Do all female mammals menstruate?
Menstruation
Most female mammals have an estrous cycle, yet only ten primates species, four bats species, the elephant shrew, and the spiny mouse have a menstrual cycle. The relatedness of these groups suggests that four distinct evolutionary events have caused menstruation to arise.
Culture and menstruation
Menstruation in synchrony with the moon is widely assumed in myths and traditions as a ritual ideal. The idea that menstruation is—or ideally ought to be—in harmony with wider cosmic rhythms is one of the most tenacious ideas central to the myths and rituals of traditional communities across the world. One of the most thoroughgoing analyses of primitive mythology ever undertaken was that of the French anthropologist Claude Lévi-Strauss, who concluded that, taken together, the indigenous myths of North and South America expressed men's worry that, unless women's periods were carefully monitored and synchronised, the universe might descend into chaos.
Menstruation (mammal)
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.
Menstruation
Many have questioned the evolution of overt menstruation in humans and related species, speculating on what advantage there could be to losing blood associated with dismantling the endometrium, rather than absorbing it, as most mammals do. Humans do, in fact, reabsorb about two-thirds of the endometrium each cycle. Some work asserts that overt menstruation occurs not because it is beneficial in itself. Rather, the fetal development of these species requires a more developed endometrium, one which is too thick to reabsorb completely. There is a correlation between species that have overt menstruation to those that have a large uterus relative to the adult female body size.
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.
Grizzly bear
Grizzly bears hibernate for 5–7 months each year except where the climate is warm, as the California grizzly did not hibernate. During this time, female grizzly bears give birth to their offspring, who then consume milk from their mother and gain strength for the remainder of the hibernation period. To prepare for hibernation, grizzlies must prepare a den, and consume an immense amount of food as they do not eat during hibernation. Grizzly bears do not defecate or urinate throughout the entire hibernation period. The male grizzly bear's hibernation ends in early to mid-March, while females emerge in April or early May.
Menstrual synchrony
Since then Matsumoto and colleagues have reported estrous asynchrony in groups of free-living chimpanzees in Mahale Mountains National Park, Tanzania. They subsequently investigated whether estrous asynchrony was adaptive for female chimpanzees. They tested three hypotheses about the adaptiveness of estrous asynchrony: (1) females become asynchronous to increase copulation frequency and opportunities for giving birth; (2) paternity confusion to reduce infanticide; and (3) sperm competition. They found no support for hypothesis (1) and partial support for hypotheses (2) and (3).
Adolescence
In females, changes in the primary sex characteristics involve growth of the uterus, vagina, and other aspects of the reproductive system. Menarche, the beginning of menstruation, is a relatively late development which follows a long series of hormonal changes. Generally, a girl is not fully fertile until several years after menarche, as regular ovulation follows menarche by about two years. Unlike males, therefore, females usually appear physically mature before they are capable of becoming pregnant.
Menstruation (mammal)
Orangutans also lack visible signs of impending ovulation. Also, it has been said that the extended estrus period of the bonobo (reproductive-age females are in heat for 75% of their menstrual cycle) has a similar effect to the lack of a "heat" in human females.
Menstrual synchrony
Strassmann investigated whether menstrual synchrony occurred in a natural fertility population of Dogon village women. Her study consisted of 122 Dogon women with an average lifetime fertility rate of 8.6 ± .3 live births per woman. Their median cycle length was 30 days, which is indistinguishable from western women. In analyzing whether menstrual synchrony occurs among Dogon women, she was aware of Wilson's methodological criticisms of previous studies and aware that menstrual synchrony isn't synchrony "per se", but rather the closeness of menstruation among women. She used Cox regression to determine whether the likelihood of menstruating was influenced by other women. She considered the levels of all the women in the village, all the women in the same lineage, and all the women in the same economic unit (i.e., they worked together). She found no significant relationship at any level, which means that there was no evidence of synchronization. She concluded that this result undermined the view that menstrual synchrony is adaptive and the view held by many anthropologists at the time that menstrual synchrony occurred in preindustrial societies.
Lactational amenorrhea
Return of menstruation following childbirth varies widely among individuals. This return does not necessarily mean a woman has begun to ovulate again. The first postpartum ovulatory cycle might occur before the first menses following childbirth or during subsequent cycles. A strong relationship has been observed between the amount of suckling and the contraceptive effect, such that the combination of feeding on demand rather than on a schedule and feeding only breast milk rather than supplementing the diet with other foods will greatly extend the period of effective contraception. In fact, it was found that among the Hutterites, more frequent bouts of nursing, in addition to maintenance of feeding in the night hours, led to longer lactational amenorrhea. An additional study that references this phenomenon cross-culturally was completed in the United Arab Emirates (UAE) and has similar findings. Mothers who breastfed exclusively longer showed a longer span of lactational amenorrhea, ranging from an average of 5.3 months in mothers who breastfed exclusively for only two months to an average of 9.6 months in mothers who did so for six months. Another factor shown to affect the length of amenorrhea was the mother's age. The older a woman was, the longer period of lactational amenorrhea she demonstrated. The same increase in length was found in multiparous women as opposed to primiparous. With regards to the use of breastfeeding as a form of contraception, most women who choose not to breastfeed will resume regular menstrual cycling within 1.5 to 2 months following parturition. Furthermore, the closer a woman's behavior is to the Seven Standards of ecological breastfeeding, the later (on average) her cycles will return. Overall, there are many factors including frequency of nursing, mother's age, parity, and introduction of supplemental foods into the infant's diet among others which can influence return of fecundity following pregnancy and childbirth and thus the contraceptive benefits of lactational amenorrhea are not always reliable but are evident and variable among women. Couples who desire spacing of 18 to 30 months between children can often achieve this through breastfeeding alone, though this is not a foolproof method as return of menses is unpredictable and conception can occur in the weeks preceding the first menses.
Mammal
The basic body type is quadruped, and most mammals use their four extremities for terrestrial locomotion; but in some, the extremities are adapted for life at sea, in the air, in trees, underground, or on two legs. Mammals range in size from the bumblebee bat to the blue whale—the largest animal on the planet. Maximum lifespan varies from two years for the shrew to 211 years for the bowhead whale. All modern mammals give birth to live young, except the five species of monotremes, which are egg-laying mammals. The most species-rich group of mammals, the cohort called placentals, have a placenta, which enables the feeding of the fetus during gestation.
Placentophagy
There is also a school of thought that holds that placentophagy naturally occurred to hide any trace of childbirth from predators in the wild. Many placental mammals participate in placentophagy, including herbivores; exceptions include mainly humans, Pinnipedia, Cetacea, and camels. Placentophagy has been observed in Insectivora, Rodentia, Chiroptera, Lagomorpha, Carnivora, Perissodactyla, Artiodactyla (with the camel as a noted exception), and Primates. Marsupials, which are an order of metatherian (pouched) mammals, resorb rather than deliver the placenta, and therefore cannot engage in placentophagia; they do, however, vigorously lick birth fluids as they are excreted.
