Why only a few animals have periods and the evolutionary reasons behind menstruation? Learn about which species menstruate and the potential benefits of this unique biological process
While humans are well-known for their menstrual cycles, they’re not the only creatures that experience this process. Surprisingly, only a small number of animal species menstruate, and scientists are still unraveling the mystery behind this phenomenon.
The Role of Menstruation in Reproduction
Menstruation is a crucial part of the human reproductive process, but it’s far from universal in the animal kingdom. In fact, out of the vast number of mammal species, only about 85 are known to have menstrual cycles. This tiny group makes up less than 2% of all mammals and includes not only primates—like chimpanzees and bonobos, which are closely related to humans—but also certain bats, elephant shrews, and most recently, spiny mice.
Evolutionary Perspective: Why Menstruation?
The fact that these animals aren’t closely related suggests that menstruation may have evolved independently in different species, indicating some evolutionary advantage. Evolutionary biologist Deena Emera from the Buck Institute for Research on Aging points out that the occurrence of menstruation in such diverse species hints at a significant, yet still not fully understood, benefit.
Beyond Menstruation: Other Forms of Reproductive Bleeding
Other animals, such as dogs, may experience bleeding during estrus, commonly referred to as being “in heat.” However, this bleeding is different from menstruation. In dogs, the increase in estrogen during fertility causes blood vessels in the vagina to dilate, resulting in minor bleeding. This process is distinct from the menstrual cycle, which involves a more complex interaction of hormones, including estrogen and progesterone.
How Hormones Drive Menstruation
In menstruating species, periods are triggered by the interplay of estrogen and progesterone, along with other hormones that prepare the body for potential pregnancy. As estrogen rises, it thickens the uterine lining and promotes the development of new blood vessels. If the egg is not fertilized, progesterone levels drop, leading to the shedding of the uterine lining in the form of menstrual blood.
In non-menstruating mammals, the uterus only undergoes these changes after pregnancy has begun, making the preparatory menstruation process in certain species an intriguing evolutionary adaptation.
The Evolutionary Question: Why Prepare the Uterus Early?
Emera raises an interesting evolutionary question: Why do menstruating animals prepare their uterus for pregnancy even before conception? The answer is still unclear, but it may relate to the reproductive strategies of these species. Notably, menstruating animals tend to give birth to smaller litters and have longer gestation periods. For example, spiny mice, with their prolonged pregnancies and small litters, must ensure the survival of each offspring. This makes early preparation of the uterine lining, with its ability to detect and respond to the viability of an embryo, a potentially life-saving adaptation.
Menstruation and Sperm Storage
Another theory, proposed by retired evolutionary biologist Robert Martin, suggests that menstruation might help with sperm storage. Some species, like bats, can store sperm for extended periods, allowing fertilization to occur long after mating. In humans, sperm can be stored for up to nine days in the reproductive tract.
However, as sperm age, they can degrade, potentially leading to chromosomal abnormalities. Menstruation might help by shedding the uterine lining and expelling old sperm, making way for fresher, healthier sperm.
The Need for Further Research of Animals
Despite various theories, the exact reasons why some animals menstruate remain uncertain. More research is needed to fully understand this complex process, not just in humans but across the animal kingdom. According to Martin, the practical applications of such research could be significant, offering new insights into reproductive health and biology.
