mammals, facilitating uniform signaling in reproductive cycles. FSHR, a G-protein-coupled receptor, mediates FSH-driven with conserved transmembrane domains and key residues (e.g., Asp224) essential for activation in both estrous and menstrual contexts; LHCGR similarly supports LH-induced and luteal function through shared structural motifs, ensuring robust hormone responsiveness from to .

Phases of the Cycle

Proestrus

The proestrus phase represents the preparatory stage of the estrous cycle in female mammals, during which the undergoes changes to support follicle maturation and impending . This phase is marked by the regression of the from the previous cycle, leading to declining progesterone levels and the initiation of follicular development in the ovaries. As a result, circulating concentrations begin to rise, setting the stage for subsequent phases. The duration of proestrus varies significantly across species, typically spanning 2-4 days in many domestic mammals, though shorter in (e.g., 12-24 hours in rats and mice) and longer in others (e.g., 6-11 days in dogs). In , it lasts 1-3 days, characterized by follicular waves where multiple follicles grow but only one dominant follicle emerges. Physiological changes include accelerated growth of ovarian follicles stimulated by rising (FSH) from the , with granulosa cells in the developing follicles increasingly producing . This estrogen surge promotes endometrial proliferation in the , thickening the lining through cellular to prepare for potential implantation, while also influencing cervical production to become more watery and fertile. Behaviorally, females during proestrus show limited sexual receptivity to males, distinguishing this phase from the receptive estrus that follows, though they may display increased playfulness or attraction in some . Common physical signs include vulvar swelling due to -induced vascularization and clear or slightly bloody , which aids in and signals approaching ; in , the vagina appears swollen and moist with nucleated epithelial cells observable in smears. These changes reflect the mounting influence on accessory reproductive structures. Hormonally, proestrus is triggered by pulsatile (GnRH) from the , which stimulates the to release escalating levels of FSH and luteinizing hormone (LH). The rising provides to the and pituitary, amplifying GnRH and secretion. This phase endpoints with a critical preovulatory LH surge, typically occurring toward the close of proestrus, which induces final maturation, within 24-36 hours, and the shift to estrus.

Estrus

Estrus, also known as "," represents the fertile phase of the estrous cycle in mammals, marked by peak sexual receptivity, , and physiological adaptations that facilitate and conception. This phase follows the preparatory follicular development in proestrus and is the period when the female is most likely to accept a for copulation, optimizing the chances of successful fertilization. In general, estrus is the shortest phase of the cycle, typically lasting 1-2 days across many mammalian species, though durations vary; for instance, it may extend to 2-3 days in or be as brief as 12-18 hours in . Physiologically, estrus is characterized by the culmination of follicular maturation, with occurring as the dominant event, triggered by a preovulatory surge in (LH) that typically happens toward the end of proestrus but results in egg release during early estrus. The reproductive tract undergoes key changes to support sperm transport and survival: cervical mucus becomes abundant, watery, and sperm-friendly with a characteristic ferning pattern that enhances , while increase to propel sperm toward the oviducts. These adaptations, driven by elevated levels reaching their maximum, create an optimal environment for fertilization shortly after . Behaviorally, females exhibit pronounced signs of receptivity during estrus, including attraction to males, increased vocalizations, and specific postures such as in or "standing heat" in many ungulates like and , where the female rigidly stands to allow mounting. These behaviors are estrogen-mediated and signal peak , often accompanied by restlessness, frequent mounting of other females, and vulvar swelling or discharge. Hormonally, estrus follows the LH surge, with concentrations at their zenith to sustain receptivity, while progesterone levels begin to rise post-ovulation as the starts forming, marking the transition to subsequent phases. The window during estrus is narrow but critical, with optimal timing aligned to the period of receptivity, as the released ova remain viable for approximately 12-24 hours in most mammals. This brief viability underscores the importance of synchronized , with survival in the tract potentially extending up to 48 hours in some to overlap with .

Metestrus

Metestrus represents the immediate post-ovulatory phase of the estrous cycle in mammals, typically lasting 2 to 4 days, as observed in such as and sheep. This brief interval follows the cessation of estrus and is characterized by the onset of luteal development following , which occurs approximately 10 to 15 hours after the end of behavioral receptivity in bovines. Physiologically, the corpus hemorrhagicum—a blood clot-filled structure—forms within the ovarian follicle's rupture site, serving as the precursor to the through luteinization of granulosa and theca cells. This early initiates low-level progesterone secretion, which gradually rises but remains insufficient for full luteal support initially, while circulating concentrations decline sharply from estrus peaks. Behaviorally, sexual receptivity diminishes quickly, with females exhibiting reduced attraction to males and no further mating attempts, signaling the end of the fertile window. In the uterus, rising progesterone prompts initial endometrial changes, including glandular proliferation and vascular remodeling, to prepare for embryo implantation should fertilization occur. However, without an , these preparatory alterations remain transient and reversible, allowing the reproductive tract to reset for subsequent cycles. If conception fails, early signs of pseudopregnancy—such as mild development—may emerge in certain like dogs, driven by sustained progesterone influence. Overall, metestrus functions as a critical bridge from the ovulatory events of estrus to the sustained luteal dominance of diestrus, stabilizing the post-ovulatory environment while maintaining reproductive plasticity.