Menopause
Menopause in the animal kingdom appears to be uncommon, but the presence of this phenomenon in different species has not been thoroughly researched. Life histories show a varying degree of senescence; rapid senescing organisms (e.g., Pacific salmon and annual plants) do not have a post-reproductive life-stage. Gradual senescence is exhibited by all placental mammalian life histories.
Menstruation
The first menstrual period occurs after the onset of pubertal growth, and is called menarche. The average age of menarche is 12 to 15. However, it may start as early as eight. The average age of the first period is generally later in the developing world, and earlier in the developed world. The average age of menarche has changed little in the United States since the 1950s.
Vagina
Vaginal secretions are primarily from the uterus, cervix, and vaginal epithelium in addition to minuscule vaginal lubrication from the Bartholin's glands upon sexual arousal. It takes little vaginal secretion to make the vagina moist; secretions may increase during sexual arousal, the middle of or a little prior to menstruation, or during pregnancy. Menstruation (also known as a "period" or "monthly") is the regular discharge of blood and mucosal tissue (known as menses) from the inner lining of the uterus through the vagina. The vaginal mucous membrane varies in thickness and composition during the menstrual cycle, which is the regular, natural change that occurs in the female reproductive system (specifically the uterus and ovaries) that makes pregnancy possible. Different hygiene products such as tampons, menstrual cups, and sanitary napkins are available to absorb or capture menstrual blood.
Embryonic diapause
Some mammals that undergo embryonic diapause include rodents, bears, armadillos, mustelids (e.g. weasels and badgers), and marsupials, (e.g. kangaroos). Some groups only have one species that undergoes embryonic diapause, such as the roe deer in the order Artiodactyla.
Woman
In terms of biology, the female sex organs are involved in the reproductive system, whereas the secondary sex characteristics are involved in nurturing children or, in some cultures, attracting a mate. The ovaries, in addition to their regulatory function producing hormones, produce female gametes called eggs which, when fertilized by male gametes (sperm), form new genetic individuals. The uterus is an organ with tissue to protect and nurture the developing fetus and muscle to expel it when giving birth. The vagina is used in copulation and birthing, although the term vagina is often colloquially and incorrectly used in the English language for the vulva or external female genitalia, which consists of (in addition to the vagina) the labia, the clitoris, and the female urethra. The breast evolved from the sweat gland to produce milk, a nutritious secretion that is the most distinctive characteristic of mammals, along with live birth. In mature women, the breast is generally more prominent than in most other mammals; this prominence, not necessary for milk production, is probably at least partially the result of sexual selection. (For other ways in which men commonly differ physically from women, see man.)
Birth
It is not only mammals that give birth. Some reptiles, amphibians, fish and invertebrates carry their developing young inside them. Some of these are ovoviviparous, with the eggs being hatched inside the mother's body, and others are viviparous, with the embryo developing inside her body, as in mammals.
Mammalian reproduction
Once the fetus is sufficiently developing, chemical signals start the process of birth, which begins with contractions of the uterus and the dilation of the cervix. The fetus then descends to the cervix, where it is pushed out into the vagina, and eventually out of the female. The newborn, which is called an infant in humans, should typically begin respiration on its own shortly after birth. Not long after, the placenta is passed as well. Most mammals eat this, as it is a good source of protein and other vital nutrients needed for caring for the young.
Culture and menstruation
In some historic cultures, a menstruating woman was considered sacred and powerful, with increased psychic abilities, and strong enough to heal the sick. According to the Cherokee, menstrual blood was a source of feminine strength and had the power to destroy enemies. In Ancient Rome, Pliny the Elder wrote that a menstruating woman who uncovers her body can scare away hailstorms, whirlwinds and lightning. If she strips naked and walks around the field, caterpillars, worms and beetles fall off the ears of corn. Menstrual blood is viewed as especially dangerous to men's power. In Africa, menstrual blood is used in the most powerful magic charms in order to both purify and destroy.
Mayan mythology explains the origin of menstruation as a punishment for violating the social rules governing marital alliance. The menstrual blood turns into snakes and insects used in black sorcery, before the Maya moon goddess is reborn from it.
The Evolution of Human Sexuality
According to Symons, while orgasm in the human female has been proposed to be an adaptation resulting from selective forces, the available evidence, which shows that the female orgasm is far from being a universal result of heterosexual intercourse and that its frequency varies greatly between cultures and between individuals, does not support that conclusion. Symons suggested that the female orgasm may be possible for female mammals because it is adaptive for males. He notes that in most mammalian species the only known function of the clitoris is to generate sensation during copulation, but saw no evidence that "the female genitals of any mammalian species have been designed by natural selection for efficiency in orgasm production." He criticizes Elizabeth Sherfey's view that the female orgasm is an adaptation, writing that her arguments are not supported by ethnographic or biological evidence. Symons proposes that male human ancestors lost the ability to detect ovulation in females by smell because females gained a reproductive advantage by concealing ovulation, and that estrus ceased to exist in humans at the same time. Observing that estrous female chimpanzees are more successful than nonestrous females in obtaining meat from males, Symons suggests that when hunting became a dominant male economic activity during human evolution, the benefits to females of receiving meat may have outweighed the costs to them of constant sexual activity, leading to women making sexual overtures to men in order to obtain meat.
Mammal
Mammals ( from Latin "mamma" "breast") are vertebrate animals constituting the class Mammalia, and characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or hair, and three middle ear bones. These characteristics distinguish them from reptiles and birds, from which they diverged in the late Triassic, 201–227 million years ago. There are around 5,450 species of mammals. The largest orders are the rodents, bats and Soricomorpha (shrews and others). The next three are the Primates (humans, apes, monkeys, and others), the Cetartiodactyla (whales and even-toed ungulates), and the Carnivora (cats, dogs, seals, and others).
Continuous breeder
Continuous breeders are animal species that can breed or mate throughout the year. This includes humans and apes (chimpanzees, gorillas, orangutans, and gibbons), who can have a child at any time of year. In continuous breeders, females are sexually receptive during estrus, at which time ovarian follicles are maturing and ovulation can occur. Evidence of ovulation, the phase during which conception is most probable, is advertised to males among many non-human primates via swelling and redness of the genitalia.
Womb (disambiguation)
The womb is a major female hormone-responsive reproductive sex organ of most mammals.
Mammal
All higher mammals (excluding monotremes) share two major adaptations for care of the young: live birth and lactation. These imply a group-wide choice of a degree of parental care. They may build nests and dig burrows to raise their young in, or feed and guard them often for a prolonged period of time. Many mammals are K-selected, and invest more time and energy into their young than do r-selected animals. When two animals mate, they both share an interest in the success of the offspring, though often to different extremes. Mammalian females exhibit some degree of maternal aggression, another example of parental care, which may be targeted against other females of the species or the young of other females; however, some mammals may "aunt" the infants of other females, and care for them. Mammalian males may play a role in child rearing, as with tenrecs, however this varies species to species, even within the same genus. For example, the males of the southern pig-tailed macaque ("Macaca nemestrina") do not participate in child care, whereas the males of the Japanese macaque ("M. fuscata") do.
