Study Renews Debate on Menopause

Menopause is a midlife milestone for all women that, in the eyes of some scientists, is as important a signature of the human species as a large brain and an opposable thumb. It may have ensured the evolutionary triumph of the human family, they say, by freeing older women to care for their grandchildren.

Yet others have argued that it amounts to no more than hot flashes, brittle bones and barren years.

New research from the University of Minnesota and Brown University, to be published today in Nature, strongly suggests that menopause simply is a consequence of aging, nothing more.

Nurturing grandmothers offered no evolutionary advantage to their kin, the researchers found in a study of lions and baboons in the wilds of Africa.

By seeking an answer to an enduring riddle of human biology, the researchers have entered an energetic debate over the meaning of life after menopause, the importance of older women, and the evolutionary role that grandmothers may have played in fostering the young.

The argument also touches contemporary sensitivities, from whether menopause should be treated medically as a kind of chronic disease, to cultural assumptions about child care and when it is appropriate to view human behavior through the prism of evolutionary biology.

Certainly, menopause presents a paradox for those who study how individuals of a species thrive by passing their genes on to future generations as often as possible.

Baffled anthropologists and evolutionary biologists long have wondered what could be the survival value of something like menopause that abruptly ends the ability to have children in midlife. That is when a woman’s ovaries usually stop functioning and the menstrual cycle ceases.

Indeed, the modern appetite for fertility treatments, such as in vitro fertilization and egg donation, is compelling evidence that the desire to have children persists even when an individual’s ability to conceive has waned.

Until now, researchers have been reluctant to simply consider a woman’s lost reproductive capacity a casualty of middle age because women are otherwise quite vigorous when menopause occurs, typically around the age of 51.

“This is a very complicated puzzle,” said Marc Tatar, an evolutionary biologist at Brown University, who helped conduct the new research.

Some have argued that menopause protects older women from the potentially fatal rigors of childbirth.

Others say it only arises because people live so much longer than in the past. Until relatively recently in human history, men and women rarely survived past 40.

But several anthropologists, based on their study of primitive hunting and gathering societies, have conceived a “grandmother hypothesis” to account for the unusual character of human menopause.

Those researchers noticed that older women who can no longer bear children of their own help their child-bearing daughters forage to feed the grandchildren. Perhaps, the scientists speculated, grandmothers were crucial to the survival of the ancient human family.

They may have been so important that natural selection dramatically extended the human lifetime after menopause. That meant a woman would not only see her own offspring safely into adulthood but would be able to pitch in to help the grandchildren survive as well.

In work published recently in the Proceedings of the National Academy of Sciences, University of Utah anthropologist Kristen Hawkes said the hypothesis not only accounts for menopause and humanity’s long life span, but also for its relatively late age of maturity, small size at weaning, and high fertility rate compared to other primates.

“Something very different is happening in humans, something that is characteristic of our species,” said Hawkes, who studies primitive human cultures in New Guinea, South America and Asia. “It is really grandmothers that could be the key to a bunch of things that we have thought of as these distinctive human capacities.”

To answer such questions about human menopause, University of Minnesota ecologist Craig Packer and Tatar looked to lions and baboons in the wild and their reproductive life cycles.

In the research published today, they used data gathered in the study of thousands of lions in Tanzania’s Serengeti National Park and hundreds of wild baboons in the nearby Gombe National Park over 35 years--the longest such wildlife surveys ever conducted.

“The reason our data is so important is that we can put menopause into the big picture, where it fits into the overall survival of a species,” Packer said.

The researchers first wanted to see how the animal’s age affected its ability to reproduce, to see if these mammals experienced anything like human menopause.

Then they wanted to test the theory that menopause may have evolved as a way to enhance the survival of the animals’ kin, as some have argued. They focused on the female of each species because the males play little or no role in child-rearing.

Even though the females of both species do help care for their daughters’ offspring--forming nurseries, defending their younger kin, and even nursing their grandchildren--there seemed no measurable impact on overall survival, Packer and Tatar said.

“The very simple answer is that the grandmothers don’t have a significant impact on their adult daughters or the survival of their grandchildren,” Packer said.

Moreover, by extending their findings to humans, the researchers concluded that evolution ensures that a woman will live at least long enough after menopause to see that her last child survives safely into adulthood and no further, the researchers concluded.

The field studies of lions, for example, revealed that a lion cub usually depends on its mother for a year after birth. On average, there is slightly less than two years between the age when a female lion’s capacity to conceive declines and the end of her expected life span, the researchers found.

By the same token, newborn baboons needed about two years of nurturing before they could survive on their own, and a female baboon lives just about five years after she is no longer able to conceive.

But other scientists said the new study, although impressive, failed to account for several distinctive human features.

Those include a woman’s ability to conceive while still caring for a child, the human habit of sharing food with children who have weaned, and what some scientists regard as the unusually long human life span after menopause--often twice the age at which menopause occurs and long after any offspring have matured.

“This also suggests that a broad array of relatives benefit from the accumulated wisdom, status and resources of post reproductive women,” said neurobiologist Paul W. Sherman at Cornell University.