Helping hands. A grandmother and her grandchild from Malawi.

Rebecca Sear

Grandma Plays Favorites

Most women have their last child before age 40. Why would Darwinian evolution favor such a cutoff, especially when most other mammals reproduce until they die? A new study finds support for the "grandmother hypothesis," the idea that older women spread their genes most effectively by helping their daughters take care of their children.

In 1998, behavioral ecologist Kristen Hawkes of the University of Utah in Salt Lake City and her colleagues proposed that grandmothers lend their skill and experience to the rearing of their grandchildren. Hawkes and others cited the Hadza, a modern foraging society in Tanzania, in which grandmothers search for tubers while their daughters are breastfeeding their babies. Given that tubers are thought to have become an important staple during the early days of human evolution, a selective advantage for "grandmothering" rather than "mothering" by older women might have arisen in our species.

Over the past decade, a number of researchers have tried to test the hypothesis by looking at the relationship between grandmothers and their grandchildren. Some studies found that when grandmothers live near their grandchildren and/or live longer, their grandchildren have higher survival rates. But other studies did not see this correlation.

To try to resolve these inconsistencies, a team led by biological anthropologist Leslie Knapp of the University of Cambridge in the United Kingdom proposed what it calls the "X-linked grandmother hypothesis." There are varying degrees of relatedness between grandmothers and their grandchildren, particularly via the X chromosome. For example, paternal grandmothers, like all women, have two X chromosomes, and they pass one of them to their sons, who in turn pass it on to their daughters. Thus, paternal grandmothers are 50% related to their granddaughters, at least as far as sex chromosomes are concerned. But paternal grandmothers have no X-chromosome relatedness to their grandsons, who get their only X from their mother's line. And maternal grandmothers are 25% related to both their granddaughters and their grandsons on their sex chromosomes, because each of those offspring has only a 25% chance of receiving a particular X chromosome from either their maternal grandmother or their maternal grandfather.

Knapp and her co-workers looked at the survival of grandchildren in seven modern-day and historical societies for which good family records were kept. The societies included rural farming villages in Japan, Ethiopia, Gambia, and Malawi, as well as towns in Germany, England, and Canada going back as far as the late 1600s, whose data were gleaned from church and other historical records. When the data from all seven populations were combined in one meta-analysis, there was a highly significant correlation between the degree of relatedness of grandmothers living with or near a family and the survival rate of their grandchildren, the team reports online today in the Proceedings of the Royal Society B. Thus paternal grandmothers were most beneficial to the survival of their granddaughters and least beneficial to the survival of their grandsons, while maternal grandmothers showed an intermediate effectiveness.

Experts are thrilled by the findings. "Wow, very interesting," says Hawkes. "The consistent results across seven populations ... seem to clarify previously inconsistent results." Lorena Madrigal, an anthropologist at the University of South Florida in Tampa, calls the study "an important contribution to a topic of great interest to evolutionary biologists."

Still unanswered, however, is the exact mechanism that apparently leads grandmothers to favor some grandchildren over others. There is no evidence that they do it deliberately, the authors point out, leaving the possibility that more closely related children give off some sort of signal such as physical resemblance or a certain smell, or that grandmothers--especially on the paternal line--are passing down some sort of "genetic imprinting" that gives their grandchildren a survival advantage.