Reproductive system
The uterus and vagina are unique to mammals with no homologue in birds, reptiles, amphibians, or fish. In place of the uterus the other vertebrate groups have an unmodified oviduct leading directly to a cloaca, which is a shared exit-hole for gametes, urine, and feces. Monotremes (i.e. platypus and echidnas), a group of egg-laying mammals, also lack a uterus and vagina, and in that respect have a reproductive system resembling that of a reptile.
Menstrual synchrony
In 1985, estrous synchrony was reported in female chimpanzees. In her study, 10 female chimpanzees were caged, at different times, in two groups of four and six in the same building. The anogenital swelling of each female was recorded daily. Synchrony was measured by calculating the absolute differences in days between (1) the day of swelling onset and (2) the day of maximum swelling. She reported a statistically significant average difference of 5.7 days for onset of swelling and 8.0 days for maximum swelling. Schank, however, noted that due to females who became pregnant and who stopped cycling, most of the data were based on only four animals. He performed a computer simulation study to calculate the expected swelling onset and maximal swelling onset difference for female chimpanzees with the reported mean estrous cycle lengths of 36.7 (with a standard deviation of 4.3) days. He reported an expected difference of 7.7 days. Thus, a maximum swelling difference of 8.0 days is about what would be expected by chance and given that only four animals contributed data to the study, a 5.7 day onset difference is not significantly less than 7.7 days.
Cervix
Female marsupials have paired uteri and cervices. Most eutherian (placental) mammal species have a single cervix and single, bipartite or bicornuate uterus. Lagomorphs, rodents, aardvarks and hyraxes have a duplex uterus and two cervices. Lagomorphs and rodents share many morphological characteristics and are grouped together in the clade Glires. Anteaters of the family myrmecophagidae are unusual in that they lack a defined cervix; they are thought to have lost the characteristic rather than other mammals developing a cervix on more than one lineage. In domestic pigs, the cervix contains a series of five interdigitating pads that hold the boar's corkscrew-shaped penis during copulation.
Mammalian reproduction
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.
Orgasm
The mechanics of male orgasm are similar in most male mammals. Females of some mammal and some non-mammal species, such as alligators, have clitorises. There has been ongoing research about the sexuality and orgasms of dolphins, a species which apparently engages in sexual intercourse for reasons other than procreation. The duration of orgasm varies considerably among different mammal species.
Operating temperature
Mammals attempt to maintain a comfortable body temperature under various conditions by thermoregulation, part of mammalian homeostasis. The lowest normal temperature of a mammal, the basal body temperature, is achieved during sleep. In women, it is affected by ovulation, causing a biphasic pattern which may be used as a component of fertility awareness.
Opossum
As a marsupial, the female opossum has a reproductive system that includes a bifurcated vagina, a divided uterus and a marsupium, which is her pouch. The average estrous cycle of the opossum is about 28 days. Opossums do possess a placenta, but it is short-lived, simple in structure, and, unlike that of placental mammals, not fully functional. The young are therefore born at a very early stage, although the gestation period is similar to that of many other small marsupials, at only 12 to 14 days. Once born, the offspring must find their way into the marsupium to hold on to and nurse from a teat. Baby opossums, like their Australian cousins, are called joeys. Female opossums often give birth to very large numbers of young, most of which fail to attach to a teat, although as many as thirteen young can attach, and therefore survive, depending on species. The young are weaned between 70 and 125 days, when they detach from the teat and leave the pouch. The opossum lifespan is unusually short for a mammal of its size, usually only one to two years in the wild and as long as four or more years in captivity. Senescence is rapid.
Menstruation
Beginning in 1971, some research suggested that menstrual cycles of cohabiting human females became synchronized (menstrual synchrony). However, there is currently significant dispute as to whether menstrual synchrony exists. A 2013 review concluded that menstrual synchrony likely does not exist.
Maternal transfer in aquatic mammals
Marine mammals are exposed to a variety of chemicals throughout their life, mostly through their diet. Once the chemicals are accumulated in the body tissues of the mammals, a portion of these chemicals in the female mammals are transferred to their offspring during gestation and lactation.
Menstrual cycle
The menstrual cycle is the regular natural change that occurs in the female reproductive system (specifically the uterus and ovaries) that makes pregnancy possible. The cycle is required for the production of oocytes, and for the preparation of the uterus for pregnancy. Up to 80% of women report having some symptoms during the one to two weeks prior to menstruation. Common symptoms include acne, tender breasts, bloating, feeling tired, irritability and mood changes. These symptoms interfere with normal life and therefore qualify as premenstrual syndrome in 20 to 30% of women. In 3 to 8%, they are severe.
The first period usually begins between twelve and fifteen years of age, a point in time known as menarche. They may occasionally start as early as eight, and this onset may still be normal. The average age of the first period is generally later in the developing world and earlier in developed world. The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women and 21 to 35 days in adults (an average of 28 days). Menstruation stops occurring after menopause which usually occurs between 45 and 55 years of age. Bleeding usually lasts around 2 to 7 days.
The menstrual cycle is governed by hormonal changes. These changes can be altered by using hormonal birth control to prevent pregnancy. Each cycle can be divided into three phases based on events in the ovary (ovarian cycle) or in the uterus (uterine cycle). The ovarian cycle consists of the follicular phase, ovulation, and luteal phase whereas the uterine cycle is divided into menstruation, proliferative phase, and secretory phase.
Female
Mammalian females bear live young (with the rare exception of monotremes, which lay eggs). Some non-mammalian species, such as guppies, have analogous reproductive structures; and some other non-mammals, such as sharks, whose eggs hatch inside their bodies, also have the appearance of bearing live young.
Lemur
Lemurs time their mating and birth seasons so that all weaning periods are synchronized to match the time of highest food availability. Weaning occurs either before or shortly after the eruption of the first permanent molars in lemurs. Mouse lemurs are able to fit their entire breeding cycle into the wet season, whereas larger lemurs, such as sifakas, must lactate for two months during the dry season. Infant survival in some species, such as Milne-Edwards' sifaka, has been shown to be directly impacted by both environmental conditions and the rank, age, and health of the mother. The breeding season is also affected by geographical location. For example, mouse lemurs give birth between September and October in their native habitat in the Southern Hemisphere, but from May through June in the captive settings in the Northern Hemisphere.
Scent factors heavily into lemur reproduction. Scent-marking activity escalates during the mating season. Pheromones may coordinate reproductive timing for females coming into estrus. Mating can be either monogamous or promiscuous for both males and females, and mating can include individuals from outside the group. Monogamous lemurs include the red-bellied lemur ("Eulemur rubriventer") and the mongoose lemur ("E. mongoz"), although the mongoose lemur has been observed mating outside of its pair bond. Monogamy is most common among nocturnal species, although some exhibit scramble competition, sexual suppression of subordinates, or competitions between males that avoid direct fighting. In mouse lemurs, males utilize sperm plugs, developed enlarged testes during the mating season, and develop size dimorphism (likely due to the enlarged testes). These indicate a mating system known as scramble competition polygyny, where males cannot defend females or the resources that might attract them.
