Estrous cycle

The estrous cycle or oestrus cycle (derived from Latin oestrus 'frenzy', originally from Greek οἶστρος oîstros 'gadfly') is the recurring physiological changes that are induced by reproductive hormones in most mammalian therian females. Estrous cycles start after sexual maturity in females and are interrupted by anestrous phases or by pregnancies. Typically, estrous cycles continue until death. Some animals may display bloody vaginal discharge, often mistaken for menstruation.

Differences from the menstrual cycle

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, species vary significantly in the detailed functioning. One difference is that animals that have estrous cycles resorb the endometrium if conception does not occur during that cycle. Animals that have menstrual cycles shed the endometrium through menstruation instead. Another difference is sexual activity. In species with estrous cycles, females are generally only sexually active during the estrus (oestrus) phase of their cycle (see below for the estrous cycle phases). This is also referred to as being "in heat". In contrast, females of species with menstrual cycles can be sexually active at any time in their cycle, even when they are not about to ovulate.

Humans have menstrual cycles rather than estrous cycles. They, 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). There are, however, subtle signs to which human males may favorably respond, including changes in a woman's scent[1] and facial appearance.[2] Some research also suggests that women tend to have more sexual thoughts and are more prone to sexual activity right before ovulation.[3][4] Animals with estrous cycles often have unmistakable outward displays of receptivity, ranging from engorged and colorful genitals to behavioral changes like mating calls.

Etymology and nomenclature

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. Euripides used oestrus to indicate 'frenzy', and to describe madness. Homer uses the word to describe panic.[5] Plato also uses it to refer to an irrational drive[6] and to describe the soul "driven and drawn by the gadfly of desire".[7] 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.[8]

The earliest use in English was with a meaning of 'frenzied passion'. In 1900, it was first used to describe 'rut in animals; heat'.[9][10]

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

Four phases

Overview of the mammal estrous cycle

Overview of the mammal estrous cycle

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


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 in 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 gets degenerated; the uterus and the vagina get distended and filled with fluid, become contractile and secrete a sanguinous fluid; the vaginal epithelium proliferates and the vaginal smear shows a large number of non-cornified nucleated epithelial cells. Variant terms for proestrus include pro-oestrus, proestrum, and pro-oestrum.


Estrus or oestrus refers to the phase when the female is sexually receptive ("in heat"). Under regulation by gonadotropic hormones, ovarian follicles mature and estrogen secretions exert their biggest influence. The female then exhibits sexually receptive behavior,[4] a situation that may be signaled by visible physiologic changes. Estrus is commonly seen in the mammalian species, including primates. It is thought that this increased sexual receptivity serves the function of helping the female obtain mates with superior genetic quality.[4] This phase is sometimes called estrum or oestrum.

In some species, the labia are reddened. Ovulation may occur spontaneously in some species. Especially among quadrupeds, a signal trait of estrus is the lordosis reflex, in which the animal spontaneously elevates her hindquarters.

Metestrus or diestrus

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 pseudo-pregnancy) 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 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.[11] For example, the mouse has a fertile postpartum estrus that occurs 14 to 24 hours following parturition.

Terminology for humans

In a medical context, human ovulation is uncommonly also referred to as estrus or oestrus, with the non-ovulating range of the menstrual cycle sometimes generally referred to as the luteal phase.[4] A more complex classification of the menstrual cycle includes two major sub-cycles with three phases each: the ovarian cycle, with the follicular, ovulation (= estrus), and luteal phases; and the uterine cycle, with the menstruation (or menses), proliferative, and secretory phases. See Menstrual cycle § Cycles and phases for more information.

Cycle variability

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. Domestication can affect estrous cycles due to changes in the environment.


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:

Species that go into heat twice per year are diestrous.

Monoestrous species, such as bears, foxes, and wolves, 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 and are able to conceive at almost any arbitrary moment (comparable with humans, who, however, have a menstrual cycle in place of an estrous cycle).

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


The female cat in heat has an estrus of 14 to 21 days and is generally characterized as an induced ovulator, in that coitus induces ovulation. However, various incidents of spontaneous ovulation have been documented in the domestic cat and various non-domestic species.[12] Without ovulation, she may enter physiological changes 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.[13]


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 is about somewhere 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.[14]


A mare may be 4 to 10 days in heat and about 14 days in diestrus. Thus a cycle may be short, around 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 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 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 lasting about 10 days. Mice and hamsters have similar behaviour.[15] The events of the cycle are strongly influenced by lighting periodicity.[9]

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.

