Fitness for grandmas

Natural History, Dec, 2007 by Stephan Reebs

[ILLUSTRATION OMITTED]

Menopause is puzzling from an evolutionary standpoint. Why should healthy women stop having children after age fifty when doing so curtails their reproductive success? Actually, it doesn't. A new study supports the hypothesis that menopause frees up older women to help care for their grandkids, whose improved survival overcomes the lost opportunity to have more children.

Daryl P. Shanley of Newcastle University in England and three colleagues analyzed an unusually complete trove of demographic data collected in two Gambian villages between 1950 and 1975. Modern medical care only became available to the villagers in 1975, and life was pretty basic before that, so Shanley says the data offer a unique glimpse into relatively natural human fertility patterns.

His analysis revealed that infant mortality during the first two years of life declined by half when a maternal grandmother was present. Predictably, moms helped even more. But infant survival was not influenced at all by fathers, siblings, grandfathers, or, notably, paternal grandmothers (it makes sense for grandmothers to devote more time to their daughters' babies than to their sons', since paternity is uncertain).

Through mathematical modelling, Shanley calculated that grandmotherly care would be cut by more than two-thirds if menopause occurred at age sixty-five instead of fifty. Delayed menopause would reduce the fitness of the grandkids--and the grandmas. (Proceedings of the Royal Society B)