Mule
A female mule that has estrus cycles and thus, in theory, could carry a fetus, is called a "molly" or "Molly mule", though the term is sometimes used to refer to female mules in general. Pregnancy is rare, but can occasionally occur naturally as well as through embryo transfer. A male mule is properly called a "horse mule", though often called a "john mule", which is the correct term for a gelded mule. A young male mule is called a "mule colt", and a young female is called a "mule filly".
Sexual swelling
To successfully ensure paternal confusion, Hrdy predicted that ovulation must be randomly distributed across the term where the sexual swelling is of maximum tumescence. This would ensure that males were unable to use the swelling as a signal of female fertility. One example of this stems from research into sooty mangabey ("Cercocebus atys"); the females of this species have been found to produce sexual swellings both when they are fertile and when they are with child. Additionally, sexual swellings are only observed during the most fertile period of a female’s menstrual cycle in 26-35 species of anthropoid primates, in contrast to Hrdy's prediction.
Pregnancy (mammals)
For mammals the gestation period is the time in which a fetus develops, beginning with fertilization and ending at birth. The duration of this period varies between species.
Lactation
In most species, milk comes out of the mother's nipples; however, the monotremes, egg-laying mammals, lack nipples and release milk through ducts in the abdomen. In only one species of mammal, the Dayak fruit bat, is milk production a normal male function.
Pygmy sperm whale
Although firm details concerning pygmy sperm whale reproduction are limited, they are believed to mate from April to September in the Southern hemisphere and March to August in the Northern hemisphere. These whales become sexually mature at age 4-5, and like virtually all mammals, are iteroparous (reproducing many times during their life). Once a female whale is impregnated, the average gestation period lasts nine to eleven months and, unusually for cetaceans, the female gives birth to a single calf head-first. Newborn calves are about in length, weighing 50 kg, and are weaned at around one year of age. They are believed to live up to age 23.
Menstrual synchrony
In mammalian mating systems generally, and among primates in particular, female spatio-temporal distribution – how clumped females are in the environment and how much they overlap their fertile periods – affects the ability of any single male to monopolize matings. The basic principle is that the more females are fertile at any one time, the harder it is for any single male to monopolize access to them, impregnating all simultaneously at the expense of rival males. In the case of nonhuman primates, once the number of co-cycling females rises above a critical threshold, a harem-holder may be unable to prevent other males from invading and mating with his females. A dominant male can maintain his monopoly only if his females stagger their fertile periods, so that he can impregnate them one at a time (see figure a, right). Suppose a group of female baboons need between them just one dominant male, desirable in view of his high-quality genes. Then, logically, they should avoid synchronizing their cycles. By the same token, if males during the course of human evolution became valued by females for additional purposes – hunting and bringing home food, for example – then females should resist being controlled by dominant male harem-holders. If males are useful partners to have and keep around, then ideally each female should have at least one for herself. Under those circumstances, according to this argument, the logical strategy would be for females to synchronize as tightly as they can (see figure b, right).
Spiny mouse
One species of spiny mouse was recently discovered to have a menstrual cycle which would make them important as a model organism to study menstruation and reproductive disorders, as Mus musculus, the common laboratory mouse, does not menstruate.
Calendar-based contraceptive methods
Written references to a "safe period" do not appear again for over a thousand years. Scientific advances prompted a number of secular thinkers to advocate periodic abstinence to avoid pregnancy: in the 1840s it was discovered that many animals ovulate during estrus. Because some animals (such as dogs) have a bloody discharge during estrus, it was assumed that menstruation was the corresponding most fertile time for women. This inaccurate theory was popularized by physicians Bischoff, Félix Archimède Pouchet, and Adam Raciborski. In 1854, an English physician named George Drysdale correctly taught his patients that the days near menstruation are the "least" fertile, but this remained the minority view for the remainder of the 19th century.
Menstrual synchrony
The !Kung (or Ju|'hoansi) hunter-gatherers of the Kalahari "believe...that if a woman sees traces of menstrual blood on another woman's leg or even is told that another woman has started her period, she will begin menstruating as well". Among the Yurok people of northwestern California, according to one ethnographic study, "all of a household's fertile women who were not pregnant menstruated at the same time..."
The Great Pheromone Myth
He argues that there is little or no scientific basis for human pheromones in the scientific literature. The vomeronasal organ, which is the sensory organ that takes in pheromone compounds in mammals such as mice and rats is a vestigial organ in humans. Menstrual synchrony has long been viewed as a physiological phenomenon in humans that could only be explained by the exchange of pheromones among women. He argues, however, that methodological critiques of menstrual cycle research and recent research indicate the menstrual synchrony does not occur among women. Finally, he reviews the literature on human pheromones and argues that there are serious methodological issues in all studies reporting human pheromones and that no human pheromone has ever been definitively identified.
Barbary macaque
Although Barbary macaques are sexually active at all points during a female’s reproductive cycle, male Barbary macaques determine a female’s most fertile period by sexual swellings on the female. Mating is most common during a female’s most fertile period. The swelling size of the female reaches a maximum around the time of ovulation, suggesting that size helps a male predict when he should mate. This is further supported by the fact that male ejaculation peaks at the same time that female sexual swelling peaks. There is not a sufficient change in female sexual behaviour around the time of ovulation in order to demonstrate to the male that the female is fertile. The swellings, therefore, appear necessary for predicting fertility.
Female
Female (♀) is the sex of an organism, or a part of an organism, that produces non-mobile ova (egg cells). Barring rare medical conditions, most female mammals, including female humans, have two X chromosomes. Female characteristics vary between different species with some species containing more well defined female characteristics. Both genetics and environment shape the prenatal development of a female.
Endometrium
The endometrial lining undergoes cyclic regeneration. Humans, apes, and some other species display the menstrual cycle, whereas most other mammals are subject to an estrous cycle. In both cases, the endometrium initially proliferates under the influence of estrogen. However, once ovulation occurs, the ovary (specifically the corpus luteum) will produce much larger amounts of progesterone. This changes the proliferative pattern of the endometrium to a secretory lining. Eventually, the secretory lining provides a hospitable environment for one or more blastocysts.
Vertebra
In many species, though not in mammals, the cervical vertebrae bear ribs. In many groups, such as lizards and saurischian dinosaurs, the cervical ribs are large; in birds, they are small and completely fused to the vertebrae. The transverse processes of mammals are homologous to the cervical ribs of other amniotes. In the whale the cervical vertebrae are typically fused, an adaptation trading flexibility for stability during swimming. All mammals except manatees and sloths have seven cervical vertebrae, whatever the length of the neck. This includes seemingly unlikely animals such as the giraffe, the camel, and the blue whale, for example. Birds usually have more cervical vertebrae with most having a highly flexible neck consisting of 13–25 vertebrae.
