Females of most mammal species advertise fertility to males with visual behavioral cues,
pheromones, or both. This period of advertised fertility is known as
oestrus, "estrus" or
heat. In the
estrous cycles of most
placentals, if no fertilization takes place, the uterus reabsorbs the endometrium. This breakdown of the endometrium without
vaginal discharge 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. 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, studies show that both males and females can detect the fertility of females through hormonal signaling and alterations in scent (
fertility awareness), but some research suggests that behavioral clues may be needed to consciously assess fertility.
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.
Evolution Most
female mammals have an
estrous cycle, yet only some primate and
bat species, the
elephant shrew, and one known species of
spiny mouse 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. 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 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 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. Since most
aneuploidy 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 endometrial stromal cells in the uterus, but only upon differentiation into
decidual cells. 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. s; blue,
Chiroptera; orange,
Afrotheria; yellow,
Rodentia. ==Animal estrous cycles==