While humans commonly associate “period” with monthly menstrual bleeding, not all mammals experience reproductive cycles in the same way.
Understanding Mammalian Reproductive Cycles
Mammalian reproductive cycles prepare the female body for potential pregnancy. They involve hormones like estrogen and progesterone, which regulate changes in the ovaries and uterus. These hormones mature an egg for ovulation and create a suitable uterine environment for embryo implantation. If pregnancy does not occur, the thickened uterine lining regresses, leading to the next cycle.
The Menstrual Cycle
The menstrual cycle involves the periodic shedding of the uterine lining (endometrium) if pregnancy does not occur, resulting in visible bleeding. While common in humans, menstruation is rare across mammals, occurring in less than 2% of species.
Beyond humans, this cycle is primarily observed in other primates, including apes and Old World and New World monkeys. Outside of primates, a few specific mammals also menstruate: certain bat species, elephant shrews, and the Cairo spiny mouse. For example, the wild fulvous fruit bat has a menstrual cycle lasting about 33 days, with bleeding typically restricted to a single day.
The Estrous Cycle
Most female mammals experience an estrous cycle, a reproductive pattern that differs from the menstrual cycle. If fertilization does not occur, the uterine lining is reabsorbed by the body, conserving nutrients and energy.
During the estrous cycle, females are sexually receptive only during “estrus” or “heat,” a period accompanied by behavioral and physiological changes indicating fertility. Common examples include dogs, cats, cows, horses, pigs, and deer. While some species like dogs may have bloody vaginal discharge during their proestrus phase, this differs from menstruation as it comes from vaginal blood vessels, not the uterine lining.
Evolutionary Reasons for Different Cycles
The existence of both menstrual and estrous cycles in mammals points to different evolutionary strategies for reproduction. One prominent hypothesis for menstruation’s evolution centers on a process called spontaneous decidualization. In menstruating species, the uterine lining prepares itself for implantation in anticipation of a fertilized egg, even before a signal from the embryo.
If the embryo is not viable or implantation fails, this pre-prepared, highly specialized lining is shed. This shedding mechanism is thought to act as a quality control, allowing the mother to “screen” embryos and avoid investing resources in unhealthy pregnancies. It also might protect the uterus from overly invasive embryos.
In contrast, the reabsorption seen in estrous cycles is considered more energetically efficient, as the body conserves resources by not shedding and rebuilding the uterine lining each cycle. The varying reproductive strategies reflect diverse adaptations to ensure successful continuation of species in different ecological niches.