News - Why a menopause?
Scientists have long wondered why only female humans and toothed whales have evolved menopause, while other mammals continue breeding almost to the end of their life span. Research published in the Proceedings of the Royal Society B suggests that the menopause has the effect of freeing women so that they are better able to look after their grandchildren, who they know are carrying their genes, and improve their survival chances.1Daryl Shanley and colleagues at the Institute for Ageing and Health at Newcastle University used data from two villages in Gambia, collected between 1950 and 1975, and found that if the grandmother was young enough to bear children, two-year-olds had a 16% chance of receiving care from a grandmother, compared with a 58% chance if the grandmother was no longer able to get pregnant. A mathematical model found that care from a maternal grandmother significantly improves the survival chances of children, particularly between the ages of one and two, when children are weaned and their mothers often become pregnant again. In other words, the benefit of grandmothers may occur immediately after grandchildren are not completely dependent on their mothers.
But there may be more to the menopause then just the ‘grandmothering benefits.’ Research by scientists at the Universities of Cambridge and Exeter, writing in the prestigious academic journal PNAS, have come up with an additional answer to this problem.2 Menopause in humans could be an adaptation that originated through reproductive conflict between generations. Our ancestors lived in small groups and members within a group competed for resources that were critical for raising their young. The effects of this competition can be seen in other animals that live in close-knit family groups, where the offspring suffer when more than one female in the group breeds. Evidence suggests that the ancestral young female would move to wherever her mate lived in order to breed, so an older mother would face competition from her daughter-in-law, whose only genetic relations are her own children. The point of life is to pass genes down the generation and there is no genetic advantage in helping their mother-in-law bear more children, because they won’t share any genes with those children. But the mother-in-law who helps her son’s wife reproduce will benefit by having 25% of her genes handed down to her grandchildren. So, the mother-in-law’s best strategy is to stop breeding, avoid competition and allow the daughter-in-law to breed, as well as helping to bring up her children.
1. Shanley D, Sear R, Mace R, Kirkwood T. Testing evolutionary theories of menopause. Proc Biol Sci 2007; 274:2943-9.
2. Cant M, Johnstone R. Reproductive conflict and the separation of reproductive generations in humans. Proc Natl Acad Sci USA 2008; 105: 5332-6.