Diestrus

Diestrus represents the of the estrous cycle in mammals, characterized by the dominance of progesterone secreted by the mature , which supports potential implantation and early or regresses if fertilization does not occur. This phase typically lasts 10-14 days in many species, making it the longest segment of the cycle, though durations vary; for instance, it spans approximately 12 days in and 14-15 days in mares. Following the brief metestrus transition, the fully matures during diestrus, inhibiting the development of new ovarian follicles through on the hypothalamic-pituitary axis. Physiologically, the elevated progesterone prepares the reproductive tract for , thickening the and promoting glandular secretions in like , while suppressing further . If occurs, progesterone sustains the and initiates development for , as seen in such as pigs and cows. Hormonally, progesterone levels rise to and maintain above 5 ng/mL—reaching 30-40 ng/mL in pigs—suppressing (GnRH), (LH), and (FSH) release, thereby preventing premature follicular growth. Behaviorally, females exhibit no sexual receptivity during diestrus, often displaying increased or, if pregnant, early maternal tendencies, as progesterone overrides estrogen-driven behaviors observed in prior phases. The phase concludes with luteolysis, the regression of the triggered by uterine-derived F2α (PGF2α) in non-pregnant animals, leading to a sharp decline in progesterone and the onset of proestrus; this process is well-documented in ruminants like , where PGF2α pulses initiate the next cycle.

Anestrus

Anestrus represents the phase of reproductive dormancy in the estrous cycle of many mammals, marked by a complete cessation of follicular development, , and behavioral estrus, with females exhibiting indifference or resistance to advances. During this period, the reproductive tract remains quiescent, including small, inactive ovaries and a with minimal endometrial activity. This phase contrasts with the active cycling periods of proestrus, estrus, metestrus, and diestrus by imposing a prolonged interval of . The duration of anestrus varies widely across species and environmental contexts, typically ranging from several weeks to months, and is often pronounced in seasonal breeders from temperate regions. For instance, in long-day breeders such as and donkeys, anestrus may last 3–5 months during autumn and winter, while in short-day breeders like sheep, it occurs over summer, spanning 2–3 months. Postpartum anestrus can extend longer, such as 12–24 months in under natural conditions, though it shortens with reduced stress. Physiologically, anestrus features suppressed secretion of (GnRH) from the , resulting in low circulating levels of (LH) and (FSH). This leads to ovarian inactivity, with follicles undergoing rather than maturation, and correspondingly low concentrations of and progesterone. Uterine involution occurs, minimizing metabolic demands on the . Key causes of anestrus include environmental cues that inhibit hypothalamic-pituitary-gonadal axis activity, such as shortened photoperiods in winter for long-day breeders, which elevate and dampen GnRH pulsatility. Nutritional deficits, signaling low energy availability through pathways like AMPK activation, further suppress GnRH release, while stress-induced elevation from the hypothalamic-pituitary-adrenal axis reinforces this inhibition. The primary role of anestrus is to promote , allowing mammals to allocate resources toward rather than during periods of environmental adversity, such as scarcity or harsh in non-breeding seasons. This adaptive strategy ensures that breeding resumes only when conditions favor offspring viability, as seen in temperate species like sheep and .

Hormonal Regulation

Major Hormones Involved

The estrous cycle in mammals is orchestrated by a coordinated network of hormones that regulate follicular development, , and preparation for potential pregnancy. These hormones originate from the , gland, ovaries, and , with their pulsatile secretion driving the cyclic changes in reproductive physiology. Key players include (GnRH), (FSH), (LH), (the primary estrogen), progesterone, and prostaglandin F2α (PGF2α). Gonadotropin-releasing hormone (GnRH) is secreted by neurons in the in a pulsatile manner, serving as the master regulator of the reproductive axis. It stimulates the to release FSH and LH, thereby initiating and synchronizing ovarian events across the cycle phases. Disruptions in GnRH pulsatility can lead to irregular estrous cycles in various species. Follicle-stimulating hormone (FSH), produced by gonadotroph cells in the , promotes the growth and maturation of ovarian follicles during the early stages of the cycle. FSH acts on granulosa cells to enhance follicular development and stimulate the production of estrogens, setting the stage for subsequent . Levels of FSH typically rise at the onset of the cycle to recruit multiple follicles, with the dominant one selected for further growth. Luteinizing hormone (LH), also secreted from the , plays a pivotal role in triggering and supporting luteal function. A surge in LH, induced by rising levels, causes the rupture of the mature follicle and the release of the , typically occurring around the estrus phase. Post-ovulation, LH maintains the , ensuring progesterone production to sustain the . Estradiol, the predominant form of , is synthesized by granulosa cells in developing ovarian follicles under the influence of FSH. It exerts on the and pituitary to induce the LH surge, promotes endometrial proliferation during proestrus, and elicits behavioral estrus in females. Peak levels correlate with receptivity to and prepare the reproductive tract for fertilization. Progesterone is primarily produced by the following , induced by LH. It maintains the uterine environment during the (diestrus), inhibits further follicular development, and supports early if conception occurs. Declining progesterone levels signal the regression of the , allowing the cycle to restart. Prostaglandin F2α (PGF2α), synthesized by the uterine , is crucial for luteolysis at the end of the . Released in pulses around days 16-18 of the cycle in like , it induces the functional and structural breakdown of the , thereby reducing progesterone and facilitating the return to follicular development. This process ensures cyclic in non-pregnant animals.

