In fact, many mammalian species menstruate, but only humans, chimpanzees, bats, and elephant shrews have a type of period called “overt bleeding”. The other animals do not have menstrual bleeding out of the vaginal opening, so the blood is absorbed by the wall inside the uterus.
What is menstruation?
Menstruation is the shedding of the endometrium layer of the uterus by tearing. This process, which repeats itself in certain periods in females who have reached reproductive age, is a case experienced in many mammals. The old wall is thrown out with bleeding and discharge. Then, the uterine wall is renewed and healthy. Also, a new structure can be built for pregnancy. Only a few animals have menstruation which occurs as the uterine wall ruptures and blood/fragments of the wall are expelled from the vaginal opening called”open menstruation”. Many mammals have menstruation as blood and parts of the uterine wall are absorbed by the uterine tissues, so no signs of bleeding are reflected outside called a “hidden period” or “closed period”.
Artwork showing women’s menstrual problems by Rupi Kaur. Her works have been blocked by Instagram twice. Instagram was withdrawn after it was appealed by Canadian university student Kaur who is the person in the photo. However, the images were blocked again later. The decision resonated all over the world. Ultimately, the ban was lifted completely. The artists opposed the obstacle by saying that they thought there was nothing to be ashamed of of menstruation and that it should be normalized.
Menstruation in Animals (Except Humans)
Only humans and chimpanzees experience bleeding as a result of tearing in the vaginal opening. Sometimes overt menstruation can be found in orangutans and gorillas. This is one of the millions of examples that confirm the close kinship between other apes and humans. An increasing amount of secret menstruation is found when looking at distant ape relatives of humans (with the exception of the rhesus monkeys).
Strepsirhini monkeys do not have periods at all, but their closest relative, the Haplorhin monkeys, menstruate (humans and all their close relatives are also monkeys from this group). The early version of menstruation occurs in tarsiers, a transitional group between Strepsirhin and Haplorhini apes as evolution predicted. In other words, we can clearly see the traces of evolution even in a subject such as menstruation. Latent menstruation occurs in non-primate mammals such as cats, dogs, horses, elephants, rats, pigs, etc.
The menstrual cycles of these living species are different. While human females menstruate every month, dogs menstruate once every 6 months, and cats are every month. Horses have menstruation once every 3 weeks and rats have once every 5 days. Although dogs sometimes bleed outward, this is not an overt menstrual because the reason for the bleeding is not the rupture of the uterine wall. However, the cracking of the capillaries in the vaginal wall of the dog, which is in the rutting period, causes bleeding.
Egg-laying animals’ menstruation completely disappeared, but most reptiles, most amphibians, most fish, and none of the birds menstruate. Since egg-laying animals do not need a uterus that has not evolved.