In humans, a “period” is the monthly shedding of the uterine lining with visible bleeding. It’s a misconception that all female animals experience this. Most animals have reproductive cycles but lack overt menstrual bleeding, showcasing diverse reproductive strategies.
Menstruation Versus Estrous Cycles
Mammalian reproductive cycles differ between menstruation and the estrous cycle. Menstruation, seen in humans, involves shedding the uterine lining (endometrium) with visible bleeding if pregnancy doesn’t occur. This prepares the uterus for potential pregnancy; if implantation fails, tissue is expelled. Most mammals have an estrous cycle where the uterine lining is reabsorbed if fertilization doesn’t happen, meaning no significant external bleeding.
Estrous cycles include “estrus” or “heat,” when the female is receptive to mating. This phase often involves behavioral changes and physical signs. Unlike menstruating species, which are receptive throughout their cycle, estrous animals only mate during “heat.”
Animals That Experience Menstruation
True menstruation, the periodic shedding of the uterine lining with visible bleeding, is rare among mammals. Humans are the most known example, but it’s also seen in most higher primates: chimpanzees, gorillas, baboons, and macaques. Cycles typically range from 21 to 37 days, with bleeding lasting a few days.
Beyond primates, menstruation occurs in a few other groups. Certain bat species (black mastiff, wild fulvous fruit) menstruate. The elephant shrew and spiny mouse (Acomys cahirinus) also do. These are independent evolutionary occurrences, highlighting the adaptation’s uniqueness.
The Widespread Estrous Cycle
The estrous cycle is the predominant reproductive pattern in most mammals. It involves hormonal and physiological changes preparing the female for pregnancy; the uterine lining is reabsorbed if conception doesn’t occur. The cycle has distinct phases: proestrus, estrus, metestrus, and diestrus, with an anestrus phase in some species.
In proestrus, follicles develop and estrogen rises, preparing the reproductive tract. This leads to estrus, or “heat,” the period of peak estrogen, ovulation, and sexual receptivity, often with behavioral or physical signs. If pregnancy occurs, the cycle stops. Otherwise, it progresses to metestrus, where the ruptured follicle forms a corpus luteum, and then diestrus, dominated by progesterone.
During diestrus, the corpus luteum produces progesterone, preparing the uterus for implantation. If pregnancy doesn’t occur, the corpus luteum regresses, progesterone declines, and the uterine lining is reabsorbed, preventing external bleeding. Common animals like dogs, cats, cows, horses, and rodents have estrous cycles. Frequencies vary: polyestrous (multiple cycles per year), seasonally polyestrous (multiple cycles during a season), or monoestrous (one cycle per year).
Evolutionary Perspectives on Reproductive Cycles
The evolution of menstruation versus estrous cycles is an ongoing scientific inquiry. One hypothesis suggests menstruation acts as a protective mechanism. Shedding the uterine lining may remove potential pathogens from sperm or screen for embryo quality, expelling non-viable ones. This “quality control” may benefit species investing heavily in fewer offspring, like humans and other primates.
Conversely, the estrous cycle’s widespread prevalence indicates its evolutionary advantages. Reabsorbing the uterine lining is energetically efficient, conserving resources lost through bleeding and rebuilding. The distinct “heat” period also limits female mating vulnerability to fertile times, saving energy and avoiding unnecessary male interactions. Menstruation’s independent evolution in unrelated mammalian lineages suggests it offers unique benefits under specific evolutionary pressures.