Molecular Mechanisms

The molecular mechanisms governing the estrous cycle operate primarily through receptor-mediated signaling and at the cellular level. The (GnRH) receptor, a prototypical G-protein-coupled receptor (GPCR), facilitates the pulsatile release of GnRH from hypothalamic neurons, activating downstream pathways to mobilize intracellular calcium and . Similarly, the (LH) and (FSH) receptors, also members of the GPCR superfamily, are expressed on granulosa and cells in the , where they couple to Gs proteins to stimulate adenylate cyclase, elevating cyclic AMP levels and promoting steroidogenesis. In parallel, and progesterone receptors function as nuclear receptors, translocating to the nucleus upon binding to modulate structure and directly influence the transcription of target genes involved in reproductive tissue proliferation and differentiation. Feedback loops at the molecular level ensure precise temporal control of the cycle. arises from escalating concentrations, which activate in neurons within the anteroventral of the , triggering a surge in GnRH and subsequent LH release to induce . Conversely, is exerted by progesterone binding to its nuclear receptors, which represses GnRH neuronal activity through inhibition of expression and enhancement of tone, thereby suppressing secretion during the . Gene regulation drives the synthesis of key hormones and adapts the cycle to physiological demands. The CYP19A1 gene, encoding the enzyme, is transcriptionally activated in ovarian granulosa cells by FSH-induced cAMP signaling via of transcription factors such as CREB, converting androgens to estrogens essential for follicular maturation. Epigenetic mechanisms, including and histone modifications, further regulate reproductive during seasonal anestrus in mammals, silencing pathways like GnRH signaling in response to prolonged exposure under short photoperiods. Recent investigations have highlighted novel molecular players in cycle dynamics. , secreted by the in response to photoperiod, modulates estrous onset in seasonal breeders through 2021 studies showing its binding to MT1/MT2 receptors in the , which alters epigenetic marks on clock genes like PER2 to synchronize reproductive timing. Additionally, microRNAs (miRNAs) contribute to luteolysis by post-transcriptionally repressing genes in the pathway. Disruptions in these mechanisms can precipitate reproductive pathologies. Inactivating mutations in the FSH receptor (FSHR) gene, such as those impairing G-protein coupling, disrupt follicular recruitment and production, resulting in irregular estrous cycles and in female mammals, as observed in murine models with homozygous variants.

Variability and Influences

Cycle Length and Frequency

The estrous cycle length refers to the interval from the onset of one estrus to the onset of the next, typically encompassing the full sequence of phases driven by hormonal fluctuations and . In mammals, this duration varies widely across , reflecting adaptations to reproductive strategies, with measurements often derived from behavioral observations, , or hormonal assays in veterinary and settings. For instance, the cycle is influenced by the rate of follicular development and maintenance, leading to consistent patterns in healthy adults but potential irregularities in other conditions. Mammals exhibit different frequencies of estrous cycles based on annual reproductive patterns. Polyestrous species, such as and pigs, experience continuous cycles throughout the year without prolonged anestrus, allowing multiple breeding opportunities. Seasonally polyestrous animals, like sheep and horses, restrict cycles to specific breeding seasons, typically aligned with environmental optima for offspring survival. Monoestrous mammals, including dogs and certain wild species like foxes, have only one or two cycles per year, with extended intervals between events to concentrate reproductive efforts. These classifications are determined by the number of cycles annually and the presence of anestrous periods. Cycle consistency can be affected by internal factors such as age and health status. In younger animals, cycles may be shorter or more irregular due to immature hypothalamic-pituitary-ovarian axis development, as observed in peripubertal and where initial cycles often deviate from adult norms. Advancing age in adults can lead to progressively shortened cycles or increased variability, linked to declining and altered hormone dynamics, particularly in mice and . Health conditions, including nutritional deficiencies or systemic diseases, further disrupt cycle regularity by impacting hormone production or follicular health, resulting in prolonged or skipped cycles. These influences underscore the importance of monitoring for reproductive management in veterinary practice.
SpeciesCycle Length (Average and Range)Frequency Type
4–5 daysPolyestrous
4–5 daysPolyestrous
21 days (19–23 days)Polyestrous
21 days (18–24 days)Polyestrous
Sheep17 days (14–19 days)Seasonally polyestrous
21 days (19–22 days)Seasonally polyestrous
7 months (4–12 months)Monoestrous (2 cycles/year)

