While men typically remain fertile throughout their lives, women do not. Middle-aged women experience menopause, marking the end of their reproductive years. However, menopause is something of an evolutionary mystery. The process of evolution is often described as survival of the fittest: individuals with traits that make them more likely to survive and reproduce pass along these traits to their offspring, causing those characteristics to become more common in the population. So, why does menopause exist? If women remained fertile throughout their lives, would that not increase their likelihood of passing along their genes? One possible explanation is called the grandmother hypothesis: at advanced age, a woman is more likely to pass along her genes by helping her existing offspring ensure her grandchildren’s survival than by having more children of her own.

Outside of humans, menopause has previously only been observed in a few species of whales. However, a team of researchers recently discovered that chimpanzees go through menopause, too. The researchers studied 185 wild female chimpanzees living in Kibale National Park, Uganda, over the course of 21 years. They found that fertility began to decrease by age 30 and no births were observed in chimpanzees older than 50, implying that the typical chimpanzee was infertile for about a fifth of her adult life. The researchers also tracked reproductive hormone levels using urine samples, finding that the changes in these hormones with age were similar to menopause in humans.

The discovery of menopause in chimpanzees is potentially a strike against the grandmother hypothesis: unlike humans and whales, elderly chimpanzees do not live with their daughters. Instead, the researchers suggest another explanation called the reproductive conflict hypothesis: if mating opportunities are limited, elderly females becoming infertile gives their younger relatives a better chance to breed. For humans, it is possible that both the grandmother and reproductive conflict hypotheses play a role in explaining the evolutionary origin of menopause.

This study was led by Brian Wood, an associate professor in the Department of Anthropology at the University of California, Los Angeles. Kevin Langergraber, an associate professor in the School of Human Evolution and Social Change at Arizona State University, is also a corresponding author.

Managing Correspondent: Emily Pass

Press Article: Wild Female Chimpanzees Go Through Menopause, Study Finds (Smithsonian Magazine)

Original Journal Article: Demographic and hormonal evidence for menopause in wild chimpanzees (Science)

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