The menstrual cycle is a fundamental aspect of human reproduction, yet it’s a phenomenon that most animals don’t experience. This raises the question: which species besides humans have periods, and what is the evolutionary purpose behind this periodic bleeding? According to Deena Emera, an evolutionary biologist at the Buck Institute for Research on Aging, scientists have identified approximately 15 mammal species that exhibit a menstrual cycle. The majority of these are primates, including our closest living relatives, chimpanzees (Pan troglodytes) and bonobos (Pan paniscus). Scientists have also observed menstrual cycles in a few bat species, elephant shrews, and most recently, spiny mice (Acomys cahirinus).
Since these animals are not closely related, the trait likely evolved convergently, implying a significant evolutionary advantage. Beyond these creatures, other animals periodically bleed through their reproductive organs. Owners of unspayed dogs often encounter blood stains on their furniture, realizing their pet is in heat, also known as estrus. However, the bleeding in dogs originates from a different source than in menstruating animals. In animals experiencing estrus, a surge in estrogen during their fertile period causes the blood vessels within the vagina to dilate. This dilation leads to a small amount of blood leaking out of the vessels and being expelled.
Menstruating animals, on the other hand, experience periods due to the combined influence of estrogen and progesterone, another hormone. Additional hormones are involved in the maturation and release of an egg in the lead-up to menstruation. Progesterone plays a crucial role in maintaining pregnancy, and in menstruating animals, its levels start to rise even before pregnancy occurs. This rise in progesterone is preceded by an increase in estrogen, which causes the uterine lining to thicken and new blood vessels to develop.
Once an egg is released, progesterone levels climb as estrogen levels decline. If pregnancy does not occur, the female’s progesterone levels drop, and the newly formed blood vessels and other tissues are shed as menstrual blood and tissue fragments. In non-menstruating mammals, progesterone levels only begin to increase after the female becomes pregnant, Emera explains. From an evolutionary perspective, this difference is intriguing.
The real question is not ‘Why do we menstruate?’ but rather ‘Why do we prepare our uterus for pregnancy before we are even pregnant?’ The exact answer remains elusive, but Emera theorizes that it could be linked to the fact that menstruating animals typically give birth to small litters. Humans, primates, bats, and elephant shrews usually have only one offspring at a time, while spiny mice have one to four pups – far fewer than most mouse species.
Menstruating animals also have longer gestation periods than their non-menstruating counterparts. For example, spiny mice have a gestation period almost twice as long as other mice. Since these animals dedicate significant time and energy to a limited number of offspring, their survival is critical. Researchers have discovered that the transformed uterine lining can detect chemical cues released by the embryo, indicating its likelihood of successful implantation. These chemical signals reflect aspects of the embryo’s viability. This quality-assurance step occurs in all mammals, but in menstruating animals that pre-build their lining, it happens much earlier.
Emera explains that when a female invests heavily in her offspring, it’s logical for systems to evolve that screen early for embryos that are less likely to survive. Robert Martin, a retired evolutionary biologist and academic guest at the University of Zurich, proposes that menstruation might also play a role in sperm storage. For instance, bats can store sperm in their reproductive tract for up to 200 days before fertilization, and humans have been documented to store sperm for up to nine days. However, prolonged sperm storage can lead to degradation, potentially causing chromosomal issues if they fertilize an egg.
Martin hypothesizes that the shedding of the uterine lining allows animals to eliminate old sperm, making way for newer, healthier sperm. While there are other theories about the reasons behind menstruation, no concrete evidence supports any one theory over the others. Martin emphasizes the need for more research on menstruation, both in humans and other animals. He believes that this area holds significant potential for practical applications.