Oestrogen 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 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 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.[16]


Buffalo have an estrous cycle of about 22 to 24 days. Buffalo are known for difficult estrous 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±0.22 (metestrus), 3.15±0.10 (early diestrus), 2.25±0.32 (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.[17]


Estrus frequencies of some other mammals:

See also


  1. ^ Kuukasjärvi, Seppo; Eriksson, C. J. Peter; Koskela, Esa; Mappes, Tapio; Nissinen, Kari; Rantala, Markus J. (1 July 2004). "Attractiveness of women's body odors over the menstrual cycle: the role of oral contraceptives and receiver sex". Behavioral Ecology. 15 (4): 579–584. doi:10.1093/beheco/arh050.
  2. ^ Roberts, S. C.; Havlicek, J.; Flegr, J.; Hruskova, M.; Little, A. C.; Jones, B. C.; Perrett, D. I.; Petrie, M. (7 August 2004). "Female facial attractiveness increases during the fertile phase of the menstrual cycle". Proceedings of the Royal Society B: Biological Sciences. 271: S270–S272. doi:10.1098/rsbl.2004.0174. PMC 1810066.
  3. ^ Bullivant, Susan B.; Sellergren, Sarah A.; Stern, Kathleen; Spencer, Natasha; Jacob, Suma; Mennella, Julie; McClintock, Martha (February 2004). "Women's sexual experience during the menstrual cycle: Identification of the sexual phase by noninvasive measurement of luteinizing hormone". Journal of Sex Research. 41 (1): 82–93. doi:10.1080/00224490409552216. PMID 15216427.
  4. ^ a b c d Geoffrey Miller (April 2007). "Ovulatory cycle effects on tip earnings by lap dancers: Economic evidence for human estrus?" (PDF). Evolution and Human Behavior (28): 375–381.
  5. ^ Panic of the suitors in Homer, Odyssey, book 22
  6. ^ Plato, Laws, 854b
  7. ^ Plato, The Republic
  8. ^ Herodotus, Histories, ch. 93.1
  9. ^ a b Freeman, Marc E. (1994). "The Neuroendocrine control of the ovarian cycle of the rat". In Knobil, E.; Neill, J. D. The Physiology of Reproduction. 2 (2nd ed.). Raven Press.
  10. ^ Heape, W. (1900). "The 'sexual season' of mammals and the relation of the 'pro-oestrum' to menstruation'". Quarterly Journal of Microscopical Science. NCBI. 44: 1:70.
  11. ^ > postpartum estrus citing: Stedman's Medical Dictionary. Copyright 2006
  12. ^ Pelican et al., 2006
  13. ^ Spindler and Wildt, 1999
  14. ^ Walter, I.; Galabova, G.; Dimov, D.; Helmreich, M. (February 2011). "The morphological basis of proestrus endometrial bleeding in canines". Theriogenology. 75 (3): 411–420. doi:10.1016/j.theriogenology.2010.04.022. Retrieved 2011-12-16.
  15. ^ McCracken, J. A.; Custer, E. E.; Lamsa, J. C. (1999). "Luteolysis: A neuroendocrine-mediated event". Physiological Reviews. 79 (2): 263–323. doi:10.1152/physrev.1999.79.2.263. PMID 10221982.
  16. ^ Yoshinaga, K. (1973). "Gonadotrophin-induced hormone secretion and structural changes in the ovary during the nonpregnant reproductive cycle". Handbook of Physiology. Endocrinology II, Part 1.
  17. ^ Qureshi, A. S.; Hussain, M.; Rehan, S.; Akbar, Z.; Rehman, N. U. (2015). "Morphometric and angiogenic changes in the corpus luteum of nili-ravi buffalo (Bubalus bubalis) during estrous cycle". Pakistan Journal of Agricultural Sciences. 52 (3): 795–800.

