Menstruation (mammal)
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Menstruation is the shedding of the uterine lining ( endometrium). It occurs on a regular basis in uninseminated sexually reproductive-age
female Female ( symbol: ♀) is the sex of an organism that produces the large non-motile ova (egg cells), the type of gamete (sex cell) that fuses with the male gamete during sexual reproduction. A female has larger gametes than a male. Females ...
s of certain mammal species. Although there is some disagreement in definitions between sources, menstruation is generally considered to be limited to
primate Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians ( monkeys and apes, the latter including ...
s. Overt menstruation (where there is bleeding from the
uterus The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The ...
through the
vagina In mammals, the vagina is the elastic, muscular part of the female genital tract. In humans, it extends from the vestibule to the cervix. The outer vaginal opening is normally partly covered by a thin layer of mucosal tissue called the hymen ...
) is found primarily in humans and close relatives such as
chimpanzees The chimpanzee (''Pan troglodytes''), also known as simply the chimp, is a species of great ape native to the forest and savannah of tropical Africa. It has four confirmed subspecies and a fifth proposed subspecies. When its close relative th ...
. It is common in simians (
Old World monkey Old World monkey is the common English name for a family of primates known taxonomically as the Cercopithecidae (). Twenty-four genera and 138 species are recognized, making it the largest primate family. Old World monkey genera include baboons ...
s,
New World monkey New World monkeys are the five families of primates that are found in the tropical regions of Mexico, Central and South America: Callitrichidae, Cebidae, Aotidae, Pitheciidae, and Atelidae. The five families are ranked together as the Ceboidea ...
s, and
ape Apes (collectively Hominoidea ) are a clade of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and as well as Europe in prehistory), which together with its sister g ...
s), but completely lacking in
strepsirrhine Strepsirrhini or Strepsirhini (; ) is a suborder of primates that includes the lemuriform primates, which consist of the lemurs of Madagascar, galagos ("bushbabies") and pottos from Africa, and the lorises from India and southeast Asia. Colle ...
primates and possibly weakly present in tarsiers. Beyond primates, it is known only in
bat Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most ...
s, the
elephant shrew Elephant shrews, also called jumping shrews or sengis, are small insectivorous mammals native to Africa, belonging to the family Macroscelididae, in the order Macroscelidea. Their traditional common English name "elephant shrew" comes from a perc ...
, and the
spiny mouse The term spiny mouse refers to any species of rodent within the genus ''Acomys''. Similar in appearance to mice of the genus '' Mus'', spiny mice are small mammals with bare, scaled tails. However, their coats are endowed with unusually stiff gu ...
species '' Acomys cahirinus''. Females of other species of
placental mammal Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguishe ...
undergo
estrous cycle The estrous cycle (, originally ) is the set of 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 anestro ...
s, in which the endometrium is completely reabsorbed by the animal (covert menstruation) at the end of its
reproductive cycle In biology, a biological life cycle (or just life cycle or lifecycle when the biological context is clear) is a series of changes in form that an organism undergoes, returning to the starting state. "The concept is closely related to those of the ...
. 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 The estrous cycle (, originally ) is the set of 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 anestro ...
period, which indicates that the animal is ready for
insemination Insemination is the introduction of sperm into a female’s reproductive system for the purpose of impregnating, also called fertilizing, the female for sexual reproduction. The sperm is introduced into the uterus of a mammal or the oviduct of a ...
.


Estrus and menstruation

Females of most mammal species advertise fertility to males with visual behavioral cues,
pheromones A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...
, or both. This period of advertised fertility is known as ''oestrus'', "estrus" or ''heat''. In species that experience estrus, females are generally only receptive to copulation while they are in heat (
dolphin A dolphin is an aquatic mammal within the infraorder Cetacea. Dolphin species belong to the families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the New World river dolphins), Pontoporiidae (the ...
s are an exception). In the
estrous cycle The estrous cycle (, originally ) is the set of 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 anestro ...
s of most
placental mammals Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguishe ...
, if no fertilization takes place, the uterus reabsorbs the endometrium. This breakdown of the endometrium without
vaginal discharge Vaginal discharge is a mixture of liquid, cells, and bacteria that lubricate and protect the vagina. This mixture is constantly produced by the cells of the vagina and cervix, and it exits the body through the vaginal opening. The composition, amou ...
is sometimes called ''covert menstruation''. Overt menstruation (where there is blood flow from the vagina) occurs primarily in humans and close evolutionary relatives such as chimpanzees. Some species, such as domestic
dog The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the wolf. Also called the domestic dog, it is derived from the extinct Pleistocene wolf, and the modern wolf is the dog's nearest living relative. Do ...
s, experience small amounts of
vaginal bleeding Vaginal bleeding is any expulsion of blood from the vagina. This bleeding may originate from the uterus, vaginal wall, or cervix. Generally, it is either part of a normal menstrual cycle or is caused by hormonal or other problems of the reproductiv ...
while approaching heat; this discharge has a different physiologic cause than menstruation.


Concealed ovulation

A few mammals do not experience obvious, visible signs of fertility ( concealed ovulation). In humans, while women can learn to recognize their own level of fertility (
fertility awareness Fertility awareness (FA) refers to a set of practices used to determine the fertile and infertile phases of a woman's menstrual cycle. Fertility awareness methods may be used to avoid pregnancy, to achieve pregnancy, or as a way to monitor gyneco ...
), whether men can detect fertility in women is debated; recent studies have given conflicting results.
Orangutan Orangutans are great apes native to the rainforests of Indonesia and Malaysia. They are now found only in parts of Borneo and Sumatra, but during the Pleistocene they ranged throughout Southeast Asia and South China. Classified in the genu ...
s 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.