Animal sexual behaviour
There is nothing in female mammals' anatomy or physiology that contradicts that stimulation of the sexual organs and mating is able to be a positive experience. For instance, the clitoris acts in the same way as with women, and scientific studies have shown that the success of reproduction is improved by stimulation of clitoris on (among other species) cows and mares in connection with insemination, because it improves the transportation of the sperm due to contractions of the inner genitalia. This probably also applies to female animals of other animal species, and contractions in the inner genitals are seen e.g. also during orgasm for women. It is therefore reasonable to assume that sexual intercourse may be linked with a positive experience for female animals.
Koinophilia is the love of the "normal" or phenotypically common (from the Greek, "koinos", meaning "the usual" or "common"). The term was introduced to scientific literature in 1990, and refers to the tendency of animals seeking a mate to prefer that mate not to have any unusual, peculiar or deviant features. Similarly, animals preferentially choose mates with low fluctuating asymmetry.
Menstrual synchrony
Researchers are divided on whether menstrual synchrony would be adaptive. McClintock has suggested that menstrual synchrony may not be adaptive but rather epiphenomenonal, lacking any biological function. Among those who postulate an adaptive function, one argument is that menstrual synchrony is only a particular aspect of the much more general phenomenon of reproductive synchrony, an occurrence familiar to ecologists studying animal populations in the wild. Whether seasonal, tidal or lunar, reproductive synchrony is a relatively common mechanism through which co-cycling females can increase the number of males included in the local breeding system.
Extended female sexuality
Although not found in all organisms, researchers have identified sexual intercourse patterns in certain animals that reflect extended female sexuality, such as in some old world primates, birds and insects. Extensive research has focused on analysing the musk shrew's rate of sexual behaviour. The only period that is associated with a drop in female receptivity to copulation is during mid to late pregnancy; yet, even at this time, occasional mating is reported. Therefore, researchers have concluded that this animal has similar sexual receptivity across infertile and fertile phases. Within primates, research has consistently found evidence of extended female sexuality in the rhesus monkey and chimpanzees. Both of these primates mate at all stages of the ovarian cycle, with only slight increases in sexual receptivity during fertile stages, and decreases during menstruation.
Mammal
The majority of mammals have seven cervical vertebrae (bones in the neck), including bats, giraffes, whales and humans. The exceptions are the manatee and the two-toed sloth, which have just six, and the three-toed sloth which has nine cervical vertebrae. All mammalian brains possess a neocortex, a brain region unique to mammals. Placental mammals have a corpus callosum, unlike monotremes and marsupials.
Mammalian reproduction
Marsupials' reproductive systems differ markedly from those of placental mammals, though it is probably the plesiomorphic condition found in viviparous mammals, including non-placental eutherians. During embryonic development, a choriovitelline placenta forms in all marsupials. In bandicoots, an additional chorioallantoic placenta forms, although it lacks the chorionic villi found in eutherian placentas.
Menarche
Menarche ( ; Greek: "mēn" "month" + "arkhē" "beginning") is the first menstrual cycle, or first menstrual bleeding, in female humans. From both social and medical perspectives, it is often considered the central event of female puberty, as it signals the possibility of fertility.
Mammalian reproduction
Gestation, called "pregnancy" in humans, is the period of time during which the fetus develops, dividing via mitosis inside the female. During this time, the fetus receives all of its nutrition and oxygenated blood from the female, filtered through the placenta, which is attached to the fetus' abdomen via an umbilical cord. This drain of nutrients can be quite taxing on the female, who is required to ingest slightly higher levels of calories. In addition, certain vitamins and other nutrients are required in greater quantities than normal, often creating abnormal eating habits. The length of gestation, called the gestation period, varies greatly from species to species; it is 40 weeks in humans, 56–60 in giraffes and 16 days in hamsters.
Menarche
Menstruation is a cultural as well as scientific phenomenon as many societies have specific rituals and cultural norms associated with it. These rituals typically begin at menarche and some are enacted during each menstruation cycle. The rituals are important in determining a status change for girls. Upon menarche and completion of the ritual, they have become a woman as defined by their culture.
Clitoris
The clitoris ( or ) is a female sex organ present in mammals, ostriches and a limited number of other animals. In humans, the visible portion - the glans - is at the front junction of the labia minora (inner lips), above the opening of the urethra. Unlike the penis, the male homologue (equivalent) to the clitoris, it usually does not contain the distal portion (or opening) of the urethra and is therefore not used for urination. The clitoris also usually lacks a reproductive function. While few animals urinate through the clitoris or use it reproductively, the spotted hyena, which has an especially large clitoris, urinates, mates, and gives birth via the organ. Some other mammals, such as lemurs and spider monkeys, also have a large clitoris.
Estrous cycle
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 and facial appearance. Some research also suggests that women tend to have more sexual thoughts and are more prone to sexual activity right before ovulation. Animals with estrous cycles often have unmistakable outward displays of receptivity, ranging from engorged and colorful genitals to behavioral changes like mating calls.
Extended female sexuality
Research is fairly consistent in the finding that species with concealed estrus mate at all stages of their ovarian cycle. For instance, mating activity in assamese macaques ("Macaca assamensis") has been investigated. The researchers analysed the levels of progesterone in the monkeys, in order to establish the ovarian stage of the female, as progesterone peaks following the fertile window. They found that the females concealed estrus and were sexually receptive during the entire mating season. Concealed estrus and sexual receptivity (at all times of the ovarian cycle) aids paternity confusion. This is because the males are unsure of who mated with the female during her fertile period, and so do not know the identity of the father.
Tide
The approximately fortnightly tidal cycle has large effects on intertidal and marine organisms. Hence their biological rhythms tend to occur in rough multiples of this period. Many other animals such as the vertebrates, display similar rhythms. Examples include gestation and egg hatching. In humans, the menstrual cycle lasts roughly a lunar month, an even multiple of the tidal period. Such parallels at least hint at the common descent of all animals from a marine ancestor.
Ovulatory shift hypothesis
Most female mammals experience reproductive fertility cycles. They typically consist of a long period of low fertility, and a brief period of high fertility just prior to and including ovulation. In humans, this is called the ovulatory cycle, or menstrual cycle. The period of high fertility is also called the fertile window, and is the only time during the cycle when sex can result in conception.
Pregnancy (mammals)
In mammals, pregnancy is the period of reproduction during which a female carries one or more live offspring from implantation in the uterus through gestation. It begins when a fertilized zygote implants in the female's uterus, and ends once it leaves the uterus.
Menopause
While not conclusive, evidence does exist to support the idea that female-biased dispersal existed in pre-modern humans. The closest living relatives to humans, chimpanzees, bonobos, and both mountain gorillas and western lowland gorillas, are female-biased dispersers. Analysis of sex specific genetic material, the non-recombining portions of the Y chromosome and mitochondrial DNA, show evidence of a prevalence of female-biased dispersal as well; however, these results could also be affected by the effective breeding numbers of males and females in local populations. Evidence of female-biased dispersion in hunter-gatherers is not definitive, with some studies supporting the idea, and others suggesting there is no strong bias towards either sex. In killer whales, both sexes mate non-locally with members of a different pod but return to the pod after copulation. Demographic data shows that a female’s mean relatedness to the group does increase over time due to increasing relatedness to males. While less well-studied, there is evidence that short-finned pilot whales, another menopausal species, also display this behavior. However, mating behavior that increases local relatedness with female age is prevalent in non-menopausal species, making it unlikely that it is the only factor that determines if menopause will evolve in a species.
