A ground-breaking study has revealed that wild chimpanzees do not routinely experience menopause, rebutting previous findings in captive animals that female chimpanzees reach reproductive senescence at 35 to 40 years of age.
Coupled with recent data from wild gorillas and orang-utans, the new finding suggests that human females are rare or even unique among primates in experiencing a lengthy post-reproductive lifespan.
"We find no evidence that menopause is common among wild chimpanzee populations. While some female chimpanzees do technically outlive their fertility, it's not at all uncommon for individuals in their 40s and 50s -- quite elderly for wild chimpanzees -- to remain reproductively active," says lead author Melissa Emery Thompson, a postdoctoral researcher in anthropology at Harvard University.
Together with recent data from wild gorillas and orang-utans, the finding -- described this week in the journal Current Biology -- suggests that human females are rare or even unique among primates in experiencing a lengthy post-reproductive lifespan.
Both wild chimpanzees and humans experience fertility declines starting in the fourth decade of life, but most other human organ systems can remain healthy and functional for many years longer, far outstripping the longevity of the reproductive system and giving many women several decades of post-reproductive life.
In chimpanzees, reproductive declines occur in tandem with overall mortality. A chimpanzee's life expectancy at birth is only 15 years, and just 7 percent of individuals live to age 40.
Emery Thompson's team has, however, found that females who reach such advanced ages tend to remain fertile to the end, with 47 percent giving birth once after age 40, including 12 percent observed to give birth twice after age 40.
"Fertility in chimpanzees declines at a similar pace to the decline in survival probability, whereas human reproduction nearly ceases at a time when mortality is still very low. This suggests that reproductive senescence in chimpanzees, unlike in humans, is consistent with the somatic aging process," the researchers write in Current Biology.
In other words, human evolution has resulted in an extended life span without complementary extended reproduction.
Emery Thompson has also suggested a reason for why reproduction has not kept pace with the general increase in human longevity. She says that it could be because there hasn't been anything for natural selection to act on, though there is heritable variation in age of menopause.
"However, it may be that the advantage older females gain by assisting their grandchildren outstrips any advantage they might get by reproducing themselves," she says.
The oldest known wild chimpanzee, who died earlier this year at approximately age 63, gave birth to her last offspring just eight years ago, at about 55.
Female chimpanzees only give birth every 6 to 8 years on average and they generally begin reproducing at age 13 to 15, which makes the chimpanzee reproductive profile much longer and flatter than that of humans, whose procreation is concentrated from age 25 to 35.
Emery Thompson and her colleagues gathered data from six wild chimpanzee populations in Tanzania, Uganda, Guinea, and Gambia. The researchers compared the chimpanzees' fertility patterns to those seen among two well-studied human foraging populations, in Botswana and Paraguay.
The study has been reported in the journal Current Biology.