Women live longer than men, and we know that longevity is promoted by natural selection to pass the maximum number of copies of their genes to the next generations. Yet women cease reproductive life earlier than men because of the menopause. Why this apparent contradiction? How is it that evolution has caused women to terminate their fertility prematurely, when they still have many years of life ahead of them?
Humans are special in this respect: menopause is a very rare phenomenon in the animal kingdom. For the time being it is only known for certain to exist in humans and two species of cetaceans, the killer whale (Orcinus orca) and the short-finned pilot whale (Globicephala macrorhynchus).
Killer whales are also grandmothers
In almost all species, lifespan is usually related to reproductive lifespan. However, in species with menopause, the two are decoupled, because females live for much longer than they reproduce. Female killer whales, for example, can live up to 90 years, but stop reproducing at around 40.
What do orcas and humans have in common that might explain the evolutionary origin of menopause? First, both species live long, social lives. In the evolution of both species, social groups have been formed by families around females and their offspring, with females often being related to each other. This way, they can help care for others’ offspring at any given time.
In addition, the young of both species need a long period of maternal care before they become independent and attempt to reproduce on their own. If, for example, an offspring needs about 20 years of dependence on its mother before it can successfully fend for itself, then having a newborn when you have less than those years of life expectancy left is not very positive from the point of view of natural selection.
This would lead to selection favouring females that stop reproducing and devote that post-reproductive life span to continuing to care for existing offspring, both their own and those of others to whom they are related, especially their granddaughters and grandsons.
And, in these matriarchal groups, the reproductive attempts of older females would come into competition with the reproduction of younger females (including their own daughters). This intergenerational conflict would encourage older females to help other females’ offspring rather than produce their own. Studies seem to support these ideas in both killer whales and pre-industrial humans.
It seems, therefore, that selection during our evolution as a species has promoted the earlier cessation of offspring production in females and, at the same time, their dedication to collaborate in the care of others who also carry copies of their genetic material.
At this point, one detail is still surprising: why is the cessation of reproduction so traumatic for many women? Menopause in itself does not mean senescence. However, the hormonal disruptions that accompany it can have negative effects on women’s physiology. Why does the ovulation period end with so much disruption?
Researchers working on this subject have advanced a very intriguing hypothesis: intragenomic conflict.
All of us have two versions of most of our genes, one inherited from our father and one from our mother. The probability for each of these versions to have an equal copy in offspring born into the social group, including their grandchildren, varies. For example, in a group of matriarchal origin, males may join from other groups and therefore won’t be related to most group members. Thus, mothers’ genes are likely related to many members of the group, whereas males may only be related to their own offspring.
Menopause promotes the cessation of offspring production in order to care for the offspring of others, but this change may not be equally interesting for genes from the father and the mother. Natural selection retains those genes that produce more copies of themselves for the next generations – and genes make more copies of themselves when they are more effective. For example, genes that cause dark eyes have predominated in humans in areas with high solar radiation – because they’re more effective – while light-eyed genes were being lost as their carriers left fewer offspring. In the case of menopause, genes do this by helping to successfully raise offspring that carry copies of the same genes.
Within the same organism, the effects of genes from the mother and the father can be very different and even opposite. Researchers have proposed that genes more likely to have copies within the social group (eg. those from the maternal line in a matriarchal group) might produce physiological effects that advance menopause – to early shift their effort to the care of relatives’ offspring. Conversely, those with less kinship in the group would be producing delaying effects – to enhance their chances of direct reproduction. This may vary between cultures depending on the extent to which social groups have been formed on the basis of the maternal or paternal line throughout their evolution.
This sort of mismatch between the genes inherited from the father and those from the mother could cause the physiological disorders of menopause.