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NSF PR 98-23 - April 22, 1998
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Infant Dependency Drives Menopause, NSF-Supported
Researcher Reports
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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