You may be surprised to hear that humans are members of a very select club of animals that have periods. So, what do humans have in common with our fellow menstruating species? Consider this your station for menstruation education.
Do Dogs Get Periods?
First, some busting of menstrual myths: dogs don’t actually menstruate. However, they can appear to, as female dogs may have bloody discharge at certain points of their estrus cycle when they are in heat. However, this bloody discharge that originates from the vagina is not the same as a period.
Do Cats Get Periods?
Also no! Cats have estrus cycles rather than menstrual cycles. Cats come into heat and become fertile during certain months of the year, with multiple estrus cycles per season. "Female cats may seem to be in heat almost constantly from late winter to early fall," veterinarian Dr Renee Rucinsky explained to Daily Paws. "There's also no bleeding when a cat is having hormonal surges."
Cats also ovulate in response to mating. In fact, it's possible for all kittens from one litter to have different fathers, a phenomenon called superfecundation.
Why Does Menstruation Happen?
The menstrual cycle is controlled by fluctuating levels of hormones including progesterone and estrogen. The body builds a nice landing spot in the uterus for an implanting embryo, then expels it if that embryo never comes. This layer of tissue is called the endometrium.
So why do some species live in a constant cycle of building up and tearing down, using up energy and resources, until they get pregnant? Many theories have been proposed, but this is what we know so far.
A variety of traits are commonly found in menstruating mammals. One is a hemochorial placenta. Among the different types of placenta, hemochorial ones are the most invasive: all layers of the endometrium are burrowed through. For example, in human pregnancies, this means that the outer layer of the amniotic sac has a direct connection to the bloodstream.
“In some mammals, like humans, the placenta is really invasive, so it invades all the way through the wall of the uterus, into the maternal tissue. In other mammals, the placenta just touches the wall of the uterus. And then there’s everything in between,” Vincent J. Lynch, associate professor of biological sciences at the University at Buffalo College of Arts and Sciences, explained.
The determined little embryos nourish themselves at the expense of their hosts – and this calls for a little evolutionary self-defense. This is where endometrial decidualization comes in: changing the endometrium to prep the uterus for an oncoming embryo.
Decidualization involves cells called endometrial stromal fibroblasts transforming into decidualized stromal cells. This involves their nucleus becoming larger and more rounded, nucleoli increasing in number, and the rough endoplasmic reticulum and Golgi apparatus expanding. The endometrium becomes nice and thick, ready for a hypothetical embryo to dig into.
A paper published this year, of which Dr Lynch is the lead author, posits that “rather than acting as a passive substrate into which the trophoblast [layer of cells that helps embryos attach to the uterus wall] invades, the endometrium directly controls trophoblast invasion […] For example, the trophoblast of mammals with hemochorial placentas, such as humans and rodents, is only permissive to invasion when the ‘window of implantation’ is opened by the endometrium.”
A drop in the level of progesterone released from the corpus luteum (the remnants of the ovarian follicle from which the egg is released) triggers menstruation, with the endometrium sloughing off and being ejected. Some researchers cite menstruation as simply an accidental side-effect of decidualization.
Another hypothesis on the origins of menstruation is the allowing selectivity of which embryos are kept and which are discarded, making sure resources are not invested in a pregnancy that is impaired. Cells in the endometrium can sense abnormalities in an embryo, and decidualized cells have been described in one paper as “biosensors that fine-tune the maternal response to individual embryos.” The same paper explains that, according to estimates, 30 to 60 percent of human embryos are discarded with the endometrium before pregnancy is noticed.
Which Animals Get Periods?
The humble Homo sapiens are primates, as are most of the other species that menstruate – including apes and both Old and New World monkeys.
Orangutans have a menstrual cycle between 29 and 32 days with three to four days of menstruation, Orangutan Foundation International states. According to Project Chimps, a sanctuary for former research chimpanzees, chimp menstrual cycles vary between 28 and 45 days depending on the individual. Gorillas have a menstrual cycle of around 30 days, primatologist Dr Susan Margulis told Slate. Studies on captive bonobos estimate they have a menstrual cycle of 34 to 35 days.
Rhesus macaques and baboons are among the monkey species that menstruate. In fact, one 2008 study suggested that baboons start menstruating earlier when their fathers spend more time with them. "For young females, because their major opponents in life are adult females and fellow juveniles, the presence of any adult male may be helpful," associate professor of biology at Duke University, Susan Alberts, the paper’s senior author, explained in a statement. “Ties between fatherly presence and early maturity may still stem from enhanced nutrition if fathers reduce any harassment their offspring experience while gathering food. It may also help reduce the stress of everyday life in a baboon group."
Some primates also display sexual swellings around ovulation (when the egg is released from the ovary). As Reproductive Biology Coordinator at Chester Zoo Dr Ronnie Cowl explains to IFLScience, rising estrogen levels prior to ovulation increase swelling size, with some species gaining up to a third of their body weight in water weight.
Bats and rodents
Menstruation has also been observed in bats and rodents. This makes up around 1.5 percent of mammalian species, with less than 0.09 percent of menstruating mammals not being primates.
“There are at least three species of bats in which menstruation has been observed,” a paper from 2020, published in the journal Biology of Reproduction explains. “The black mastiff bat Molossus ater and wild fulvous fruit bat Rousettus leschenaultii are the ones presenting the most human-like characteristics.”
“Menstruation is found in few of the world's 4,000 species of mammals which is why this discovery is so remarkable,” Professor Paul Racey, co-author of a 2007 paper describing menstruation in the wild fulvous fruit bat, said in a statement at the time. "The finding is significant because it extends our knowledge of the reproductive biology of what is a large group of mammals.”
Menstruation has been observed in elephant shrews, a family of rodents from Africa. Another rodent species recently discovered to menstruate is the Cairo spiny mouse (Acomys cahirinus), native to North Africa. A 2017 paper described the first evidence of this, with the authors explaining that “Observations of blood at the vaginal opening of nonpregnant female spiny mice in our breeding colony led to an investigation of the changes of endometrial structure and physiology during the reproductive cycle.”
Do Animals Get PMS?
Some species have been observed to show behavioral changes at certain points in their cycle. In 1985, observations of baboons in Amboseli National Park, Kenya, revealed that females exhibited different behavior shortly before the onset of their period.
''They spend more time in trees – and in the open savannah there aren't many trees. They don't have neighbors up there,'' Dr Glenn Hausfater of the University of Missouri explained to the New York Times at the time, adding that “the males don't behave differently toward the females; the females are just less responsive to their neighbors.''
''Why were we surprised? I guess because nobody had looked for this before.''
The premenstrual primates spent over twice their usual time in the treetops, and 15 percent more time feeding than usual.
The spiny mouse may also change its behavior around its period. “There’s already been a little bit of preliminary research using the spiny mouse to try and work out whether they suffer from PMS, and it seems like they do,” Dr Natalie Cooper, senior researcher at the Natural History Museum, explained in a live talk in 2021. “They find that they’re a little bit sad when they have their periods. They tend to vocalize more, they don’t move around so much, and at different points of their cycle they eat different amounts and things like that.”
“This is something we need to do more research on, but one of the really great things about the spiny mouse being discovered to menstruate is that we can start doing more research, and hopefully that will help people who do suffer from PMS quite badly.”
When asked how the rodents’ menstruation was missed, co-author of the paper announcing the discovery, Dr Hayley Dickinson, echoed Dr Hausfater’s comments from 1985: “The answer, as with many discoveries in science, is that no one really looked,” she told Nature.