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Feature

Anne Pusey

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.