Further reading

  • Spindler, R. E.; Wildt, D. E. (1999). "Circannual variations in intraovarian oocyte but not epididymal sperm quality in the domestic cat". Biology of Reproduction. 61: 188–194. doi:10.1095/biolreprod61.1.188.
  • Pelican, K.; Wildt, D.; Pukazhenthi, B.; Howard, J. G. (2006). "Ovarian control for assisted reproduction in the domestic cat and wild felids". Theriogenology. 66: 37–48. doi:10.1016/j.theriogenology.2006.03.013.

External links

Buru babirusa

The Moluccan babirusa (Babyrousa babyrussa), also known as the Buru babirusa, golden babirusa or hairy babirusa, is a wild pig-like animal native to the island of Buru and the two Sula Islands of Mangole and Taliabu, all belonging to Indonesia. Traditionally, this relatively small species included the other babirusas as subspecies, but it has been recommended treating them as separate species based on differences in their morphology. As also suggested by its alternative common names, the Buru Babirusa has relatively long thick, gold-brown body-hair – a feature not shared by the other extant babirusas.In absence of detailed data on B. babyrussa, it is generally assumed that its habitat and ecology are similar to that of B. celebensis (north Sulawesi babirusa). Furthermore, as all babirusas were considered conspecific under the scientific name B. babyrussa until 2001, data collected before that is consistently listed under the name B. babyrussa, though the vast majority actually refers to B. celebensis (by far the best known species of babirusa). Babirusas tend to occupy tropical rainforests, river banks and various natural ponds rich in water plants. They are omnivorous and feed on various leaves, roots, fruits, invertebrates and small vertebrates. Their jaws and teeth are strong enough to crack any kind of nuts. Babirusas lack the rostral bone in their nose, which is a tool used by other wild pigs for digging. Therefore, they prefer feeding on roots in soft muddy or sandy soils. Cannibalism was reported among babirusas, feeding on the young of their own species or other mammals. North Sulawesi babirusas form groups with up to a dozen of individuals, especially when raising their young. Older males might live individually.The north Sulawesi babirusa reach sexual maturity when they are 5–10 months old. Their estrous cycle is 28–42 days, and the gestation period lasts 150–157 days. The females have two rows of teats and will give birth to 1–2 piglets weighing 380–1050 grams and measuring 15–20 cm, which they will nurse until the age of 6–8 months. The lifespan is about 24 years.The restricted habitat of the Buru babirusa, with the total area within 20,000 km², and its gradual loss due to logging persuaded the International Union for Conservation of Nature to declare the species as vulnerable. Hunting by the local population is another cause of concern. Whereas it is unpopular among Muslim communities for religious reasons, it is widely hunted by the indigenous people of Buru, which are predominantly Christian. The meat of Buru babirusa has low fat (only 1.27% compared to 5–15% for domestic pigs) and is regarded as a delicacy. It is also preferred by the locals to the meat of other wild pigs or deer in terms of texture and flavor. The establishment of two protected areas on Buru, Gunung Kapalat Mada (1,380 km²) and Waeapo (50 km²), partly aim at preserving the habitat of the Buru babirusa. This species also enjoys full protection under Indonesian law since 1931.The babirusa has been documented to produce infertile hybrids with the domestic pig.

Calabar angwantibo

The Calabar angwantibo (Arctocebus calabarensis), also known as the Calabar potto, is a strepsirrhine primate of the family Lorisidae. It shares the genus Arctocebus with the golden angwantibo (Arctocebus aureus). It is closely related to the potto (Perodicticus potto) and to the various lorises.

The Calabar angwantibo lives in the rain forests of west Africa, particularly in tree-fall zones. In areas where the forest has been cleared, it has been known to live on farmland. Its range covers Cameroon, Nigeria and Equatorial Guinea. The species takes its name from the Nigerian city of Calabar.

The Calabar angwantibo weighs between 266 and 465 grams. It has orangish-yellow fur on its back, grey or white fur on its belly, and a distinctive white line on its forehead and nose. Like other lorids, this angwantibo has a very short index finger, which allows it to get a strong grip on tree branches. The second toe on each foot has a specialised claw that the angwantibo uses for grooming. The Calabar angwantibo is the only primate to have a functioning nictitating membrane (third eyelid).The Calabar angwantibo is nocturnal and arboreal. It stays considerably lower in the trees than the other nocturnal strepsirrhines in its range, and is typically found between 5 and 15 metres above ground. It moves by climbing very slowly through the trees, always grasping branches with at least three of its limbs at a time. During the day the angwantibo sleeps under dense foliage, hanging from a branch.

