All female placental mammals have a uterus lining (“endometrium”) that builds up when the animal is fertile. If the egg is not fertilized, most mammals reabsorb this lining into their uterus (“covert menstruation”), however a few mammals, including humans, shed this lining outside of their bodies (“overt menstruation”).
Covert Menstruation (“Estrous Cycle”): Animals with covert menstruation reabsorb the endometrium into their uteruses each estrous cycle. Animals with an estrous cycle are only fertile during the time in their cycle when they are ovulating. Some animals release an egg with each estrous cycle (e.g. rats, mice, guinea pigs, monkeys, and sheep) . However other animals are “induced ovulators,” requiring sex in order to release an egg each cycle (e.g rabbits, cats, ferrets, minks, camels, llamas).
Overt Menstruation (“Menstrual Cycle”): Animals (such as humans) with covert menstruation shed some of their endometrium out of their body through their vagina each cycle, if their egg has not been fertilized. Animals with a menstrual cycle are fertile throughout most of their cycle (unlike animals with an estrous cycle who are only fertile during ovulation). It is interesting to note that humans actually reabsorb about two-thirds of the uterus lining each cycle (“covert menstruation”). Therefore only one-third of the human uterus lining is “overtly menstruated”, ie. shed externally via menstruation during each menstrual cycle. This is pretty incredible – this means that a woman loses on average 35mls of menstrual fluid each cycle (via overt menstruation) however she actually has an average of 105mls menstrual fluid in her uterine lining before menstruation.
Why do humans (and some other animals) menstruate overtly?
Overt menstruation occurs in certain species because the fetuses of those species require a more developed uterine lining (endometrium), one which is too thick to completely reabsorb. It appears that overt menstruation occurs in species that have a large uterus relative to the adult female body size.
A more developed endometrium is necessary in humans (and some other mammals) because the placenta of those species is more ‘aggressive’. Animal placentas are classified into three groups, based on the number of tissues separating the maternal from the fetal blood. Within the mammals, there is variation in how deeply the placenta is sunk into the uterus. Some species are epithelochorial; the connection is entirely superficial. Others are endotheliochorial, in which the placenta pierces the uterine epithelium tissue. And others, the most invasive, are hemochorial, and actually breach maternal blood vessels (see below for a slideshow of pictures). Humans are hemochorial (as are rabbits, guinea pigs, mice, rats etc). All of the mammalian species that menstruate are also hemochorial. Females build up that thickened uterine lining to protect and insulate themselves from the greedy embryo and its selfish placenta. In species with especially invasive embryos, it’s too late to wait for the moment of implantation — instead, they build up the wall pre-emptively, before and in case of fertilization. Then, if fertilization doesn’t occur, the universal process of responding to declining progesterone levels by sloughing off the lining occurs.
Another process that may go on is that the lining of the uterus could be a sensor for fetal quality, detecting chromosomal abnormalities and allowing them to be spontaneously aborted early. There is some evidence for this: women vary in their degree of decidualization (having a thick endometrium at fertilization), and women with reduced decidualization have been found to become pregnant more often, but also exhibit pregnancy failure more often. So having a prepared uterus not only helps to fend off overly-aggressive fetuses, it allows mom a greater ability to be selective in which fetuses she carries to term.
The Evolution of Overt Menstruation in Placental Mammals
See the Phrenology chart below showing the distribution of menstruation in placental mammals and the inferred states of ancestral lineages. Menstruating species/lineages are colored in pink, non- menstruating species/lineages in black. Species in which the character state is not known are not colored, and lineages of equivocal state are represented with black lines. Note that there is strong evidence for three independent origins of menstruation among placental mammals.