Yanomami women
The start of "menstruation" symbolizes the beginning of womanhood. Girls typically start menstruation around the age of 12-15. Girls are often betrothed before menarche and the marriage may only be consummated once the girl starts menstruating, though the taboo is often violated and many girls become sexually active before then. The Yanomami word for menstruation ("roo") translates literally as "squatting" in English, as they use no pads or cloths to absorb the blood. Due to the belief that menstrual blood is poisonous and dangerous, girls are kept hidden away in a small tent-like structure constructed of a screen of leaves. A deep hole is built in the structure over which girls squat, to "rid themselves" of their blood. These structures are regarded as isolation screens.
Sperm whale
The sperm whale is a pelagic mammal with a worldwide range, and will migrate seasonally for feeding and breeding. Females and young males live together in groups, while mature males (bulls) live solitary lives outside of the mating season. The females cooperate to protect and nurse their young. Females give birth every four to twenty years, and care for the calves for more than a decade. A mature sperm whale has few natural predators, although calves and weakened adults are sometimes killed by pods of orcas (killer whales).
Menstrual cycle
Breastfeeding causes negative feedback to occur on pulse secretion of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH). Depending on the strength of the negative feedback, breastfeeding women may experience complete suppression of follicular development, but no ovulation, or normal menstrual cycle may resume. Suppression of ovulation is more likely when suckling occurs more frequently. The production of prolactin in response to suckling is important to maintaining lactational amenorrhea. On average, women who are fully breastfeeding whose infants suckle frequently experience a return of menstruation at fourteen and a half months postpartum. There is a wide range of response among individual breastfeeding women, however, with some experiencing return of menstruation at two months and others remaining amenorrheic for up to 42 months postpartum.
Mammal
In male placentals, the penis is used both for urination and copulation. Depending on the species, an erection may be fueled by blood flow into vascular, spongy tissue or by muscular action. A penis may be contained in a sheath when not erect, and some placentals also have a penis bone (baculum). Marsupials typically have forked penises while the monotreme penis generally has four heads with only two functioning. The testes of most mammals descend into the scrotum which is typically posterior to the penis but is often anterior in marsupials. Female mammals generally have a clitoris, labia majora and labia minora on the outside, while the internal system contains paired oviducts, 1-2 uteri, 1-2 cervices and a vagina. Marsupials have two lateral vaginas and a medial vagina. The "vagina" of monotremes is better understood as a "urogenital sinus". The uterine systems of placental mammals can vary between a duplex, were there are two uteri and cervices which open into the vagina, a bipartite, were two uterine horns have a single cervix that connects to the vagina, a bicornuate, which consists where two uterine horns that are connected distally but separate medially creating a Y-shape, and a simplex, which has a single uterus.
Most mammals are viviparous, giving birth to live young. However, the five species of monotreme, the platypus and the four species of echidna, 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.
Animal sexual behaviour
When animal sexual behaviour is reproductively motivated, it is often termed "mating" or "copulation"; for most non-human mammals, mating and copulation occur at oestrus (the most fertile period in the mammalian female's reproductive cycle), which increases the chances of successful impregnation. Some animal sexual behaviour involves competition, sometimes fighting, between multiple males. Females often select males for mating only if they appear strong and able to protect themselves. The male that wins a fight may also have the chance to mate with a larger number of females and will therefore pass on his genes to their offspring.
Marsupial
The evolution of reproduction in marsupials, and speculation about the ancestral state of mammalian reproduction, have engaged discussion since the end of the 19th century. Both sexes possess a cloaca, which is connected to a urogenital sac used to store waste before expulsion. The bladder of marsupials functions as a site to concentrate urine and empties into the common urogenital sinus in both females and males.
Menstruation
Breastfeeding causes negative feedback to occur on pulse secretion of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH). Depending on the strength of the negative feedback, breastfeeding women may experience complete suppression of follicular development, follicular development but no ovulation, or normal menstrual cycles may resume. Suppression of ovulation is more likely when suckling occurs more frequently. The production of prolactin in response to suckling is important to maintaining lactational amenorrhea. On average, women who are fully breastfeeding whose infants suckle frequently experience a return of menstruation at fourteen and a half months postpartum. There is a wide range of response among individual breastfeeding women, however, with some experiencing return of menstruation at two months and others remaining amenorrheic for up to 42 months postpartum.
Female
Most mammalian females have two copies of the X chromosome as opposed to the male which carries only one X and one smaller Y chromosome (but some mammals, such as the platypus, have different combinations). To compensate for the difference in size, one of the female's X chromosomes is randomly inactivated in each cell of placental mammals while the paternally derived X is inactivated in marsupials. In birds and some reptiles, by contrast, it is the female which is heterozygous and carries a Z and a W chromosome whilst the male carries two Z chromosomes. Intersex conditions can also give rise to other combinations, such as XO or XXX in mammals, which are still considered as female so long as they do not contain a Y chromosome, except for specific cases of testosterone deficiency/insensitivity in XY individuals while in the womb. However, these conditions frequently result in sterility.
Estrous 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.
Mammal
Most mammals, including the six most species-rich orders, belong to the placental group. The three largest orders in numbers of species are Rodentia: mice, rats, porcupines, beavers, capybaras and other gnawing mammals; Chiroptera: bats; and Soricomorpha: shrews, moles and solenodons. The next three biggest orders, depending on the biological classification scheme used, are the Primates including the apes, monkeys and lemurs; the Cetartiodactyla including whales and even-toed ungulates; and the Carnivora which includes cats, dogs, weasels, bears, seals and allies. According to "Mammal Species of the World", 5,416 species were identified in 2006. These were grouped into 1,229 genera, 153 families and 29 orders. In 2008, the International Union for Conservation of Nature (IUCN) completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species. According to a research published in the "Journal of Mammalogy" in 2018, the number of recognized mammal species is 6,495 species included 96 recently extinct.
Nipple
Almost all mammals have nipples. Why males have nipples has been the subject of scientific research. Differences among the sexes (called sexual dimorphism) within a given species are considered by evolutionary biologists to be mostly the result of sexual selection, directly or indirectly. For traits where there is no difference among the sexes, evolutionary biologists assume that there has been no advantage to one of the sexes losing the trait.
Monotreme
The key anatomical difference between monotremes and other mammals gives them their name; "monotreme" means “single opening” in Greek, referring to the single duct (the cloaca) for their urinary, defecatory, and reproductive systems. Monotremes, like reptiles, have a single cloaca; marsupials also have a separate genital tract; whereas most placental mammal females have separate openings for reproduction (the vagina), urination (the urethra), and defecation (the anus). In monotremes, only semen passes through the penis; urine is excreted through the cloaca. The monotreme penis is similar to that of turtles, and is covered by a preputial sac.