Environmental and Seasonal Factors

The estrous cycle in mammals is highly sensitive to photoperiod, the relative lengths of day and night, which acts as a predictive cue to synchronize with seasonal environmental changes. In long-day breeders, extended daylight suppresses pineal secretion, reducing its inhibitory effects on the hypothalamic-pituitary axis and thereby promoting GnRH release to initiate cyclicity. Conversely, in short-day breeders, shortened photoperiods elevate nocturnal production, which stimulates GnRH neurons and triggers the onset of estrous activity. This -mediated mechanism allows animals to anticipate favorable breeding conditions, such as increased availability. Nutritional inputs, particularly energy balance, exert significant control over cycle onset and maintenance via adipocyte-derived , which conveys peripheral energy status to the . Positive energy balance elevates leptin levels, facilitating GnRH pulsatility and supporting regular cyclicity, whereas undernutrition induces leptin deficiency, hypersensitizing the system and suppressing reproductive hormones to conserve resources. This results in prolonged anestrus, as the prioritizes survival over during caloric deficits. Acute or disrupts cyclicity primarily through hormones like , which directly inhibit GnRH from hypothalamic neurons. Elevated reduces GnRH pulse frequency, particularly during the when ovarian steroids are present, delaying and extending inter-cycle intervals. This stress response integrates with other cues to halt under adverse conditions. Evolutionarily, these environmental regulators have shaped the estrous cycle to align breeding with periods of resource abundance, ensuring that and rearing coincide with maximal food availability for enhanced juvenile survival rates. Such adaptations reflect selective pressures favoring predictive synchronization over opportunistic timing in variable habitats. Post-2020 studies underscore the vulnerability of seasonal breeders to , where altered photoperiods and intensified heat stress desynchronize cycles by disrupting rhythms and elevating baseline , leading to reduced estrus detection and in affected populations. These shifts pose risks to as environmental predictability declines.

Species-Specific Variations

In Companion Animals

The estrous cycle in companion animals, particularly cats and dogs, exhibits distinct patterns that influence breeding practices and health management in veterinary settings. In cats (Felis catus), the cycle is seasonally polyestrous, with typically three cycles occurring per breeding season, which is triggered by increasing daylight lengths of at least 12 hours daily. Estrus lasts an average of 7–10 days, during which display pronounced behavioral signs such as frequent vocalization, increased affection, and posture in response to dorsolateral stroking. is induced, occurring 24–48 hours after vaginal from , which triggers a (LH) surge via neural reflexes; without such , may re-enter estrus every 15–21 days. In dogs (Canis familiaris), the estrous cycle is monoestrous, featuring a single breeding period annually followed by a prolonged anestrus phase averaging 7 months (ranging from 4–13 months). Proestrus typically lasts about 9 days (3–21 days), characterized by vulvar swelling and serosanguinous , while estrus follows for another 9 days on average (4–21 days), marking the fertile period when bitches accept . Hormonal fluctuations, particularly rising estrogen in proestrus and progesterone post-ovulation, can increase skin sensitivity, cause itchiness or rashes, exacerbate existing allergies, and lead to direct skin irritation, often prompting increased grooming behaviors such as excessive licking of the vulva and scratching. A phenomenon known as "silent heat" can occur, where ovarian activity proceeds without obvious external signs like bleeding or behavioral changes, potentially leading to unintended pregnancies if undetected. A common clinical concern in both species is , a progesterone-mediated uterine that arises during diestrus due to endometrial proliferation and immune suppression from sustained progesterone exposure post-. This condition is more prevalent in intact females over 5 years old and underscores the importance of spaying in non-breeding pets. For breeding management, serves as a key diagnostic tool, particularly in dogs, where serial sampling every 2–3 days reveals increasing cornified epithelial cells (≥70–90% superficial cells) to pinpoint optimal timing around .

In Livestock

In livestock, the estrous cycle plays a critical role in reproductive for like , , and , where efficient breeding directly impacts . exhibit a polyestrous cycle averaging 21 days in length, with estrus typically lasting about 18 hours during which females display mounting behavior by standing to be mounted by other cows or bulls. Postpartum anestrus in generally spans 30 to 60 days, allowing recovery before the resumption of cyclicity. Horses, in contrast, are seasonally polyestrous, with cycles occurring primarily from to in the , driven by increasing daylight lengths. Their estrous cycles average 21 days, and a notable feature is "foal heat," which occurs 7 to 12 days after foaling and enables rapid rebreeding in managed herds. The estrous cycle in closely resembles that of , with cycles averaging 22 days, but features a prolonged anestrus period influenced by harsh environmental conditions such as and limited , often resulting in biennial breeding patterns in wild populations. Agricultural practices in often leverage estrous to achieve uniform calving seasons, enhancing market efficiency through consistent calf weights and reduced labor demands during birthing. In cows, prolonged anestrus can arise from negative energy balance during early , where high milk production outpaces energy intake, delaying ovarian resumption and impacting herd fertility.

