The idea that all female mammals experience a “period” is a common misconception. Mammalian reproductive cycles are diverse, reflecting various biological strategies for preparing the uterus for potential pregnancy. While some mammals menstruate, many others follow different patterns.
Understanding Mammalian Reproductive Cycles
Female mammals primarily utilize two distinct reproductive strategies: the menstrual cycle and the estrous cycle. Both cycles involve hormonal fluctuations that prepare the female reproductive system for the possibility of conception and pregnancy. The fundamental difference lies in what happens to the uterine lining, or endometrium, if pregnancy does not occur.
These cycles are controlled by a complex interplay of hormones that regulate ovarian activity and uterine changes. The mechanisms by which they achieve and reset this state vary significantly between species.
Mammals That Experience Menstruation
Menstruation involves the cyclical shedding of the uterine lining (endometrium) through vaginal bleeding when pregnancy does not occur. This is a relatively rare phenomenon among mammals. Humans are the most well-known menstruating species. Other primates, including apes (like chimpanzees and gorillas) and Old World and New World monkeys, also menstruate.
However, menstruation is not universal even among primates; some, like lemurs, do not menstruate, while tarsiers may experience only a weakly present cycle. Outside of primates, a few other mammalian groups also exhibit menstruation, including certain bat species, the elephant shrew, and the spiny mouse (Acomys cahirinus). Its occurrence in these distantly related groups suggests independent evolution multiple times.
The Estrous Cycle: The More Common Pattern
The estrous cycle represents the reproductive pattern found in the vast majority of female mammals. In this cycle, if pregnancy does not occur, the uterine lining is reabsorbed by the body rather than shed. This process is sometimes referred to as “covert menstruation” because there is no external bleeding.
A characteristic of the estrous cycle is the period of “estrus,” commonly known as “heat,” when the female is receptive to mating. During estrus, females display behavioral and physiological cues to signal their fertility. Animals like dogs, cats, cows, pigs, horses, and rodents all follow an estrous cycle, with variations in cycle length and frequency depending on the species. Some animals are “polyestrous,” cycling multiple times a year, while others are “monoestrous,” having only one cycle annually.
Why the Difference? Evolutionary Perspectives
The evolutionary reasons behind the divergence of menstrual and estrous cycles are a subject of ongoing scientific inquiry. One theory, the “energy conservation hypothesis,” suggests that reabsorbing the uterine lining, as seen in estrous cycles, is less energetically costly than shedding and rebuilding it, which occurs during menstruation. This hypothesis proposes that shedding the endometrium is metabolically demanding, and reabsorption conserves resources, making it a more efficient strategy for most mammals.
Conversely, other theories propose advantages for menstruation. The “uterine defense hypothesis” suggests that shedding the uterine lining helps remove potential pathogens introduced during mating, protecting the uterus from infection. This periodic cleansing could be beneficial in species where mating is frequent or promiscuous. Another perspective, the “maternal-fetal conflict hypothesis,” posits that menstruation allows the mother to “screen” embryos. By shedding the lining if an embryo is not viable or fails to implant properly, the mother avoids investing resources in a potentially unsuccessful pregnancy.