Menstrual synchrony
Menstrual or estrous synchrony has been reported in other species including Norway rats, hamsters, chimpanzees, and Golden lion tamarins. In non-human primates, the term may also refer to the degree of overlap of menstrual or estrous cycles, which is the overlap of estrous or menses of two or more females in a group due, for example, to seasonal breeding.
Milk
The females of all mammal species can by definition produce milk, but cow's milk dominates commercial production. In 2011, FAO estimates 85% of all milk worldwide was produced from cows. Human milk is not produced or distributed industrially or commercially; however, human milk banks collect donated human breastmilk and redistribute it to infants who may benefit from human milk for various reasons (premature neonates, babies with allergies, metabolic diseases, etc.) but who cannot breastfeed.
Endangered mammals of India
The mammals are the class of vertebrate animals characterized by the presence of mammary glands, which in females produce milk for the nourishment of young; the presence of hair or fur; specialized teeth; the presence of a neocortex region in the brain; and endothermic or "warm-blooded" bodies. The brain regulates endothermic and circulatory system, including a four-chambered heart. Mammals encompass some 5,500 species (including Humans), distributed in about 1,200 genera, 152 families and up to 46 orders, though this varies with the classification scheme.
Reproductive suppression
Yellow, banded and dwarf mongooses live in families similar to wolf packs with scent marking done almost exclusively by the dominant pair. Dwarf mongooses show synchronous estrous but only the dominant female regularly gives birth. Subordinate females do not conceive or abort early, but do lactate after the dominant female gives birth and participate in communal nursing. When the subordinate females leave the parental pack they will become fully reproductively active in new groups they help establish. In captivity more than one dwarf mongoose in a group can give birth but the young of subordinates do not survive. However, if the size of their cage is increased and density is lower, reproductive activity of subordinates resumes. As in wolves, pheromonal control over the reproductive cycle of the mongoose is implicated as the mechanism for reproductive suppression.
The house mouse ("Mus musculus") is territorial with a single dominant male marking its territorial boundary with urine while subordinate males do not urine mark. A female's rate of urine marking is elevated in response to a dominant male's urine marking, and his urine contains pheromones that accelerate her ovulation. The presence of a dominant adult male and his urine are crucial for young females to initiate puberty. Females isolated from males often do not experience puberty or have unstable ovulatory cycles. Contact with the urine of a dominant female signals a young prepubescent female to delay the onset of puberty. When housed with only an adult male, female mice reach puberty within 25 days, but puberty will not occur until 40–50 days if they are housed with adult females. Being grouped with other females suppresses the young female's serum LH and prolactin. Exposure to the male elevates their LH levels followed by increased serum estradiol. Female mice have a short ovulatory cycle when housed with males and a long cycle when housed with only other females.
Hymen
In the 16th and 17th centuries, medical researchers mistakenly saw the presence or absence of the hymen as founding evidence of physical diseases such as "womb-fury", i.e., (female) hysteria. If not cured, womb-fury would, according to doctors practicing at the time, result in death.
Due to similar reproductive system development, many mammals have hymens, including chimpanzees, elephants, manatees, whales, horses and llamas.
Paternal care
The mechanisms that underlie the onset of parental behaviors in female mammals have been characterized in a variety of species. In mammals, females undergo endocrine changes during gestation and lactation that "prime" mothers to respond maternally towards their offspring.
Menopause
Menopause has been observed in several species of nonhuman primates, including rhesus monkeys and chimpanzees. Menopause also has been reported in a variety of other vertebrate species including elephants, short-finned pilot whales, killer whales, narwhals, beluga whales, the guppy, the platyfish, the budgerigar, the laboratory rat and mouse, and the opossum. However, with the exception of the short-finned pilot whale, killer whale, narwhals, and beluga whales, such examples tend to be from captive individuals, and thus they are not necessarily representative of what happens in natural populations in the wild.
Mammal
Nearly all mammals are endothermic ("warm-blooded"). Most mammals also have hair to help keep them warm. Like birds, mammals can forage or hunt in weather and climates too cold for ectothermic ("cold-blooded") reptiles and insects. Endothermy requires plenty of food energy, so mammals eat more food per unit of body weight than most reptiles. Small insectivorous mammals eat prodigious amounts for their size. A rare exception, the naked mole-rat produces little metabolic heat, so it is considered an operational poikilotherm. Birds are also endothermic, so endothermy is not unique to mammals.
Yanomami women
Up until the time of menstruation, girls are treated as children, and are only responsible for assisting their mothers in household work. When they approach the age of menstruation, they are sought out by males as potential wives. Puberty is not seen as a significant time period with male Yanomami children, but it is considered very important for females. After menstruating for the first time, the girls are expected to leave childhood and enter adulthood, and take on the responsibilities of a grown Yanomami woman. After a young girl gets her period, she is forbidden from showing her genitalia and must keep herself covered with a loincloth.
Culture and menstruation
Across the continent of Africa, a wide variety of menstruation-related customs have been recorded.
A cloth torn from the traditional wrap (chitenge) is worn, part tied around the waist and part looped under the crutch, to catch menstrual fluid. Menarche (the first menstrual cycle at puberty) is traditionally treated as a sign that the girl is probably ready for sex and marriage, as well as for adult duties in the household. Initiation rites on menarche include instruction on sex and marital relations as well as on menstrual management. This is conducted by older women. It is taboo to talk about menstruation with men, or to learn from one's own mother. 
In some portions of South Asia, there is a menstrual taboo, with it frequently being considered impure. Restrictions on movement, behaviour and eating are frequently placed. More than one-third of girls across South Asia do not go to school during menstruation. Some of that is due to lack of safe and comfortable toilets at school (lack of water, sanitation and hygiene in schools in developing countries).
A small study in a rural area in Laos (Savannakhet) found that menstruation is considered taboo and shameful. This makes it difficult for sharing knowledge in schools and in homes. Also, there is a low level of menstrual hygiene management. This has a negative effects on the female's social opportunities in achieving a good health, move around freely and to go to school. Some menstruating women (16%) wear double-layer skirts (sinhs) while in the private sphere, compared to 54% who wear disposable pads.
Hoover-Drickamer effect
A similar phenomenon of menstrual synchrony has been investigated multiple times to determine whether or not it even exists, and if it is adaptive in nature. The same cannot be said of variations in duration of estrus. The rationale behind investigations into menstrual synchrony could also be applied to duration of estrus, including male monopolization of fertile females. While one argument against menstrual synchrony is to reduce interfemale competition, particular differences between species in terms of mating period and length may influence how useful the ability to mediate and change estrus is in the first place. Given that mice can breed year round, it may be adaptive for females to be able to influence each other's frequency of sexual receptivity in order to increase the likelihood of overall species survival.
