Scientific American posts an article about why menopause may have been favored by natural selection. The prevailing argument, called the "grandmother hypothesis," states that non-reproductive women enhance their inclusive fitness by caring for existing children and grandchildren.
A new study says there is a problem with the inclusive fitness argument for menopause:
"The problem is that these grandmother benefits aren't big enough to ever favor stopping breeding between the ages of 40 and 50," says Michael Cant, an evolutionary biologist at the University of Exeter in England and co-author of a new study on the genesis of menopause published this week in Proceedings of the National Academy of Sciences USA. "When you look at data from hunter-gatherers and other natural fertility populations, the sums just don't add up." Grandmothers do benefit their descendants, he says, but the genetic payoff is small compared with those of producing another child.Cant and Rufus Johnstone offer a new hypothesis based on reproductive competition between generations. Their model is based on the idea that reproductive-age women migrate to new communities with less similar genetic make-ups:
The rapid senescence of the human female reproductive system coincides with the age at which, in natural fertility populations, women are expected to encounter reproductive competition from breeding females of the next generation. Several lines of evidence suggest that in ancestral hominids, this younger generation typically comprised immigrant females. In these circumstances, relatedness asymmetries within families are predicted to give younger females a decisive advantage in reproductive conflict with older females. A model incorporating both the costs of reproductive competition and the benefits of grandmothering can account for the timing of reproductive cessation in humans and so offers an improved understanding of the evolution of menopause.The Scientific American article continues:
The mother of the grandmother hypothesis, anthropologist Kirsten Hawkes of the University of Utah, says Cant and Johnstone are right to focus on intergenerational conflict. Elephants have babies in their 60s, and some whales give birth in their 80s. "It's clearly something selection can adjust," she says. "So explaining why it hasn't in us has to be part of the story." But she disputes their claim that female-bias dispersal is, in fact, the universal human/ape residence pattern, pointing out that half of the young female chimps at anthropologist Jane Goodall's Gombe Stream Research Center remain with their mothers, and that recent studies show that hunter-gatherers often live with the wife's family as well.