Estrous cycle

Gonadotropin-releasing hormone (GnRH) stimulates the synthesis and secretion of the follicle-stimulating hormone (FSH) and luteinizing hormone (LH), leading to ovulation

Oestrus also is the biological genus name of the gadfly.

Estrous cycle, or oestrous cycle, is the reproductive cycle found in most mammalian placental females whereby there are recurring periods when the female is fertile and sexually receptive (estrus) interrupted by periods in which the female is not fertile and not sexually receptive (anestrus). Animals that have estrous cycles reabsorb the endometrium (inner membrane of the mammalian uterus) if conception does not occur during that cycle. The estrous cycle is contrasted with the menstrual cycle in which the endometrium is shed through menstruation when pregnancy does not occur and in which the female may be sexually receptive at any time during the cycle.

There are many variations among animals in terms of their estrous cycles. Some may undergo estrous only one time a year during a particular season (white-tailed deer, foxes), while others may undergo a succession of cycles during a certain time of the year if they do not become pregnant (horses, sheep), and others may undergo cycles throughout the whole year (mice, cows, pigs). Some may be "induced ovulators," with cyclic receptivity but requiring mating in order to ovulate and undergo the luteal phase (rabbits, ferrets), and some require specific stimuli such as physical contact to even come into behavioral estrus (certain moles).

As experienced by most mammals, the estrous cycle allows the animals to place most of their mating energy into particular periods, and generally it is obvious to the males when females are in estrous. In some cases, estrus is limited to times that will allow birth when there are optimal environmental conditions for survival of the young, such as white-tailed deer mating in the autumn and giving birth in the spring. In contrast, the unique reproductive cycle of human beings, allowing sexual activity at any time and without overt external signs to signal estrual receptivity, fosters bonding between the male and female of a pair on the basis of sexual intimacy and love and facilitates overall social interactions in culture.

Overview

Mammals share the same reproductive system, including the regulatory hypothalamic system that releases gonadotropin releasing hormone (GnRH) in pulses, the pituitary that secretes follicle stimulating hormone (FSH) and luteinizing hormone (LH), and the ovary itself that releases sex hormones, including estrogens and progesterone. However, species vary significantly in the detailed functioning.

The estrous cycle comprises the recurring physiologic changes that are induced by reproductive hormones in most mammalian placental females. They typically have a period in which the female is sexually receptive and fertile, known as estrus (adjective is estrous), and a period in which the female is not sexually receptive and fertile. Estrous cycles start after puberty in sexually mature females and typically continue until death. The estrus phase is also referred to as being "in heat."

While most female placental mammals (dogs, cats, elephants, New World monkeys, and so on) under an estrous cycle, humans and the great apes (chimpanzees, gorillas, and orangutans) experience a true menstrual cycle. 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 instead. The term menstruation refers to the periodic discharge of blood and cells from the lining of a non-pregnant uterus. (Some animals that undergo estrus, such as domestic cats and dogs, do produce a very short and mild flow of blood. However, due to its small amount, and personal cleanliness in cats, it passes largely unnoticed by pet owners.)

Another difference is sexual activity. In species with estrous cycles, females are generally only sexually active during the estrus phase of their cycle. In contrast, females of species with menstrual cycles can be sexually active at any time in their cycle, even when they are not about to ovulate. This facilitates the formation of pair bonds. Note that humans also, unlike all other species, do not have any obvious external signs to signal estrual receptivity at ovulation (concealed ovulation). This combined with sexual activity not being restricted to a particular time fosters receptivity to sexual intimacy at any time, thus promoting pair bonding and allowing sex, considered by many religious traditions as ideally a holy act, to encourage and cement the bonds of love between husband and wife.

Etymology and nomenclature

The term estrous cycle or oestrous cycle is derived from Latin oestrus (frenzy, gadfly) and originally from Greek οἶστρος (gadfly, breeze, sting, mad impulse). Short (1994) notes that the term has a rather unusual origin in that the Greek word means gadfly or warble fly, which are insects that are parasitic on cattle. Short speculates that one possible origin for the use of the term for the estrous cycle is that it was observed that the cattle sometimes reacted to these flies in ways reflective of "coming into heat," by being hyperactive, frequently bellowing, and charging, and so forth (Short 1984).

Perhaps more specifically, the Greek term refers to the gadfly that Hera sent to torment Io, who had been won in her heifer form by Zeus. Euripides used "oestrus" to indicate "frenzy", and to describe madness. Homer uses the word to describe panic of the suitors in Odyssey book 22. Plato also uses it to refer to an irrational drive (Plato, Laws, 854b) and to describe the soul "driven and drawn by the gadfly of desire" (Plato, The Republic). Somewhat more closely aligned to current meaning and usage of "estrus," Herodotus (Histories ch.93.1) uses oistros to describe the desire of fish to spawn.

