The evolutionary benefit (or not) of the period
Arona Jones
The process of menstruation can be anything from inconvenient to incapacitating. In shedding the lining, the body is also losing valuable energy. This naturally raises questions as to the advantages of menstruation. Furthermore, if it is beneficial, why do so few animals do it?
Humans, and their closest animal relatives (alongside certain species of bats) are the only creatures to shed the menstrual lining, in the process known as the period. Most other mammals have a cyclical build-up and then breakdown of uterine lining, but the lining is reabsorbed into the body instead of being shed - known as ‘covert menstruation’.
First, a brief explanation of the cycle of which menstruation is a key part. To prepare for pregnancy, the lining of the womb thickens. An egg is then released, and if fertilised, can implant in the thickened lining. More often than not, it remains unfertilised and is discarded along with the lining. Menstruation occurs roughly every 28 days (coincidentally, almost the same length as a lunar month) and typically lasts between two to seven days. The whole process is controlled by an (easily upset) balance of hormones, the body’s natural regulatory chemicals. It brings with it various side effects which can have a significant negative impact on daily life. As a result, the reasons for menstruation are still somewhat uncertain, although there are some theories as to its purpose.
It has been suggested that menstruation is simply an unfortunate by-product rather than an intended effect. Beverly Strassmann of the University of Michigan argued that to understand menstruation, we must understand why the reproductive cycle, which occurs in some form throughout the animal kingdom, happens at all. The energy costs of maintaining a sufficient lining inside the womb at all times are high. As such, it is undoubtedly more economical to tear down and later regrow the lining. She then said that the reason the blood is shed rather than reabsorbed like in most other animals is a simple matter of efficiency - the volume of blood that would need to be reabsorbed is too high.
A possible reason for the thickness of the lining, and therefore the prevention of its reabsorption is in reaction to the ‘aggression’ of the embryo. All embryos will ‘burrow’ into the womb lining, seeking nutrients. How actively it does this varies from species to species, and it just so happens that in humans and other primates it is particularly aggressive. Hence the thick womb lining, and therefore the need to discard it.
Menstruation may also be a ‘disposal mechanism’ for failed pregnancies. Human embryos have been found to have a higher than average susceptibility to genetic abnormality, which leads to increased numbers of pregnancies failing in the first few weeks after conception. The energy cost of supporting a pregnancy is incredibly high, so menstruation may simply be a way to ensure a failed pregnancy does not waste more resources than necessary. It could also prevent infection.
Regardless of the purpose of menstruation, it is notable that the process is in nature a rare event. Female mammals and, historically, humans, spend much of their time either pregnant or nursing a baby, leading to the rarity of menstruation, since the cycle only occurs between pregnancies. It is the changing lifestyle of humans, and greatly reduced average number of pregnancies for an individual woman, which has spotlighted what is most likely an inefficient by-product of human reproductive evolution. After all, a process that is only intended to occur infrequently does not need to be greatly efficient.
Humans, and their closest animal relatives (alongside certain species of bats) are the only creatures to shed the menstrual lining, in the process known as the period. Most other mammals have a cyclical build-up and then breakdown of uterine lining, but the lining is reabsorbed into the body instead of being shed - known as ‘covert menstruation’.
First, a brief explanation of the cycle of which menstruation is a key part. To prepare for pregnancy, the lining of the womb thickens. An egg is then released, and if fertilised, can implant in the thickened lining. More often than not, it remains unfertilised and is discarded along with the lining. Menstruation occurs roughly every 28 days (coincidentally, almost the same length as a lunar month) and typically lasts between two to seven days. The whole process is controlled by an (easily upset) balance of hormones, the body’s natural regulatory chemicals. It brings with it various side effects which can have a significant negative impact on daily life. As a result, the reasons for menstruation are still somewhat uncertain, although there are some theories as to its purpose.
It has been suggested that menstruation is simply an unfortunate by-product rather than an intended effect. Beverly Strassmann of the University of Michigan argued that to understand menstruation, we must understand why the reproductive cycle, which occurs in some form throughout the animal kingdom, happens at all. The energy costs of maintaining a sufficient lining inside the womb at all times are high. As such, it is undoubtedly more economical to tear down and later regrow the lining. She then said that the reason the blood is shed rather than reabsorbed like in most other animals is a simple matter of efficiency - the volume of blood that would need to be reabsorbed is too high.
A possible reason for the thickness of the lining, and therefore the prevention of its reabsorption is in reaction to the ‘aggression’ of the embryo. All embryos will ‘burrow’ into the womb lining, seeking nutrients. How actively it does this varies from species to species, and it just so happens that in humans and other primates it is particularly aggressive. Hence the thick womb lining, and therefore the need to discard it.
Menstruation may also be a ‘disposal mechanism’ for failed pregnancies. Human embryos have been found to have a higher than average susceptibility to genetic abnormality, which leads to increased numbers of pregnancies failing in the first few weeks after conception. The energy cost of supporting a pregnancy is incredibly high, so menstruation may simply be a way to ensure a failed pregnancy does not waste more resources than necessary. It could also prevent infection.
Regardless of the purpose of menstruation, it is notable that the process is in nature a rare event. Female mammals and, historically, humans, spend much of their time either pregnant or nursing a baby, leading to the rarity of menstruation, since the cycle only occurs between pregnancies. It is the changing lifestyle of humans, and greatly reduced average number of pregnancies for an individual woman, which has spotlighted what is most likely an inefficient by-product of human reproductive evolution. After all, a process that is only intended to occur infrequently does not need to be greatly efficient.