Leptin
In mice, and to a lesser extent in humans, leptin is required for male and female fertility. Ovulatory cycles in females are linked to energy balance (positive or negative depending on whether a female is losing or gaining weight) and energy flux (how much energy is consumed and expended) much more than energy status (fat levels). When energy balance is highly negative (meaning the woman is starving) or energy flux is very high (meaning the woman is exercising at extreme levels, but still consuming enough calories), the ovarian cycle stops and females stop menstruating. Only if a female has an extremely low body fat percentage does energy status affect menstruation. Leptin levels outside an ideal range may have a negative effect on egg quality and outcome during "in vitro" fertilization. Leptin is involved in reproduction by stimulating gonadotropin-releasing hormone from the hypothalamus.
Human placentophagy
Maternal placentophagy is defined as "a mother’s ingestion of her own placenta postpartum, in any form, at any time". Of the more than 4000 species of placental mammals, most, including herbivores, regularly engage in maternal placentophagy, thought to be an instinct to hide any trace of childbirth from predators in the wild. The exceptions to placentophagy include mainly humans, Pinnipedia, Cetacea, and camels.
Embryonic diapause
Obligate diapause is also known as seasonal delayed implantation and is a mechanism that allows mammals to time the birth of their offspring for favorable environmental conditions. This mechanism occurs as a regular part of the reproductive cycle in mammals such as armadillos, all species of pinniped, many mustelids, all ursids, one species of fruit bat, and the roe deer.
Raja (festival)
According to popular belief as women menstruate, which is a sign of fertility, so also Mother Earth menstruates. So all three days of the festival are considered to be the menstruating period of Mother Earth. During the festival all agricultural operations remain suspended. As in Hindu homes menstruating women remain secluded because of impurity and do not even touch anything and are given full rest, so also the Mother Earth is given full rest for three days for which all agricultural operations are stopped. Significantly, it is a festival of the unmarried girls, the potential mothers. They all observe the restrictions prescribed for a menstruating woman. The very first day, they rise before dawn, do their hair, anoint their bodies with turmeric paste and oil and then take the purificatory bath in a river or tank. Peculiarly, bathing for the rest two days is prohibited. They don't walk bare-foot do not scratch the earth, do not grind, do not tear anything apart, do not cut and do not cook. During all the three consecutive days they are seen in the best of dresses and decorations, eating cakes and rich food at the houses of friends and relatives, spending long cheery hours, moving up and down on improvised swings, rending the village sky with their merry impromptu songs.
Lactational amenorrhea
Amenorrhea itself is not necessarily an indicator of infecundity, as the return of ovarian cycling is a gradual process and full fecundity may occur before or after first postpartum menses. Additionally, spotting or the appearance of first postpartum menses can be a result of either lochia or estrogen withdrawal and not actual ovulation. lactational amenorrhea has evolved as a mechanisms for preserving the health of the mother. This period of infecundity allows the mother to focus her energy on breastfeeding as well as allow time for her body to heal between births. The frequency and durations of the feedings determine how long the mother will continue to be infecund during breastfeeding. However there is variation across different cultures. The Turkana, Gainj, and Quechua societies all breastfeed on demand until their child is around 2 years old. The timing of returned ovulation for these women is however varied. Because of this interbirth intervals vary across these three societies.
Induced ovulation (animals)
Spontaneous ovulation is the ovulatory process in which the maturing ovarian follicles secrete ovarian steroids to generate pulsatile GnRH (the neuropeptide which controls all vertebrate reproductive function) release into the median eminence (the area which connects the hypothalamus to the anterior pituitary gland) to ultimately cause a pre-ovulatory LH surge. Spontaneously ovulating species go through menstrual cycles and are fertile at certain times based on what part of the cycle they are in. Species in which the females are spontaneous ovulators include rats, mice, guinea pigs, sheep, monkeys, and humans.
Menstruation (mammal)
All female placental mammals have a uterine lining that builds up when the animal is fertile, but is dismantled (menstruated) when the animal is infertile. Some anthropologists have questioned the energy cost of rebuilding the endometrium every fertility cycle. However, anthropologist Beverly Strassmann has proposed that the energy savings of not having to "continuously" maintain the uterine lining more than offsets energy cost of having to rebuild the lining in the next fertility cycle, even in species such as humans where much of the lining is lost through bleeding (overt menstruation) rather than reabsorbed (covert menstruation). However, even in humans, much of it is re-absorbed.
Menstrual cycle
Stimulated by gradually increasing amounts of estrogen in the follicular phase, discharges of blood (menses) flow stop, and the lining of the uterus thickens. Follicles in the ovary begin developing under the influence of a complex interplay of hormones, and after several days one or occasionally two become dominant (non-dominant follicles shrink and die). Approximately mid-cycle, 24–36 hours after the luteinizing hormone (LH) surges, the dominant follicle releases an ovocyte, in an event called ovulation. After ovulation, the ovocyte only lives for 24 hours or less without fertilization while the remains of the dominant follicle in the ovary become a corpus luteum; this body has a primary function of producing large amounts of progesterone. Under the influence of progesterone, the uterine lining changes to prepare for potential implantation of an embryo to establish a pregnancy. If implantation does not occur within approximately two weeks, the corpus luteum will involute, causing a sharp drop in levels of both progesterone and estrogen. The hormone drop causes the uterus to shed its lining in a process termed menstruation. Menstruation also occurs in closely related primates (apes and monkeys).
Placentalia
Placentalia ("Placentals") is one of the three extant subdivisions of the class of animals Mammalia; the other two are Monotremata and Marsupialia. The Placentals are partly distinguishable from other mammals in that the fetus is carried in the uterus of its mother to a relatively late stage of development. It is somewhat of a misnomer since marsupials also nourish their fetuses via a placenta.
Female
The sex of most mammals, including humans, is genetically determined by the XY sex-determination system where males have X and Y (as opposed to X and X) sex chromosomes. During reproduction, the male contributes either an X sperm or a Y sperm, while the female always contributes an X egg. A Y sperm and an X egg produce a male, while an X sperm and an X egg produce a female. The ZW sex-determination system, where males have ZZ (as opposed to ZW) sex chromosomes, is found in birds, reptiles and some insects and other organisms. Members of Hymenoptera, such as ants and bees, are determined by haplodiploidy, where most males are haploid and females and some sterile males are diploid.
Menstrual synchrony
This idea was extended to humans in a study by Stern and McClintock. They investigated whether a coupled-oscillator mechanism first reported for Norway rats (see section below on rats) could also exist in humans. The coupled-oscillator hypothesis in humans proposed that human females release and receive pheromones that regulate the length of their menstrual cycles. This was assumed to occur without consciously detecting any odor. The study was conducted by collecting compounds from axillae (underarms) of donor women at prescribed phases during their menstrual cycles (i.e., the follicular phase, ovulatory phase, and luteal phase), and applying the compounds daily under the noses of recipient women. In order to collect the axillary compounds, the donor women wore cotton pads under their arms for at least 8 hours, and then the pads were cut into smaller squares, frozen to preserve the scent, and readied for distribution to the recipients. The recipients were split into two groups, and were exposed to the compounds via application of the thawed axillary pad under their noses daily.