The earliest use in English is of "frenzied passion." In 1900, it was first used to describe "rut in animals, heat" (Freeman 1994; Heape 1900).

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) a mammal may be described as "in estrus" when it is in that particular part of the estrous or menstrual cycle. Estrum is sometimes used as a synonym for estrus.

Phases of the estrous cycle

The following are five periods used to describe the estrous cycle, as developed by Walter Heape. However, while this terminology is helpful in describing the cyclic events in laboratory rodents, Short (1984) maintains that it is not particularly useful for describing events in larger domestic animals.

• Proestrus. One or several follicles of the ovary are starting to grow. Their number is specific for the species. Typically this phase can last as little as one day or as long as 3 weeks, depending on the species. Under the influence of estrogen, the lining in the uterus (endometrium) starts to develop. Some animals may experience vaginal secretions that could be bloody. The female is not yet sexually receptive.

• Estrus. Estrus refers to the phase when the female is sexually receptive ("in heat," or "on heat" in British English). Under regulation by gonadotropic hormones, ovarian follicles 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 reflex, in which the animal spontaneously elevates her hindquarters. In some species, the vulvae are reddened. Ovulation may occur spontaneously in some species (for example, cow), while in others it is induced by copulation (for example, 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.

• Metestrus. During this phase, the signs of estrogen stimulation subside and the corpus luteum starts to form. The uterine lining begins to secrete small amounts of progesterone. This phase typically is brief and may last 1 to 5 days. In some animals bleeding may be noted due to declining estrogen levels.

• Diestrus. Diestrus is characterized by the activity of the corpus luteum that produces progesterone. In the absence of pregnancy, the diestrus phase (also termed pseudo-pregnancy) terminates with the regression of the corpus luteum. The lining in the uterus is not shed, but will be reorganized for the next cycle.

• Anestrus. Anestrus refers to the phase when the sexual cycle rests. This is typically a seasonal event and controlled by light exposure through the pineal gland that releases melatonin. Melatonin may repress stimulation of reproduction in long-day breeders and stimulate reproduction in short-day breeders. Melatonin is thought to act by regulating hypothalamic pulse activity of GnRH. Anestrus is induced by time of year, pregnancy, lactation, significant illness, and possibly age.

Cycle variability

Frequency

Cycle variability differs among species, but typically cycles are more frequent in smaller animals. Even within species significant variability can be observed, thus cats may undergo an estrous cycle of 3 to 7 weeks. Domestication can affect estrous cycles due to changes in the environment. The following is a classification scheme based on the number of cycles per year.

• Seasonally monoestrous. Monoestrous species, such as bears, foxes, wolves, roe deer, and white-tailed deer, undergo estrus one time a year at a predictable time (Short 1994).

• Diestrous. These are species that go into estrus twice per year, such as most dogs.

• Polyestrous. Polyestrous species are those can go through a succession of estrous cycles during the year. For example, cats, cows, and pigs can go into estrus several times a year if not pregnant.
  • Seasonally polyestrous animals are those that can go through a success of estrous cycles during a particular time of the year. For example, horses, sheep, goats, and the red deer (elk) go into estrus more than one time during a particular season if not mated. These can be divided into short-day and long-day breeders:
    • Short-day breeders, such as sheep, goats, foxes, white-tailed deer, and elk, are sexually active in fall or winter.
    • Long-day breeders, such as horses and hamsters, are sexually active in spring and summer.

Spontaneous versus induced ovulators

Mammals also can be classified according to whether ovulation occurs spontaneously or not.

• Spontaneous ovulators. Spontaneous ovulating species are those that do not require an act of coitus to induce follicle rupture (Short 1984) by rather a series of hormonal events result in the occurrence of ovulation. Most mammals fall in this category (Short 1984), including rats, mice, guinea pigs, monkeys, and sheep (and humans among those that under a menstrual cycle). In these species, ovarian steroids (from maturing ovarian follicles) induce a pattern of GnRH release that stimulates a preovulatory LH surge (Bakker and Baum 2000).

• Induced ovulators. Induced ovulators, or "reflex ovulators," are those that require the the act of copulation to stimulate ovulation. Coitus provides a discharge of GnRH that leads to release of LH and induces ovulation. Sexual receptivity occurs in cycles that are spontaneous, but ovulation does not occur without copulation (Becker). Females of the European rabbit (Oryctolagus cuniculus)—and presumably other rabbits—are reflex (or induced) ovulators that require the act of copulation to stimulate ovulation, which occurs about 12 hours after mating (Smith 2004). Other examples of induced ovulators include cats, ferrets, minks, and camels. Induced ovulators do not always have overt signs of estrus, such as in the case of the koala (Takahashi et al. 2008).