The Calabar angwantibo's diet consists mainly of insects, especially caterpillars, but it also eats some fruit. It will eat strong-smelling insects that other animals reject. Before eating a caterpillar, the angwantibo wipes it carefully with its hands to remove any poisonous barbs.

When confronted by a predator, the Calabar angwantibo will roll up into a ball, but keep its mouth open beneath its armpit. If the attacker persists, the angwantibo will bite it and not let go.Calabar angwantibos forage for food alone, but each male's territory overlaps that of several females. Angwantibos reinforce social bonds through mutual grooming and scent-marking. Mating takes place only in the final phase of the female's estrous cycle, and is performed hanging upside-down from a branch. The female gives birth to a single infant after a gestation period of 131 to 136 days; the young are normally born between January and April. Infants are born with their eyes open and can cling to their mother's fur right away.

Corpus luteum

The corpus luteum (Latin for "yellow body"; plural corpora lutea) is a temporary endocrine structure in female ovaries and is involved in the production of relatively high levels of progesterone, moderate levels of estradiol and inhibin A, and small amounts of estrogen. It is the remains of the ovarian follicle that has released a mature ovum during a previous ovulation.The corpus luteum is colored as a result of concentrating carotenoids (including lutein) from the diet and secretes a moderate amount of estrogen to inhibit further release of gonadotropin-releasing hormone (GnRH) and thus secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). A new corpus luteum develops with each menstrual cycle.


Decidualization is a process that results in significant changes to cells of the endometrium in preparation for, and during, pregnancy. This includes morphological and functional changes (the decidual reaction) to endometrial stromal cells (ESCs), the presence of decidual white blood cells (leukocytes), and vascular changes to maternal arteries. The sum of these changes results in the endometrium changing into a structure called the decidua. In humans, the decidua is shed during the third phase of birth.Decidualization plays an important role in promoting placenta formation between a mother and her fetus by mediating the invasiveness of trophoblast cells. It also triggers the production of cellular and molecular factors that result in structural changes, or remodeling, of maternal spiral arteries. Decidualization is required in some mammalian species where embryo implantation and trophoblast cell invasion of the endometrium occurs, also known as hemochorial placentation. This allows maternal blood to come into direct contact with the fetal chorion, a membrane between the fetal and maternal tissues, and allows for nutrient and gas exchange. However, decidualization-like reactions have also been observed in some species that don't display hemochorial placentation.In humans, decidualization occurs after ovulation during the menstrual cycle. After implantation of the embryo, the decidua further develops to mediate the process of placentation. In the event no embryo is implanted, the decidualized endometrial lining is shed or, as is the case with species that follow the estrous cycle, absorbed. In menstruating species, decidualization is spontaneous and occurs as a result of maternal hormones. In non-menstruating species, decidualization is non-spontaneous, meaning it only happens after there external signals from an implanted embryo.

Dog breeding

Dog breeding is the practice of mating selected dogs with the intent to maintain or produce specific qualities and characteristics. When dogs reproduce without such human intervention, their offspring's characteristics are determined by natural selection, while "dog breeding" refers specifically to the artificial selection of dogs, in which dogs are intentionally bred by their owners. Breeding relies on the science of genetics, so the breeder with a knowledge of canine genetics, health, and the intended use for the dogs attempts to breed suitable dogs.


The endometrium is the inner epithelial layer, along with its mucous membrane, of the mammalian uterus. It has a basal layer and a functional layer; the functional layer thickens and then is shed during menstruation in humans, as well as some other mammals including apes, Old World monkeys, some species of bat, and the elephant shrew. In most other mammals the endometrium is reabsorbed in the estrous cycle. During pregnancy, the glands and blood vessels in the endometrium further increase in size and number. Vascular spaces fuse and become interconnected, forming the placenta, which supplies oxygen and nutrition to the embryo and fetus. The speculated presence of an endometrial microbiota

has been argued against.

Estrous synchronization

In agriculture, estrous synchronization is used (particularly in the dairy and beef industries) to facilitate breeding by artificial insemination.

