HUMANS CAN live to over seventy years old, yet female fertility begins a precipitous decline after about forty years of age. This is unusual since most other species continue reproducing into old age. Some have proposed our extension of life past reproductive years is due to a survival benefit for grandchildren with grandmothers who contribute to their care. However, this benefit is not enough to explain the initial cessation of reproduction.
Cant and Johnstone have a new proposal that helps to explain menopause. They believe that the role of reproductive conflict in human populations has been overlooked. Since there are a limited number of resources and these tend to be shared widely in human populations, women producing infants are in conflict with other mothers over limited resources. If reproduction continues through the lifespan, women will end up in conflict with their daughters over group resources. In most species selection favors the suppression of reproduction in young adults (daughters who would reproduce at the expense of their mothers). One factor about humans that may make a difference is unusual human dispersal patterns.
Among our closest living relatives there is a bias towards female dispersion. Historically female dispersion has been more common than male dispersion, and remains so in hunter-gatherer societies. We can also track patterns of male and female dispersion from looking at mitochondrial genomes, passed on maternally, and the Y chromosome, passed on paternally. These have different distributions and are consistent with greater female dispersion.
If female dispersion is ancestral for our species, the role of menopause becomes more clear. A new female in a population is unrelated to the rest of the group, and there is no evolutionary disincentive for her to compete with them for resources for child-rearing. However, the older females in the group now have an incentive to cease reproduction and concentrate on rearing grandchildren.
Females leave their natal groups at maturity, pair with a male of similar age, and join his natal social group. Consequently, when a young female first arrives in the group, she has no other genetic relatives present. This female can choose to breed herself and produce offspring to whom she is related by 1/2 or to refrain from breeding and assist the breeding attempts of the older female (i.e., the mother of her mate), who produces offspring to whom the younger female is unrelated. The difference in relatedness to her own offspring versus those produced by helping is therefore 1/2. The older female, by contrast, can choose to breed and produce offspring of relatedness 1/2 or refrain from breeding and help to rear grandoffspring, to whom she is related by (1 – p)/4. The difference in relatedness to offspring produced by breeding versus helping for the older female is therefore 1/2 – (1 – p)/4, or (1 + p)/4, which means that as long as there is any chance that her son fathered her putative grandchildren (i.e., p < 1), the difference in relatedness to offspring produced by breeding rather than helping is lower for the older female than for the younger female. As a result, a younger female will have an advantage in reproductive competition with older females because she is insensitive to the costs she inflicts on an older female by breeding and because older females have more to gain by helping. By contrast, where dispersal is male-biased [as in most social mammals (54)], relatedness asymmetries within the group will favor older females in reproductive conflict with their daughters.
According to a game theory model the cessation of reproduction with the maturation of the next generation was an evolutionarily stable strategy even without the added benefits of grandmothering. The authors suggest that these selective forces were in effect during the Middle and Upper Pleistocene, at the same time that human lifespan was increasing. The result was to extend our species lifespan, but not reproductive years. Our early cessation of reproduction yet increased lifespan are explained jointly by the benefits of helping to raise grandchildren and the costs of continuing to produce children in competition with the next generation.
Cant, M.A., Johnstone, R.A. (2008). Reproductive conflict and the separation of reproductive generations in humans.. Proceedings of the National Academy of Sciences, USA, 105(14), 5332-5336. DOI: 10.1073/pnas.0711911105