Menopause in Wild Chimpanzees: A New Discovery
Hormonal Changes and Reproductive Decline
In a groundbreaking study published in the journal Science, researchers have documented menopause in wild female chimpanzees for the first time. Menopause, the natural end of menstrual cycles, was previously only known to occur in humans and a few species of toothed whales.
The study followed 185 female chimpanzees in Uganda’s Kibale National Park for 21 years. The researchers found that fertility declined after the animals turned 30, and none of them gave birth after turning 50.
Hormone levels also changed in post-menopausal chimpanzees, mirroring the changes seen in humans. Levels of follicle-stimulating hormone and luteinizing hormone increased, while levels of estrogens and progestins decreased. These hormonal shifts indicate that menopause ends reproduction in chimpanzees at around the age of 50.
Ecological and Social Factors
The researchers speculate that menopause may be specific to the Ngogo chimpanzees, which live longer lives due to a lack of predators and a lot of available food. The Ngogo chimps have also been studied more extensively than other groups.
Alternatively, menopause may have been more common in chimpanzees before human impacts, like logging and introduced diseases, began affecting the animals’ mortality.
Evolutionary Implications
The findings challenge the “grandmother hypothesis,” which suggests that some animals live past their reproductive years to help raise the offspring of their offspring. In chimpanzees, this is not possible because they do not live with their daughters.
Instead, the researchers propose that older females may stop reproducing to prevent them from competing with younger females for chances to breed. This “kin competition hypothesis” suggests that menopause evolved to reduce competition within the group.
Comparison to Humans and Other Chimpanzee Species
The study suggests that the last common ancestor of humans and chimpanzees may have gone through menopause. To get a better idea of how menopause evolved, scientists could study how common it is across different chimpanzee communities, as well as whether bonobos—a species that, along with chimpanzees, are humans’ closest living relatives—also live long after they stop reproducing.
Significance
The discovery of menopause in wild chimpanzees provides new insights into the evolution of this phenomenon and its potential role in shaping social and reproductive strategies in primates. It also highlights the importance of long-term research in understanding the complexities of animal behavior and physiology.
