Estrous cycle

Oestrus is also 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 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, fox), 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," requiring mating in order to ovulate and undergo the luteal phase (rabbits, ferrets), and some require physical contact to come into behavior estrus (certain moles).

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, etc.) 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.

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.

Some animals, such as domestic cats and dogs, do produce a very short and mild flow of blood after estrus. However, due to its small amount (and personal cleanliness in cats) it passes largely unnoticed by pet owners.

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 accurately, 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.

The four phases of the estrous cycle

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 (e.g. cow), while in others it is induced by copulation (e.g. cat). If there is no copulation in an induced ovulator, estrus may continue for many days, followed by 'interestrus,' and the estrus phase starts again until copulation and ovulation occur.

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 characterised 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 reorganised 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

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.

Frequency

Some species, such as cats, cows and pigs, are polyestrous and can go into heat several times a year. Seasonally polyestrous animals have more than one estrous cycles during a specific time of the year and can be divided into short-day and long-day breeders:

• Short-day breeders, such as sheep, goats, deer, foxes, elk—are sexually active in fall or winter.
• Long-day breeders, such as horses and hamsters, are sexually active in spring and summer.

Species that go into heat twice per year, such as most dogs, are diestrous.

Monoestrous species, such as bears, foxes, and wolves, have only one breeding season a year, typically in spring to allow growth of the offspring during the warm season to survive the next winter.

A few mammalian species, such as rabbits, do not have an estrous cycle and are able to conceive at almost any arbitrary moment.

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).

Specific species

Cats

The female cat in heat has an estrus of 14-21 days and is an induced ovulator. Without copulation she may enter interestrus before reentering estrus. With copulation and in the absence of pregnancy, cycles occur about every three weeks. Cats are polyestrous but experience a seasonal anestrus in autumn and 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 7-13 days, depending on the size and maturity of the dog.

Horses

For more information, see the article on Horse reproduction.

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

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

Rats

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 which lasts about 10 days. Mice and hamsters have similar behaviour. The events of the cycle are strongly influenced by lighting periodicity.

A set of follicles start to develop near the end of proestrus and grow at a nearly constant rate until the beginning of the subsequent estrus when the growth rates accelerate eightfold. They then ovulate about 109 hours after starting growth. Oestrogen peaks at about 11am on the day of proestrus. Between then and midnight there is a surge in progesterone, LH and FSH, and ovulation occurs at about 4am on the next, estrus day. The following day, metestrus, is called early diestrus or diestrus I by some authors. During this day the 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.

Others

Estrus frequencies of some other mammals:

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

References

ISBN links support NWE through referral fees

• 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.

Neuroendocrine Regulation of GnRH Release in Induced Ovulators*1 Purchase the full-text article Julie Bakker and Michael J. Baum http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WFS-45F4KCS-9&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=e1c950b53d9b3fe9a85512b3a1cf7ebc Frontiers in Neuroendocrinology Volume 21, Issue 3, July 2000, Pages 220-262

http://www3.interscience.wiley.com/journal/121473807/abstract?CRETRY=1&SRETRY=0 Zoo Biology Published Online: 21 Oct 2008 Copyright © 2008 Wiley-Liss, Inc., A Wiley Company Evaluating the utility of an accelerometer and urinary hormone analysis as indicators of estrus in a Zoo-housed koala (Phascolarctos cinereus) Maressa Takahashi 1, Jennifer R. Tobey 1 *, Corinne Bacon Pisacane 1, Chris Hamlin Andrus

Short, R. V. 1984. Oestrous and menstrual cycles. Pages 115-153 in Reproduction in Mammals: Hormonal Control of Reproduction By Colin Russell Austin, Roger Valentine Short, John R. Fuller Published by Cambridge University Press, 1984 ISBN 0521275946, 9780521275941

External links

• Systematic overview
• Etymology
• Cat estrous cycle
• Horse estrous cycle
• NY times 'Lap-Dance Science'

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