In Laboratory and Wild Animals

The estrous cycle in laboratory s is polyestrous, typically lasting 4 to 5 days, with a short proestrus phase of approximately 12 hours characterized by rising levels and preparation for . This cycle consists of four phases—proestrus, estrus (24-48 hours), metestrus (6-8 hours), and diestrus (48-72 hours)—allowing for frequent reproductive opportunities under controlled conditions. Vaginal smear cytology is a standard non-invasive method for staging these phases, relying on the appearance of nucleated epithelial cells during proestrus, cornified anucleated cells in estrus, leukocytes in metestrus, and a mix of leukocytes and epithelial cells in diestrus. Due to its regularity and ease of monitoring, the rat model is widely used in hormone studies to investigate reproductive physiology, including regulation and ovulatory mechanisms. Similar to rats, laboratory mice exhibit a 4- to 5-day polyestrous cycle divided into proestrus, estrus, metestrus, and diestrus, with occurring during estrus and enabling precise staging based on cell morphology shifts. In Syrian hamsters, the cycle is consistently 4 days long, with estrus marked by sexual receptivity in the evening, but these animals display seasonal variations as long-day breeders, entering reproductive quiescence under short photoperiods mimicking winter, which suppresses cyclicity. These laboratory provide reliable models for dissecting estrous dynamics, with environmental factors like photoperiod influencing cycle regularity in a manner analogous to broader variability seen across species. In wild species, estrous cycles often align with seasonal breeding to optimize survival. are monoestrous, with a single breeding season in fall triggered by decreasing day length, featuring an estrous cycle of about 28 days that may recur up to six times if conception fails, but limited to the autumn rut period. African elephants exhibit irregular estrous cycles averaging 13 to 17 weeks, with luteal phases of 8 to 10 weeks and follicular phases of 4 to 7 weeks, often disrupted by factors like or , contributing to extended inter-birth intervals of about 4 years in the wild. These patterns highlight evolutionary adaptations for resource-limited environments, contrasting the continuous cycling in laboratory models. Laboratory serve as key models for developing drugs, enabling tests of compounds that synchronize or induce estrous phases to mimic reproductive therapies. Evolutionary comparisons using wild like deer and reveal how estrous timing enhances offspring survival through seasonal synchronization, informing cross- insights into reproductive resilience. Recent 2022 studies on have advanced understanding of regulation, showing that ablation of in neurons disrupts onset and by impairing pulsatility, while stereological analyses link pituitary adenylate cyclase-activating polypeptide to altered neuron structure during embryonic development. These findings underscore 's role in fine-tuning estrous cyclicity for potential therapeutic targeting in reproductive disorders.

Applications in Veterinary Medicine

Estrous Synchronization

Estrous synchronization involves the use of hormonal treatments to align the estrous cycles of a group of females, enabling coordinated breeding and (AI) at predetermined times, primarily in such as to enhance reproductive efficiency. This approach targets key physiological events in the estrous cycle, including luteolysis and , to compress the breeding period and improve . The primary method for inducing luteolysis uses prostaglandin F2α (PGF2α), which causes regression of the , thereby resetting the cycle in cycling animals. For ovulation control, (GnRH) is administered to induce a luteinizing hormone surge, promoting follicular maturation and . A seminal protocol, the Ovsynch method, combines these hormones in a sequence: an initial GnRH injection on day 0 to synchronize follicle wave emergence, followed by PGF2α on day 7 for luteolysis, and a second GnRH injection 48-56 hours later to trigger , with timed AI occurring 16-20 hours after the final GnRH. The CO-Synch protocol, a variation of Ovsynch, simplifies timing by administering the second GnRH concurrently with timed AI approximately 66 hours after PGF2α, reducing the need for precise heat detection. The 7-day CO-Synch protocol, often combined with a controlled internal drug release (CIDR) insert for progestin supplementation, further enhances synchronization in postpartum or anestrous cows. FDA-approved products include Lutalyse (dinoprost tromethamine, a PGF2α analog) at a dosage of 25 mg (5 mL) intramuscularly per cow, and Cystorelin (, a GnRH analog) at 100 mcg (2 mL) intramuscularly per cow. These protocols achieve pregnancy rates of 50-70% following fixed-time AI, depending on factors such as cow body condition, postpartum interval, and protocol adherence, with Ovsynch often yielding 54-72% in well-managed . Benefits include shortened breeding seasons from 60 to 45 days or less, increased calf crop uniformity, and earlier conceptions that boost overall productivity by concentrating calving within a narrower window. Regarding safety, the FDA provides resources emphasizing proper administration to minimize residues and risks, with updates in 2021 protocols confirming the established profiles of these hormones when used as labeled.