Follicular phase

The follicular phase is the phase of the estrous cycle, (or, in humans and great apes, the menstrual cycle) during which follicles in the ovary mature. It ends with ovulation. The main hormone controlling this stage is estradiol. There are several steps in the process of follicular development that occur in the follicular phase: Initiation, Recruitment and progression, Pre-ovulatory maturation and Ovulation. These processes occur in different areas of the body and will be briefly explained below.

Horse breeding

Horse breeding is reproduction in horses, and particularly the human-directed process of selective breeding of animals, particularly purebred horses of a given breed. Planned matings can be used to produce specifically desired characteristics in domesticated horses. Furthermore, modern breeding management and technologies can increase the rate of conception, a healthy pregnancy, and successful foaling.

List of MeSH codes (G08)

The following is a list of the "G" codes for MeSH. It is a product of the United States National Library of Medicine.

Source for content is here. (File "2006 MeSH Trees".)

Luteal phase

The luteal phase is the latter phase of the menstrual cycle (in humans and a few other animals) or the earlier phase of the estrous cycle (in other placental mammals). It begins with the formation of the corpus luteum and ends in either pregnancy or luteolysis. The main hormone associated with this stage is progesterone, which is significantly higher during the luteal phase than other phases of the cycle. Another is the endocannabinoid anandamide AEA, where the lowest plasma AEA level is observed in the luteal phase. The opposite of the luteal phase, the rest of the two weeks, is called the follicular phase.

Mammalian reproduction

Most mammals are viviparous, giving birth to live young. However, the five species of monotreme, the platypuses and the echidnas, lay eggs. The monotremes have a sex determination system different from that of most other mammals. In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal.The mammary glands of mammals are specialized to produce milk, a liquid used by newborns as their primary source of nutrition. The monotremes branched early from other mammals and do not have the nipples seen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly.

Viviparous mammals are in the subclass Theria; those living today are in the Marsupialia and Placentalia infraclasses. A marsupial has a short gestation period, typically shorter than its estrous cycle, and gives birth to an underdeveloped (altricial) newborn that then undergoes further development; in many species, this takes place within a pouch-like sac, the marsupium, located in the front of the mother's abdomen. Some placentals, e.g. guinea pig, give birth to fully developed (precocial) young, usually after long gestation periods, while some others, e.g. mouse, give birth to underdeveloped young.

Mammary tumor

A mammary tumor is a neoplasm originating in the mammary gland. It is a common finding in older female dogs and cats that are not spayed, but they are found in other animals as well. The mammary glands in dogs and cats are associated with their nipples and extend from the underside of the chest to the groin on both sides of the midline. There are many differences between mammary tumors in animals and breast cancer in humans, including tumor type, malignancy, and treatment options. The prevalence in dogs is about three times that of women. In dogs, mammary tumors are the second most common tumor (after skin tumors) over all and the most common tumor in female dogs with a reported incidence of 3.4%. Multiple studies have documented that spaying female dogs when young greatly decreases their risk of developing mammary neoplasia when aged. Compared with female dogs left intact, those spayed before puberty have 0.5% of the risk, those spayed after one estrous cycle have 8.0% of the risk, and dogs spayed after two estrous cycles have 26.0% of the risk of developing mammary neoplasia later in life. Overall, unspayed female dogs have a seven times greater risk of developing mammary neoplasia than do those that are spayed. While the benefit of spaying decreases with each estrous cycle, some benefit has been demonstrated in female dogs even up to 9 years of age. There is a much lower risk (about 1 percent) in male dogs and a risk in cats about half that of dogs.

Menstruation (mammal)

Menstruation in mammals is the shedding of the uterine lining (endometrium). It occurs on a regular basis in sexually reproductive-age females of certain mammal species.

Although there is some disagreement in definitions between sources, menstruation is generally considered to be limited to primates. Overt menstruation (where there is bleeding from the uterus through the vagina) is found primarily in humans and close relatives such as chimpanzees. It is common in simians (Old World monkeys, and apes), but completely lacking in strepsirrhine primates and possibly weakly present in tarsiers. Beyond primates, it is known only in bats and the elephant shrew.Females of other species of placental mammal undergo estrous cycles, in which the endometrium is completely reabsorbed by the animal (covert menstruation) at the end of its reproductive cycle. Many zoologists regard this as different from a "true" menstrual cycle. Female domestic animals used for breeding —for example dogs, pigs, cattle, or horses— are monitored for physical signs of an estrous cycle period, which indicates that the animal is ready for insemination.

