Humans experience monthly menstruation, a visible shedding of the uterine lining. This raises a fundamental question: why do most other animals not have periods? The reproductive processes in the vast majority of other mammalian species are less overtly apparent, leading to misconceptions about their physiological cycles.
Mammalian Reproductive Cycles
Mammals exhibit two primary reproductive cycles that prepare the female body for potential pregnancy: the estrous cycle and the menstrual cycle. Both cycles involve hormones that regulate ovarian activity and prepare the uterus for an embryo. The distinction lies in how the uterine lining, or endometrium, is managed if pregnancy does not occur.
The Menstrual Cycle
The menstrual cycle is characterized by the periodic shedding of the uterine lining (endometrium) if pregnancy does not occur. This process results in visible bleeding. Biologically, the endometrium thickens in preparation for a fertilized egg, developing a rich blood supply and glandular tissue. If an egg is not fertilized or does not implant, hormone levels, particularly progesterone, decline, leading to the breakdown and expulsion of this prepared lining.
This process is observed in a limited group of mammals. Beyond humans, menstruation occurs in most Old World and New World monkeys and apes, including chimpanzees and baboons. It is also found in a few non-primate species, such as certain bats (e.g., Seba’s short-tailed bat, Pallas’s long-tongued bat), the elephant shrew, and the spiny mouse (Acomys cahirinus).
The Estrous Cycle
The estrous cycle is the predominant reproductive cycle among most non-primate mammals, including dogs, cats, cows, horses, and rodents. In this cycle, the uterine lining thickens for pregnancy. If conception does not occur, the endometrium is largely reabsorbed by the animal’s body rather than being shed externally. This reabsorption means there is typically no visible bleeding, which differentiates it from menstruation.
During the estrous cycle, females are sexually receptive only during a specific phase called estrus, often referred to as “heat.” This period of receptivity coincides with ovulation, signaling the optimal time for mating. The duration and frequency of estrous cycles vary widely across species; some animals are polyestrous, cycling multiple times a year, while others are monoestrous, cycling only once annually.
Evolutionary Perspectives on Uterine Management
Several scientific hypotheses explain the evolution of menstruation in some species and its absence in most others. One theory suggests that reabsorbing the uterine lining, as seen in the estrous cycle, might be more energy-efficient for most mammals. Shedding and regenerating the endometrium, as in menstruation, is an energetically costly process.
Another idea is the pathogen defense hypothesis, which proposes that menstruation serves to expel potential pathogens that could enter the reproductive tract during mating. Menstrual blood, in this view, acts as a flush, carrying away microbes and delivering immune cells to the uterine cavity. This mechanism could protect the uterus from infection, particularly in species with frequent mating or high pathogen exposure.
A theory centers on spontaneous decidualization and maternal-fetal conflict. In menstruating species, the uterus spontaneously prepares a specialized lining (decidua) for implantation before an embryo arrives. If no implantation occurs, this lining is shed. This “spontaneous decidualization” gives the mother more control over embryo quality, allowing the maternal system to screen and reject genetically compromised or non-optimally invasive embryos, preventing resource investment into non-viable pregnancies.