Reproductive Management Techniques

Reproductive techniques in veterinary practice encompass a range of interventions aimed at optimizing outcomes in animals exhibiting estrous cycles, beyond basic methods. These techniques focus on accurate detection of estrus, targeted induction of for advanced reproductive procedures, suppression of cycles for , nutritional enhancements to boost reproductive vigor, and innovative genetic approaches to modulate cycle-related traits. Such methods are particularly vital in , companion animals, and , where precise timing and control directly influence breeding success and . Detection of estrus remains a of effective reproductive , relying on non-invasive tools to identify receptive females. Tail paint, applied to the tailhead or rump, serves as a visual indicator of mounting activity, with removal or smearing signaling potential estrus; studies in heifers demonstrate that spray-formulated paints reduce false positives from or rubbing behaviors compared to traditional chalk-based options, achieving detection efficiencies exceeding 94% when combined with behavioral observation. Teaser males, typically sterilized bulls via or epididymectomy, enhance detection by exhibiting strong while preventing unwanted pregnancies; these animals identify standing estrus with rates up to 97%, outperforming visual checks in and herds, though selection of healthy, trainable individuals is essential to minimize transmission risks. Ultrasound imaging provides a precise, real-time method for monitoring follicular development and confirming , visualizing the sudden disappearance of a dominant follicle (9-20 mm in diameter) and subsequent formation as early as day 3 post- in cows; transrectal ultrasonography tracks 2-4 follicular waves per cycle, enabling timed interventions in and sheep. Induction techniques target superovulation to maximize yield for programs, particularly in valuable breeds. Administration of (hCG), which mimics , during the early promotes accessory corpora lutea formation and elevates progesterone levels, improving embryo viability; in sheep, combining hCG with (FSH) protocols has increased rates and lambing outcomes, with single injections yielding up to 85% conception in superovulated donors under subtropical conditions. Contraceptive strategies, such as deslorelin acetate implants, offer reversible suppression of estrous cycles in wildlife and managed populations to prevent overbreeding. These (GnRH) agonists downregulate reproductive hormones, halting estrus for extended periods; in lionesses and tigers, 9.4 mg deslorelin implants effectively induced contraception lasting over 2 years without adverse health effects, providing a humane alternative to surgical sterilization in captive and free-ranging felids. Nutritional aids like flushing diets elevate and protein intake to amplify estrus expression and . Short-term supplementation (2-3 weeks pre-breeding) with concentrates and forages, such as enset leaves combined with commercial feeds, shortens estrus onset, boosts response rates, and enhances conception to 85% in ewes; high-energy flushing in Doyogena sheep increased by 151% through improved body condition and rates, underscoring its role in optimizing reproductive performance under resource-limited conditions. Emerging gene-editing technologies, including , hold promise for precise control of reproductive traits in , targeting genes influencing and litter size. Post-2020 research has applied these tools to investigate genes involved in , such as those affecting pathways and sex-specific outcomes, supporting sustainable production through integration with traditional breeding. As of 2025, gene-edited resistant to diseases like PRRS are nearing FDA approval, with ongoing studies exploring broader reproductive enhancements.