Ovandrotone albumin

Ovandrotone albumin (INN, BAN) (brand names Fecundin, Ovastim), also known as polyandroalbumin, as well as ovandrotone:human serum albumin conjugate, is an immunogen and vaccine against androstenedione that is used in veterinary medicine to increase the ovulation rate and number of lambs born to ewes. It is a conjugate of ovandrotone (androstenedione-7α-carboxyethylthioether) and human serum albumin. The drug was developed by 1981 and was introduced in Australia and New Zealand in 1983.Ovandrotone albumin produces transient immunity against androstenedione, and the generation of antibodies against androstenedione presumably decreases circulating levels of androstenedione. This is thought to result in reduced negative feedback on the hypothalamic-pituitary-gonadal axis and increased gonadotropin secretion, which in turn improves fertility and fecundity. Indeed, ovandrotone albumin has been found to significantly increase luteinizing hormone levels throughout the estrous cycle in ewes.


Superfetation (also spelled superfoetation – see fetus) is the simultaneous occurrence of more than one stage of developing offspring in the same animal. It is not believed that it occurs naturally in humans. There have been 10 reported cases of possible superfetation in humans.In mammals, it manifests as the formation of an embryo from a different estrous cycle while another embryo or fetus is already present in the uterus. When two separate instances of fertilisation occur during the same menstrual cycle, it is known as superfecundation.

Superfetation is claimed to be common in some species of animals. In mammals, it can occur only where there are two uteri, or where the estrous cycle continues through pregnancy.

Syrian hamster breeding

Syrian hamster breeding is selective breeding on Syrian hamsters. The practice of breeding requires an understanding of care for the Syrian hamster, knowledge about Syrian hamster variations, a plan selective breeding, scheduling of the female body cycle, and the ability to manage a colony of hamsters.

Vaginal cytology

Vaginal cytology is a microscopic examination of cells from the vaginal epithelium. In mammals, it shows the stages of the menstrual or estrous cycle because the vaginal epithelium changes in response to sex hormone levels; practically, it is used to distinguish when a female canine is at a particular point in the estrous cycle. In normal vaginal smear, lactational cells, navicular cells, endocervical cells, endometrial cells, trophoblastic cells and leucocytes may be present.

Vandenbergh effect

The Vandenbergh effect is a phenomenon reported by J.G. Vandenbergh et al. in 1975, in which an early induction of the first estrous cycle in prepubertal female mice occurs as a result of exposure to the pheromone-laden urine of a sexually mature (dominant) male mouse.Physiologically, the exposure to male urine induces the release of GnRH, which provokes the first estrus. The Vandenbergh effect has also been seen with exposure to adult female mice. When an immature female mouse is exposed to the urine of mature female mouse, estrus is delayed in the prepubertal female. In this situation, GnRH is inhibited and therefore delays puberty in the juvenile female mouse.The Vandenbergh effect is caused by pheromones found in a male’s urine. The male does not have to be present for this effect to take place; the urine alone is sufficient. These pheromones are detected by the vomeronasal organ in the septum of the female’s nose. This occurs because the female body will only take the step to begin puberty if there are available mates around. She will not waste energy on puberty if there is no possibility of finding a mate.In addition to GnRH, exogenous estradiol has recently implicated as having a role in the Vandenbergh effect. Utilizing tritium-labeled estradiol implanted in male mice, researchers have been able to trace the pathways the estradiol takes once transmitted to a female. The estradiol was found in a multitude of regions within the females and appeared to enter her circulation nasally and through the skin. Their findings suggested that some aspects of the Vandenbergh effect as well as the Bruce effect may be related to exogenous estradiol from males.Additional studies have looked into the validity of estradiol’s role in the Vandenbergh effect by means of exogenous estradiol placed in castrated rats. Castrated males were injected with either a control (oil) or estradiol in the oil vehicle. As expected, urinary androgens in the castrated males were below normal levels. Additionally, castration by itself rid the males of the capacity to induce growth of the uterus in prepubescent females or to disrupt implantation of the blastocyst in females that had previously been inseminated. However, when resupplied with estradiol, the castrated males regained the ability to induce uterine growth or halt blastocyst implantation. These studies further support estradiol’s role in mediating the Vandenbergh effect (as well as the Bruce effect).

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