• Induced estrus and ovulation. A few species undergo behavior estrus and ovulation only in response to certain stimuli. For example, several species of voles remain reproductively inactive until there is stimuli for a male. The females require physical contact to begin behavior estrus and mating to promote ovulation. Examples include prairie voles (Microtus ochrogaster), montane voles (M. montanus), and meadow moles (M. pennsylvanicus).

Specific species

Cats. The female cat in heat has an estrus of 14 to 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 late winter.

Dogs. 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 to 13 days, depending on the size and maturity of the dog.

Horses. A mare may be 4 to 10 days in heat and about 14 days in diestrus. Thus a cycle may be short, such as 3 weeks. Horses mate in spring and summer, autumn is a transition time, and anestrus rules the winter.

A feature of the fertility cycle of horses and other large herd animals is that it is usually affected by the seasons. The number of hours daily that light enters the eye of the animal affects the brain, which governs the release of certain precursors and hormones. When daylight hours are few, these animals "shut down," become anestrous, and do not become fertile. As the days grow longer, the longer periods of daylight cause the hormones that activate the breeding cycle to be released. As it happens, this has a sort of utility for these animals in that, given a gestation period of about eleven months, it prevents them from having young when the cold of winter would make their survival risky.

Rats. Rats typically have rapid cycle times of 4 to 5 days. Although they ovulate spontaneously, they do not develop a fully functioning corpus luteum unless they receive coital stimulation. Fertile mating leads to pregnancy in this way, but infertile mating leads to a state of pseudopregnancy that lasts about 10 days. Mice and hamsters have similar behavior (McCracken et al. 1999). The events of the cycle are strongly influenced by lighting periodicity (Freeman 1994).

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 11 am on the day of proestrus. Between then and midnight there is a surge in progesterone, LH and FSH, and ovulation occurs at about 4 am on the next, estrus day. The following day, metestrus, is called early diestrus or diestrus I by some authors. During this day, the corpora lutea grow to a maximal volume, achieved within 24 hours of ovulation. They remain at that size for 3 days, halve in size before the metestrus of the next cycle and then shrink abruptly before estrus of the cycle after that. Thus the ovaries of cycling rats contain three different sets of corpora lutea at different phases of development (Yoshingaga 1973).

Others

Estrus frequencies of some other mammals:

• Ewe—17 days
• Bovine—21 days
• Goat—21 days
• Sow—21 days
• Elephant—16 weeks

References

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• Bakker, J., and M.J. Baum. 2000. Neuroendocrine regulation of GnRH release in induced ovulators. Frontiers in Neuroendocrinology 21(3): 220-262.
• Becker, J. B. n.d. Chapter 3: Hormonal influences on female sex behavior. University of Michigan, Psychology Course 531. Retrieved December 22, 2008.
• Freeman, M. E. 1994. The Neuroendocrine control of the ovarian cycle of the rat. In E. Knobil and J. D. Neill (eds.), The Physiology of Reproduction. Raven Press. ISBN 0781700868.
• Heape, W. 1900. The "sexual season" of mammals and the relation of the "pro-oestrum" to menstruation. Q J Micr Sci 44: 1-70.
• McCracken, J. A., E. E. Custer, and J. C. Lamsa. 1999. Luteolysis: A neuroendocrine-mediated event. Physiol. Rev. 79(2): 263–323. PMID 10221982. Retrieved December 22, 2008.
• Short, R. V. 1984. Oestrous and menstrual cycles. Pages 115-153 in C. R. Austin, R. V. Short, and J. R. Fuller, Reproduction in Mammals: Hormonal Control of Reproduction. Cambridge University Press. ISBN 0521275946.
• Smith, A.T. 2004. Lagomorpha (Pikas, rabbits, and hares). Pages 479 to 489 in B. Grzimek et al., Grzimek's Animal Life Encyclopedia. Detroit, MI: Thomson/Gale. ISBN 0787657921.
• Takahashi, M., J. R. Tobey, C. Bacon Pisacane, and C. Hamlin Andrus. 2008. Evaluating the utility of an accelerometer and urinary hormone analysis as indicators of estrus in a Zoo-housed koala (Phascolarctos cinereus). Zoo Biology. Retrieved December 22, 2008.
• Yoshinaga, K. 1973. Gonadotrophin-induced hormone secretion and structural changes in the ovary during the nonpregnant reproductive cycle. Handbook of Physiology, Endocrinology II. American Physiological Society.

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