References

  1. https://www.uwyo.edu/wjm/repro/estrous.htm
  2. https://www.uwyo.edu/wjm/repro/menstrua.htm
  3. https://pmc.ncbi.nlm.nih.gov/articles/PMC1789810/
  4. https://embryology.med.unsw.edu.au/embryology/index.php/Estrous_Cycle
  5. https://www.sciencedirect.com/topics/medicine-and-dentistry/estrus-cycle
  6. https://www.intechopen.com/chapters/55696
  7. https://pmc.ncbi.nlm.nih.gov/articles/PMC7071652/
  8. https://www.etymonline.com/word/estrus
  9. https://www.etymonline.com/word/cycle
  10. https://wellcomecollection.org/works/p97unrys
  11. https://doi.org/10.1177/0192623315570339
  12. https://www.merriam-webster.com/dictionary/estrus
  13. https://pmc.ncbi.nlm.nih.gov/articles/PMC7765733/
  14. https://pmc.ncbi.nlm.nih.gov/articles/PMC6541669/
  15. https://www.extension.purdue.edu/extmedia/as/as-559-w.pdf
  16. https://www.eolss.net/sample-chapters/c10/E5-15-20-03.pdf
  17. https://pmc.ncbi.nlm.nih.gov/articles/PMC3528014/
  18. https://pmc.ncbi.nlm.nih.gov/articles/PMC4963617/
  19. https://agrilife.org/animalscience/files/2012/04/ANSC-631-Bazer-Spring-2014-Estrous-Menstrual-Cycles-Part-1.pdf
  20. https://my.clevelandclinic.org/health/articles/23439-ovulation
  21. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/estrous-cycle
  22. https://febs.onlinelibrary.wiley.com/doi/10.1111/febs.15649
  23. https://dairy-cattle.extension.org/what-is-the-basic-estrous-cycle-of-the-cow/
  24. https://www.vet.cornell.edu/departments-centers-and-institutes/riney-canine-health-center/canine-health-information/dog-estrous-cycles
  25. https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/estrus-cycle
  26. https://beefrepro.org/wp-content/uploads/2020/09/02-Moorey-SE.pdf
  27. https://www.vet.k-state.edu/academics/student-faculty-handbook/studentorgs/aadpDocs/Estrous_Cycle_physiology1.pdf
  28. https://www.groupe-esa.com/ladmec/bricks_modules/brick03/co/ZBO_Brick03_4.html
  29. https://krex.k-state.edu/server/api/core/bitstreams/f6baae72-d9d8-468f-aa03-d10a53601b04/content
  30. https://vetmed-education-a2.sites.medinfo.ufl.edu/wordpress/files/2024/12/Phase-I-VEM-5111E-Reproduction-Spring-2025.pdf
  31. https://scholarworks.uark.edu/cgi/viewcontent.cgi?article=7230&context=etd
  32. https://pmc.ncbi.nlm.nih.gov/articles/PMC10393238/
  33. https://extension.missouri.edu/publications/g2015
  34. https://pmc.ncbi.nlm.nih.gov/articles/PMC1456201/
  35. https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/metestrus
  36. https://www.sciencedirect.com/topics/veterinary-science-and-veterinary-medicine/estrus
  37. https://www.sciencedirect.com/topics/veterinary-science-and-veterinary-medicine/estrous-cycle
  38. https://www.sciencedirect.com/science/article/pii/B0323017134500030
  39. https://www.sciencedirect.com/science/article/pii/B9780128096338205207
  40. https://www.sciencedirect.com/science/article/pii/B9780122639517500065
  41. https://www.sciencedirect.com/topics/medicine-and-dentistry/anestrus
  42. https://pmc.ncbi.nlm.nih.gov/articles/PMC2781850/
  43. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/anestrus
  44. https://doi.org/10.3389/fvets.2025.1633945
  45. https://www.ncbi.nlm.nih.gov/books/NBK279070/
  46. https://beef.unl.edu/learning/estrous.shtml
  47. https://www.ncbi.nlm.nih.gov/books/NBK535442/
  48. https://www.dcrcouncil.org/wp-content/uploads/2017/04/Back-to-the-Basics_Explaining-the-Estrous-Cycle.pdf
  49. https://www.ncbi.nlm.nih.gov/books/NBK279054/
  50. https://ansci.wisc.edu/jjp1/ansci_repro/lec/lec9_cycles/lec9out.html
  51. https://www.ncbi.nlm.nih.gov/books/NBK539704/
  52. https://www.ncbi.nlm.nih.gov/books/NBK558960/
  53. https://pmc.ncbi.nlm.nih.gov/articles/PMC9219485/
  54. https://pmc.ncbi.nlm.nih.gov/articles/PMC3185177/
  55. https://pmc.ncbi.nlm.nih.gov/articles/PMC7565105/
  56. https://rbej.biomedcentral.com/articles/10.1186/1477-7827-7-150
  57. https://pmc.ncbi.nlm.nih.gov/articles/PMC9364933/
  58. https://pmc.ncbi.nlm.nih.gov/articles/PMC28006/
  59. https://www.pnas.org/doi/10.1073/pnas.0603006103
  60. https://pmc.ncbi.nlm.nih.gov/articles/PMC11226475/
  61. https://ovarianresearch.biomedcentral.com/articles/10.1186/s13048-021-00880-3
  62. https://veterinaria.unsa.ba/journal/index.php/vfs/article/download/428/400
  63. https://academic.oup.com/endo/article/155/6/2287/2422375
  64. https://www.vetscraft.com/factors-affecting-oestrous-cycle/
  65. https://www.merckvetmanual.com/management-and-nutrition/management-of-reproduction-sheep/reproductive-physiology-of-sheep
  66. https://fieldreport.caes.uga.edu/wp-content/uploads/2025/08/B-1434_2.pdf
  67. https://pmc.ncbi.nlm.nih.gov/articles/PMC9536053/
  68. https://www.pnas.org/doi/10.1073/pnas.0904747106
  69. https://pmc.ncbi.nlm.nih.gov/articles/PMC2630911/
