Researchers have found no evidence that chimpanzees in the wild undergo menopause in the way that women do, according to a new report published online on December 13th in Current Biology, a Cell Press publication. That’s despite the fact that reproduction tends to peter out at a similar age in both species.
“It is important to distinguish reproductive senescence, which is something animals are expected to experience if they live long enough, from menopause, which is a very unique trait that occurs because reproductive function declines much more rapidly than declines in other bodily systems,” said Melissa Emery Thompson of Harvard University. “This study of reproductive senescence indicates that chimpanzees do not routinely experience menopause.”
Therefore, she continued, scientists will have to “look to other unique features of human biology and socioecology to help explain why humans have menopause.”
Human menopause is remarkable in that reproductive deterioration is markedly accelerated relative to the aging of the rest of the body, leaving an extended post-reproductive period for many women, the researchers said. The explanation for that pattern has remained unclear, in part because comparative data from closely related species had been inadequate. Earlier studies of chimpanzees are based on very small samples and have not provided clear conclusions about the fertility of aging females, she said. And those studies have not examined whether reproductive declines in chimpanzees exceed the pace of general aging, as in humans, or occur in parallel with declines in overall health, as in many other animals.
To remedy those problems, Emery Thompson teamed up with researchers from six long-term chimpanzee research sites across Africa. “By combining our data, we were able to examine the effects of age on fertility rates in chimpanzees,” she said.
They found that chimpanzee and human birth rates show similar patterns of decline after the age of 40, suggesting that the “biological clock” has been relatively conserved over the course of human evolution. However, in contrast to humans, chimpanzee fertility tends to drop along with their chances of surviving, with healthy females maintaining high birth rates late into life.
“When we look at only the healthiest individuals, it looks like chimpanzees may actually be reproducing better than humans in their forties,” Thompson said. “The oldest chimpanzee known to give birth in the wild is estimated to have been 55. She began reproductive cycling again shortly before her death at the age of 63.”
They thus find no evidence that menopause is a typical characteristic of chimpanzee life histories.
“The adaptive significance of human menopause, or post-reproductive lifespan, is still debated,” the researchers concluded. “This study provides greater evolutionary context to this debate.” Along with recent data from wild gorillas and orangutans, the findings in chimpanzees indicate that “menopause is not a part of the life cycle of living apes but has been uniquely derived in the human lineage.”