Chimps share many traits that we consider to be uniquely human, but now a new study suggests that the menopause really does set humans apart from other apes.

A detailed look at long-term fertility data from six populations of chimpanzees, compared with similar data from populations of hunter-gatherer humans, shows that both chimp and human birth rates have similar patterns of reproductive decline after the age of 40.

But where chimp survival drops along with fertility, humans stop reproducing and continue to live for a long time. Some chimps in their 40s are in fact better at reproducing than humans at that age. And contrary to the general case in humans, in chimps old females are preferred by males.

“Human life history is in fact one of the most radical departures from the apes,” says Melissa Emery Thompson, at Harvard University, Cambridge, Massachusetts, US, who led the research.


Aged mums

“We 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.”

Emery Thompson gathered data from colleagues working on wild chimps at sites across Africa, and compared fertility patterns with that of human foragers – the Kung people of Botswana, and Aché of Paraguay.

Their finding that birth rates of both chimps and humans decline after 40 suggests that the “biological clock” is a feature that has been conserved over the course of human evolution.

Healthy chimps over 40 reproduced quite well, the team found. And whereas when a woman over 60 has children after IVF it makes headline news, it is not unusual for old chimps to give birth.

“Females in the wild and in captivity have given birth in their 50s and the oldest living captive female, who is about 69, gave birth past the age of 60,” says Emery Thompson.

One wild chimp, called Auntie Rose, was fertile until she died aged 63, and still had males fighting over her, she notes.

Sexy but bald

“Male chimpanzees are consistently more sexually interested in older females, even those like Auntie Rose who was nearly bald,” says Emery Thompson. “This is a definite difference from humans.” Male chimps might prefer older females as their age might be a good indication of genetic fitness.

As to why evolution has not favoured any extension of human reproduction that would complement our extended life, Emory Thompson says that is an open question, though grandmothers in hunter-gatherer societies bring in more calories than they actually require.

Women with living mothers have higher birth rates than those without, supporting the idea that “grandmothering” may be more genetically profitable than having children in late life.

Apart from humans, only some species of whales have an extended period of post-reproductive life that could be called menopausal. Pilot whales stop breeding at around 40, for example, and live for several decades longer.

Journal reference: Current Biology (DOI: 10.1016/j.cub.2007.11.033)