By Nancy Bird
We’ve all heard (and perhaps been annoyed by) the phrase ‘your biological clock is ticking’, but why do women go through the menopause at all, when men can keep fathering children into their 80s and 90s?
The menopause is an incredibly rare phenomenon in the animal kingdom, seen only in humans and a few species of whale. This rarity makes sense when you consider it from an evolutionary perspective. What would be the benefit to a woman of not being able to reproduce for her whole lifetime? She would most likely have fewer children, pass on fewer copies of her genes, and so any gene causing the menopause should be removed by natural selection. Then, why does it exist at all?
The Grandmother Hypothesis
An answer to this mystery was proposed by Hamilton in 1966 and dubbed ‘the grandmother hypothesis’1. The theory is that in human societies, grandmothers help their grandchildren by caring for them, providing food for them, or teaching them important life skills. Consequently, a post-menopausal lifespan is beneficial because it means a woman will have more, or healthier grandchildren.
The hypothesis gained great popularity in the 80s and 90s, and evidence for it was provided by anthropological studies. One study in rural Gambia showed that children with a maternal grandmother grew taller and were more likely to survive than children without, adding support to the grandmother hypothesis2.
Are there other explanations for the menopause that do not rely so much on caring grannies? It has been suggested that perhaps there is a fundamental biological constraint on older women giving birth. Yet, there is an abundance of older mothers elsewhere in the animal kingdom, with elephants giving birth well into their 60s.
Another explanation may lie in our social lives. Humans today live in tight-knit family groups, and probably have for much of our history. Families tend to share common resources and are therefore in competition with each other when times are hard. Since babies are extremely dependent on both food and care, this competition is particularly fierce between women who are rearing children3. Perhaps this neediness of human youngsters means that multiple family members cannot have children at one time?
A mother rarely keeps reproducing once her daughter has started. In the study of rural Gambia, 93% of maternal grandmothers, and all paternal grandmothers, had stopped reproducing before the birth of their first grandchild2. This is in contrast with other apes, where there is often a large overlap in the timespan over which mothers and daughters are reproducing.
So, we have explained why there are likely to be limits on the timespan that women can reproduce. However, one question remains: why is it that we go through the menopause, rather than perhaps starting puberty later? Both these solutions prevent direct competition between generations, and the latter is more common in other animals. This is certainly true in many social insects, where the older queen can reproduce while younger worker generations are sterile.
For humans, the answer to why older women are the ones that stop reproducing lies in relatedness. Chimps, bonobos, and the majority modern hunter gatherer societies show a patrilocal pattern of movement, meaning that young women move to live with the family of their new partners. This means that rather than being in direct competition with her own mother, a woman is competing for resources with her mother-in-law3,4.
As anyone who is married can probably attest, this changes the family dynamics completely. This is because a mother-in-law is related to her daughter-in-law’s children (her grandchildren), but a woman is not at all related to her mother-in-law’s children (her partner’s siblings). Therefore, in any family conflict, it is very unlikely that a woman will give up having children to make way for her mother-in-law, as they do not share any genes. However, the latter is related to her daughter-in-law’s children, and so is more likely to make sacrifices for them.
The within-family competition theory explains why we experience the menopause without the need for kind-hearted grannies caring for their grandchildren. Our evolutionary history could have been full of conflict between warring generations, leading to the evolution of an almost unique reproductive phenomenon. However, this does not necessarily mean that the role of grandmothers is not important, and there are plenty of other theories about why the menopause evolved out there. Perhaps, we should all be thinking more about this amazing, and seldom talked about aspect of evolution unique to humans.
- Hawkes, K. et al., (1998). Grandmothering, menopause, and the evolution of human life histories. , 95 (3). doi.org/10.1073/pnas.95.3.1336
- Sear R., Mace R., & McGregor I. (2000). Maternal grandmothers improve nutritional status and survival of children in rural Gambia. Proc. R. Soc. B., 267 (1453). doi.org/10.1098/rspb.2000.1190
- Cant M. & Johnstone R. (2008). Reproductive conflict and the separation of reproductive generations in humans. Proc. Natl Acad. Sci. USA 105, 5332–5336. doi:10.1073/pnas.0711911105
- Johnstone R. & Cant M. (2010). The evolution of menopause in cetaceans and humans: the role of demography. R. Soc. B., 277 (1701). doi.org/10.1098/rspb.2010.0988