  70. https://rep.bioscientifica.com/view/journals/rep/152/1/R1.xml
  71. https://www.sciencedirect.com/science/article/pii/S2950409025000437
  72. https://www.merckvetmanual.com/cat-owners/reproductive-disorders-of-cats/the-gonads-and-genital-tract-of-cats
  73. https://journals.sagepub.com/doi/10.1177/1098612X221079706
  74. https://www.merckvetmanual.com/reproductive-system/reproductive-diseases-of-the-female-small-animal/reproductive-management-of-the-female-small-animal
  75. https://pmc.ncbi.nlm.nih.gov/articles/PMC11292940/
  76. https://www.merckvetmanual.com/dog-owners/reproductive-disorders-of-dogs/management-of-reproduction-in-dogs
  77. https://www.msdvetmanual.com/dog-owners/skin-disorders-of-dogs/whole-body-disorders-that-affect-the-skin-in-dogs
  78. https://veterinarypartner.vin.com/doc/?id=11652016&pid=19239
  79. https://vcahospitals.com/know-your-pet/infertility-in-female-dogs
  80. https://www.merckvetmanual.com/reproductive-system/reproductive-diseases-of-the-female-small-animal/cystic-endometrial-hyperplasia-pyometra-complex-in-small-animals
  81. https://www.merckvetmanual.com/management-and-nutrition/management-of-reproduction-dogs-and-cats/breeding-management-of-bitches
  82. https://extension.msstate.edu/publications/the-estrous-cycle-cattle
  83. https://www.aces.edu/blog/topics/dairy/detecting-estrus-in-dairy-cattle/
  84. https://www.canr.msu.edu/uploads/236/58763/estrouscycle.pdf
  85. https://extension.sdstate.edu/sites/default/files/2020-09/P-00167.pdf
  86. https://extension.okstate.edu/fact-sheets/reproductive-management-of-the-mare.html
  87. https://extension.missouri.edu/sites/default/files/legacy_media/wysiwyg/Extensiondata/Pub/pdf/agguides/ansci/g02790.pdf
  88. https://pubmed.ncbi.nlm.nih.gov/1787475/
  89. https://www.sccpzp.org/wp-content/uploads/2024/03/1993_Remote-monitoring-of-ovulation-and-pregnancy-of-yellowstone-bison.pdf
  90. https://beef.unl.edu/estrus-synchronization-considerations-beef-herds/
  91. http://therio.vetmed.lsu.edu/bovine_anestrus.htm
  92. https://pmc.ncbi.nlm.nih.gov/articles/PMC11504324/
  93. https://arccjournals.com/journal/indian-journal-of-animal-research/B-5159
  94. https://www.cureus.com/articles/378612-laboratory-rat-vaginal-cytology-a-quick-method-of-staging-unstained-cells
  95. https://journals.sagepub.com/doi/10.1177/0192623315607668
  96. https://www.biorxiv.org/content/10.1101/2024.09.18.613702v1.full-text
  97. https://dep.nj.gov/njfw/hunting/biology-of-the-white-tailed-deer/
  98. https://rbej.biomedcentral.com/articles/10.1186/1477-7827-10-63
  99. https://pubmed.ncbi.nlm.nih.gov/35945211/
  100. https://pubmed.ncbi.nlm.nih.gov/36387875/
  101. https://beef.unl.edu/reproduction/benefits-of-estrous-synchronization/
  102. https://www.iowabeefcenter.org/estrussynch/pgnrh.pdf
  103. https://www.fda.gov/animal-veterinary/product-safety-information/cattle-estrous-cycle-and-fda-approved-animal-drugs-control-and-synchronize-estrus-resource-producers
  104. https://pmc.ncbi.nlm.nih.gov/articles/PMC5894419/
  105. https://www.researchgate.net/figure/llustration-of-the-Ovsynch-and-CO-Synch-synchronization-protocols-with-and-without-48-h_fig1_12125897
  106. https://clinicaltheriogenology.net/index.php/CT/article/view/11650/19258
  107. https://www.zoetisus.com/content/_assets/docs/vmips/package-inserts/lutalyse-injection-for-cattle-prescribing-information.pdf
  108. https://dailymed.nlm.nih.gov/dailymed/fda/fdaDrugXsl.cfm?setid=50cf9edb-f318-41d2-9b98-6ea3410e8128
  109. https://pubmed.ncbi.nlm.nih.gov/9655570/
  110. https://www.sciencedirect.com/science/article/pii/S1080744615318325
  111. https://www.iowabeefcenter.org/bch/EstrusSynchBeef.pdf
  112. https://www.ag.ndsu.edu/news/newsreleases/2025/april/estrous-synchronization-with-natural-service-offers-many-benefits-to-the-cowherd
  113. https://bookstore.ksre.ksu.edu/pubs/2021-protocols-for-synchronization-of-estrus-and-ovulation-in-beef-cows-and-heifers_MF2573.pdf
  114. https://bovine-ojs-tamu.tdl.org/bovine/article/view/1778/7756
  115. https://pmc.ncbi.nlm.nih.gov/articles/PMC6137838/
  116. https://bovine-ojs-tamu.tdl.org/aabp/article/view/8344
  117. https://pmc.ncbi.nlm.nih.gov/articles/PMC8953989/
  118. https://www.researchgate.net/publication/248883716_The_use_of_deslorelin_implants_for_the_long-term_contraception_of_lionesses_and_tigers
  119. https://pmc.ncbi.nlm.nih.gov/articles/PMC10304144/
  120. https://onlinelibrary.wiley.com/doi/pdf/10.1002/mrd.23620
  121. https://www.nature.com/articles/s41467-021-27227-2
  122. https://www.science.org/content/article/poised-be-first-widely-consumed-gene-edited-animals-virus-resistant-pigs-trot-toward
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