Evolution

Most
female mammals Female (symbol: ♀) is the sex of an organism that produces the large non-motile ova (egg cells), the type of gamete (sex cell) that fuses with the male gamete during sexual reproduction. A female has larger gametes than a male. Females ...
have an
estrous cycle The estrous cycle (, originally ) is the set of 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 anestro ...
, yet only ten primate species, four
bat Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most ...
species, the
elephant shrew Elephant shrews, also called jumping shrews or sengis, are small insectivorous mammals native to Africa, belonging to the family Macroscelididae, in the order Macroscelidea. Their traditional common English name "elephant shrew" comes from a perc ...
, and one known species of
spiny mouse The term spiny mouse refers to any species of rodent within the genus ''Acomys''. Similar in appearance to mice of the genus '' Mus'', spiny mice are small mammals with bare, scaled tails. However, their coats are endowed with unusually stiff gu ...
have a menstrual cycle. As these groups are not closely related, it is likely that four distinct evolutionary events have caused menstruation to arise. There are varying views on evolution of overt menstruation in humans and related species, and the evolutionary advantages in losing blood associated with dismantling the uterine lining rather than absorbing it, as most mammals do. The reason is likely related to differences in the ovulation process. Most female
placental mammals Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguishe ...
have a uterine lining that builds up when the animal begins ovulation, and later further increases in thickness and blood flow after a fertilized egg has successfully implanted. This final process of thickening is known as
decidualization 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), ...
, and is usually triggered by hormones released by the embryo. In humans, decidualization happens spontaneously at the beginning of each menstrual cycle, triggered by hormonal signals from the ovaries. For this reason, the human uterine lining becomes fully thickened during each cycle as a defense to trophoblast penetration of the endometrial wall, regardless of whether an egg becomes fertilized or successfully implants in the uterus. This produces more unneeded material per cycle than in non-menstruating mammals, which may explain why the extra material is not simply reabsorbed as done by those species. In essence, menstruating animals treat every estrous cycle as a possible pregnancy by thickening the protective layer around the endometrial wall, while non-menstruating placental mammals do not begin the pregnancy process until a fertilized egg has implanted in the uterine wall. For this reason, it is speculated that menstruation is not a trait which provides any evolutionary advantage. Instead it is a side effect of spontaneous decidualization, which evolved in some placental mammals due to its advantages over non-spontaneous decidualization. Spontaneous decidualization allows for more maternal control in the maternal-fetal conflict by increasing selectivity over the implanted embryo. This may be necessary in humans and other primates, due to the abnormally large number of genetic disorders in these species. Since most
aneuploidy Aneuploidy is the presence of an abnormal number of chromosomes in a cell, for example a human cell having 45 or 47 chromosomes instead of the usual 46. It does not include a difference of one or more complete sets of chromosomes. A cell with any ...
events result in stillbirth or miscarriage, there is an evolutionary advantage to ending the pregnancy early, rather than nurturing a fetus that will later miscarry. There is evidence to show that some abnormalities in the developing embryo can be detected by cells in the uterus. This triggers epigenetic changes that prevent formation of the placenta, which prevents the embryo from implanting and leaves it to be removed in the next menstruation. This failsafe mode is not possible in species where decidualization is controlled by hormonal triggers from the embryo. This is sometimes referred to as the choosy uterus theory, and it is theorized that this positive outweighs the negative impacts of menstruation in species with high aneuploidy rates and hence a high number of 'doomed' embryos.


Animal estrous cycles

The female will ovulate spontaneously and be receptive to the male to be bred (express estrus) at regular biologically defined intervals. The female is receptive to males only while experiencing estrus. For breeding livestock, there are a number of advantages to be gained by finding methods to induce ovulation on a planned schedule, and thus synchronize the estrus cycle between many female animals. If animals can be bred on the same schedule, it increases convenience for the livestock owner, since the young animals will be at the same stage of development. Also, if
artificial insemination Artificial insemination is the deliberate introduction of sperm into a female's cervix or uterine cavity for the purpose of achieving a pregnancy through in vivo fertilization by means other than sexual intercourse. It is a fertility treatment ...
(AI) is used for breeding, the AI technician's time can be used more efficiently, by breeding several females at the same time. In order to induce estrus, a variety of techniques have been tried in recent years, involving both more natural, and more hormonal based methods.Synchronized mating and lambing in spring-bred Merino sheep: the use of progestogen-impregnated intravaginal sponges and teaser rams
/big> () () G. L. HUNTER, P. C. BELONJE and C. H. VAN NIEKERK, Department of Agricultural Technical Services, Stellenbosch, Agroanimalia 3,133-140 (1971)
Different ways of injecting or feeding hormones to livestock are costly, and have variable success rates. Average length (days) of estrus and estrous cycles:


See also

* Endometrium *
Estrous cycle The estrous cycle (, originally ) is the set of 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 anestro ...
* Breeding pair * Reproduction * Animal husbandry


References


External links

* * * {{cite web , first1=Jerry , last1=May , first2=Ron , last2=Bates , url=http://www.thepigsite.com/articles/2090/managing-the-sow-and-gilt-estrous-cycle , title=Managing the Sow and Gilt Estrous Cycle Animal breeding Animal physiology Zoology Menstrual cycle Mammal female reproductive system