Grandmother Theory Takes a Hit

Last February, the idea that it's advantageous for human females to live long after menopause so they can help feed their grandchildren--a notion taken from studies of African hunter-gatherers--captured public attention. But now a study of old female lions and baboons, published in this week's issue of Nature, challenges this "grandmother hypothesis."

Craig Packer, a biologist at the University of Minnesota, St. Paul, found that grandmother lions and baboons did help feed and protect their grandchildren--but their investment had no impact on the youngsters' survival. And loss of fertility did not boost the amount of care older lions or baboons gave to their grandchildren--in fact, only lions who were still nursing their own cubs were also able to nurse grand-cubs. In Packer's view, "menopause isn't adaptive. It has no function." Rather, it's simply a consequence of the aging of female reproductive systems.

How long these females survive after their fertility drops, Packer says, appears to be determined by the needs of their children rather than their grandchildren: Lion cubs are vulnerable for only 1 year, so their mothers don't need to live much longer. But baboons orphaned at age 2 usually die, so a baboon mother's life-span is 5 years beyond her last pregnancy--somewhat longer than the time in which she has a dependent infant.

But one of the authors of the grandmother hypothesis, anthropologist Kristen Hawkes of the University of Utah, Salt Lake City, doesn't think these findings say much about human females. She notes that just a few percent of adult female lions and baboons survive past menopause, compared to more than 80% of women hunter-gatherers--and they often survive into their 70s, well beyond the decade needed to ensure their own offspring's survival. This shows that "we're really odd in that we live so long after menopause," says Hawkes. She adds that human grandmothers, with their provisioning of weaned grandchildren, are doing "a very special kind of thing."