Infant Dependency Drives Menopause, NSF-Supported Researcher Reports
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Why is there menopause? Some researchers have claimed that women need child-rearing help from their mothers, and that menopause frees older women to pitch in. Others have claimed that menopause is just an unavoidable consequence of aging.
Writing in the April 23rd issue of Nature, University of Minnesota ecology professor Craig Packer says evidence from lions and baboons points to menopause as a simple result of aging. The timing of menopause, however, is set by how long a species needs to raise last-born infants to the age of independence, he says. Packer's research is funded by the National Science Foundation (NSF).
"This finding points out the importance of long-term, careful field studies involving the behavior, life history and reproductive success of individuals in social groups," says Penny Kukuk, program officer in NSF's division of environmental biology. "Studies that follow individuals throughout their lifetimes and that determine numbers of offspring they leave behind are crucial to finding answers to questions such as why there is menopause."
The results suggest that "there's no evolutionary benefit to menopause -- it's simply that there's no cost," said Packer. That is, as an individual ages, the reproductive system is the first to go, but that's okay at the point when the individual won't live long enough raise an additional baby, he said. "Since humans have a more prolonged period of infant dependency than other species, we'd expect menopause to occur earlier in life."
The theory predicts that reproductive decline will begin once the mother's life expectancy drops below the time required to raise additional offspring. For example, if women in pre-technological societies could expect to live 50 years, and if a child, in order to survive, needed its mother until the age of 10, then reproductive decline could begin at age 40. Packer's data illustrates this concept in baboons and lions. Female baboons don't live past age 26 or 27, and their infants require at least two years of maternal care. Baboon reproductive rates decline around age 21, which allows ample time for the youngest infant to reach independence.
Packer said that he found no evidence that menopause frees older females to help younger females raise offspring. Such behavior should result in higher survival of individuals whose grandmothers are still alive but no longer reproductive. Although grandmother lions and baboons both engage in what's called kin-directed behavior, they had no measurable impact on the survival or reproduction of their grandchildren or adult daughters.
It has also been suggested that menopause evolved in women to avoid the increased dangers of childbirth in middle age. But Packer saw no evidence for this in elderly baboons and lions. Menopause occurs in several other species, including non-human primates, rodents, whales, dogs, rabbits, elephants and domestic livestock. It appears to be a universal feature of mammalian females. "Since female mammals are the primary caretakers of infants, we would expect menopause to evolve whenever child-rearing is extensive and prolonged," Packer said.
Packer's co-authors are Marc Tatar, assistant professor in the Brown University department of ecology and evolutionary biology, and Antony Collins, a research director at Gombe Stream Research Centre in Tanzania.
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