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.
[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.
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.
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.
[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".
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.
Under regulation by gonadotropic hormones, ovarian follicles mature and estrogen secretions exert their biggest influence.
The female then exhibits sexually receptive behavior,[12] a situation that may be signaled by visible physiologic changes.
This is typically a seasonal event and controlled by light exposure through the pineal gland that releases melatonin.
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.
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.
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.
[21] Horses mate in spring and summer; autumn is a transition time, and anestrus occurs during winter.
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.
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.
[22] Although they ovulate spontaneously, they do not develop a fully functioning corpus luteum unless they receive coital stimulation.
[26] Other behaviors of the female during standing estrus may change, including, but not limited to: nervousness, swollen vulva, or attempting to mount other animals.
[26] While visual and behavioral cues are helpful to the male cattle, estrous stages cannot be determined by the human eye.
[27][28] 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.
[32] Additionally, heat stress has been linked to impairment of follicular development, especially impactful to the first-wave dominant follicle.
[33] Future synchronization programs are planning to focus on the impact of heat stress on fertilization and embryonic death rates after artificial insemination.
[34] 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.