Anne Pusey, director of the Jane Goodall Institute's Center for Primate Studies at the University, is an expert on chimpanzee behavior.
Chiimps and the change of life
A study on chimps renews debate on why menopause happens
By Deane Morrison
December 14, 2007
To some it brings blessed relief; to others, a heartbreaking
loss of possibility. But whatever its personal significance,
menopause has long been a subject of debate. The big question for
researchers is why it evolved in the first place. Clues to human
evolution are often found in our closest relatives, chimpanzees,
whose ancestors split from ours sometime in the past seven million
years. Now, a study of 172 wild chimps stirs the waters by
reporting no evidence for a menopause-like event in the apes. If
upheld, this finding would hand biologists the task of explaining
how the timing of a major reproductive event evolved separately in
two closely related primates. The study draws on data from
chimpanzees at several sites in Africa, including Gombe National
Park, where Jane Goodall began her pioneering work in 1960. The
Gombe data is now stored at the University, in the Jane Goodall
Institute's Center for Primate Studies. Its director, ecology
professor Anne Pusey, is an author of the paper, which was
published Dec. 13, 2007, online in Current Biology. The first
author is Melissa Emery Thompson of Harvard University.
A big difference
The researchers compared the chimps to hunter-gatherers in two
tribes: the !Kung of Botswana and the Ache of Paraguay. In chimps
and women, fertility, measured as births per female per year,
declined similarly with advancing age, coming to a halt near the
age of 50. Chimps tended to die off as they lost fertility, and so
few survived beyond their reproductive period. But for women,
survivorship didn't drop as steeply as fertility. Close to 40
percent of Ache and 60 percent of !Kung lived past reproductive
age, with about 20 percent of !Kung living into their seventies.
"This pins down that wild chimpanzees don't have a long
post-reproductive life," says Pusey. "But even now, we don't have a
good fix on wild chimp lifespan. The big question is, are people
really different from apes, other primates, and other mammals in
general with respect to the timing of reproductive [decline] versus
[general bodily decline]?" According to Emery Thompson, the decline
in fertility is something all animals are expected to experience if
they live long enough. But menopause is unique in that it occurs
because declines in reproductive function happen much faster than
declines in other bodily systems. And indeed, the survivorship and
fertility curves in the paper show a clear post-reproductive life
for women but not chimps.
"The big question is, are people really
different from apes, other primates, and other mammals in general
with respect to the timing of reproductive [decline] versus
[general bodily decline]?"
Therefore, she says, scientists will have to "look to other unique
features of human biology and socioecology to help explain why
humans have menopause."
Or maybe not so big
Not so fast, says University ecology professor Craig Packer. He has
studied the latest data, but says he is uncomfortable with saying
that the longevity of chimpanzees is known. "We don't know enough
about the end of life in chimps in the wild or captivity," he says.
"Studies at Gombe have been going on only 47 years [since Jane
Goodall began them in 1960]. In the paper, they had to estimate
some females' ages. "Also, in every field study, [chimpanzees] are
disturbed in some way. In Gombe it's epidemic disease like
pneumonia and polio. In other places, the chimps are horribly
mangled by poachers' snares. We know of at least one captive chimp
that lived into his seventies. I think it's too soon to say with
any confidence where [in the life course] reproductive decline
occurs in chimps." Packer says he'd like to see fossil records from
before the dawn of agriculture and technology to settle the
question of what the primitive human lifespan is. People rarely
lived past their forties in agricultural societies around the 1800s
or so, he says, and "even in hunter-gatherers, an occasional
anti-malarial pill will extend life a lot." Packer says
post-reproductive life occurs in other nonhuman primates, rodents,
whales, dogs, rabbits, elephants, and domestic livestock and
appears to be a universal feature of mammalian females. In a
previous study of baboons and lions, he found that females tended
to live past reproductive age, and the reason could well be tied to
the needs of offspring. For example, female baboons don't live past
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.
Similarly, lion cubs need only one year of maternal care, and lion
maternity drops at age 14, at which point life expectancy is 1.8
years. Humans, of course, require a lot more time. What the age of
independence might have been in pre-technological times is hard to
say, but Pusey says it could have been sometime in the teens.
Meanwhile, chimps seem to need only three years of maternal care to
get on their feet. So from that point of view it's no surprise that
post-reproductive life is longer in humans.
A matter of definition?
In a way, the argument seems to be over what menopause is.
Supposing women do enjoy a much longer period of post-reproductive
life than their ape counterparts; does that mean something
different is going on with women? Or is human "menopause" a
manifestation of the same evolutionary forces as a chimp's short
post-reproductive life? That is, could the length of
post-reproductive life in both species be driven by the same thing,
namely, the needs of offspring, who happen to be much more
demanding in our species? Or, says Packer, "Maybe humans have just
applied their greater intellect to looking after their elders."
Still, says Pusey, it is remarkable that many chimpanzees reproduce
well into their forties or fifties. In the future, she wants to
determine just how long a young chimp needs its mother in order to
survive and thrive. That will predict how long the
post-reproductive interval in the species ought to be if the needs
of offspring are driving its evolution.