WE鈥橰E getting used to chimps doing things we once considered unique to humans. Now one human feature that really distinguishes us from other apes has been identified: the menopause.
A comparison of the fertility patterns of six populations of chimpanzees with those of modern hunter-gatherers 鈥� who have no access to modern healthcare 鈥� shows that both chimp and human birth rates begin to decline after 40. But whereas chimps begin to die of old age when their fertility drops, human females live for a long time after they stop reproducing.
鈥淗uman life history is in fact one of the most radical departures from the apes,鈥� says Melissa Emery Thompson of the anthropology department at Harvard University, who led the research. 鈥淲e live longer than expected for our size; we have vastly higher reproductive costs yet manage to reproduce much faster; we mature very slowly, and we have this peculiar post-reproductive period that distinguishes us from most other mammals.鈥�
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Emery Thompson put together data from long-term studies of wild chimps at sites across Africa, and compared their fertility patterns with those of the !Kung people of Botswana and the Ache of Paraguay. The team鈥檚 finding that birth rates in both chimps and humans decline after 40 suggests that the 鈥渂iological clock鈥� is a primate feature that has been retained in humans (Current Biology, ).
But unlike humans, healthy female chimps over 40 still reproduced quite well, the team found, and chimps are known to have given birth in their 50s. What鈥檚 more male chimps, unlike the average man, tend to prefer mating with older females, presumably because they have proved their genetic fitness. One wild chimp, called Auntie Rose, was fertile until she died at 63, and males were still fighting over her.
鈥淢ale chimpanzees are consistently more sexually interested in older females, even those like Auntie Rose who was nearly bald,鈥� says Emery Thompson. 鈥淭his is a definite difference from humans.鈥�
Why evolution did not favour extended human reproduction is an open question. But grandmothers are useful, and in hunter-gatherer societies they bring in more food than they need for themselves.
Apart from humans, only toothed whales have a post-reproductive life that could be called menopausal.
