Why do humans endure the biological cost of a menstrual cycle?
In short: Menstruation is a biological rarity. Humans are among a select few mammal species that menstruate, and we do so more frequently than any other animal. Despite the physical inconvenience and the apparent waste of nutrients, this process remains a fundamental part of our evolutionary history.
The menstrual cycle is far from a universal mammalian trait. While many species undergo hormonal shifts, humans belong to a very small group of mammals that experience menstruation. Interestingly, we are not just part of this group; we are the most frequent menstruators on Earth, performing this cycle more often than any other known animal species. This rarity makes the cycle a subject of intense evolutionary study, as researchers look for the biological advantage behind such a unique trait.
From a purely metabolic standpoint, the process appears counterintuitive. Menstruation involves the shedding of uterine lining, which results in a loss of nutrients. This biological cost, paired with the physical discomfort often associated with the cycle, raises significant evolutionary questions about why such a seemingly inefficient mechanism has been preserved through natural selection. Beyond the loss of nutrients, the cycle can bring about premenstrual syndrome, or PMS. This condition involves various physical and emotional shifts, such as bloating, headaches, or mood swings, which typically subside once the period begins.
The cycle itself is characterized by significant physiological variability. While a 28-day cycle is often cited as the standard, many women experience cycles ranging from 21 to 35 days. The physical manifestation of the period—the bleeding from the vagina—typically lasts between 2 and 7 days, with the heaviest flow usually occurring in the first 48 hours. During this time, a person may lose between 20ml and 90ml of blood, which is roughly equivalent to one to five tablespoons. The color of the blood can also change, appearing pink or brown on lighter days. For most, this cycle begins around age 12, though regular patterns are typically established by the time a person reaches 16 to 18 years old.
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Sources
- Summary by Intellect · source: Why do women have periods?
- Also: www.nhs.uk — Periods
- Also: Why do women have periods?
- Also: Why do women have periods?
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