NEW YORK -- Scientists may have found the answers to two questions that might nag a woman entering menopause: Why is this happening? And why now?
The proposed answers, from 30 years of observing lions and baboons in the wilds of Tanzania: No particular reason. And: For the kids' sake.
The researchers concluded that menopause -- instead of conferring a longterm evolutionary advantage -- is just a result of aging, like failing eyesight or thinning hair. But they did find evidence that evolution has at least timed menopause so that a female will have lifespan left to help raise her last babies into adulthood.
The study focused on why women and other female mammals have menopause at all, and why a woman's reproductive system takes early retirement when the rest of her body is humming right along.
Those questions have long been puzzling, because evolutionary theory says organisms are driven to pass their genes on to future generations as often as possible. That makes menopause hard to explain.
Some researchers have proposed that menopause might actually have given an advantage in ages past. Maybe it pays to stop having kids early, so Mom will have enough lifespan left to raise her last newborns rather than leaving them as orphans.
In fact, by focusing on her present kids and her grandchildren, a menopausal female might actually increase the number of her descendants who reach adulthood and reproduce, sending her genes farther down the line, some researchers say.
But the new work found no such advantage from menopause in lions and baboons, ecology professor Craig Packer of the University of Minnesota and colleagues elsewhere report in Thursday's issue of the journal Nature.
The presence of post-menopausal grandmas made no difference in how many offspring their daughters had or in how well those offspring survived.
There was, as Packer put it, no "granny effect."
The researchers studied lions and baboons because both species have social systems where such an effect might appear, Packer said. Baboon mothers influence the social rank of their daughters, for example, and female lions help defend the territory and nurse each other's offspring.
The researchers concluded that menopause is just another result of aging. The timing of menopause, though, seems well-chosen by evolution, the researchers found.
A lion cub needs Mom for a year after birth, and there's 1.8 years between the age when reproductive decline begins in a female and the end of her expected lifespan, they found. Newborn baboons need two years of nurturing, which fits in the five years between the first reproductive decline and the end of Mom's expected lifespan.
As for women, if one assume that their babies needed them for about 10 years after birth in ancient times, it would make sense that menopause was pushed back into what we now consider mid-life, the researchers wrote.
Kristen Hawkes, an anthropologist at the University of Utah, agreed that menopause probably is just a consequence of aging.