Chimpanzees experience menopause like humans

Menopause, a phenomenon observed in humans and a select few mammal species, including some toothed whales, has now been identified in chimpanzees as well. Wood et al. conducted a study on chimpanzees in Uganda, revealing conclusive evidence of menopause in females aged over 50. Interestingly, unlike humans and toothed whales, post-reproductive female chimps in this population do not partake in raising offspring, hinting at unique evolutionary drivers behind this biological process. This discovery sheds new light on our understanding of menopause across species.

Why does evolution favor menopause or the survival of individuals who can no longer reproduce? This perplexing question has intrigued scientists for years. While post-reproductive females are rare among mammals in the wild, they exist in humans and a select few whale species, making them a fascinating yet elusive subject of study. To shed light on this mystery, scientists turn to our closest primate relatives, chimpanzees. By analyzing demographic and hormonal data, scientists aim to unravel the secrets of post-reproductive life spans and their physiological mechanisms in these fascinating creatures. 

Scientists conducted a fascinating study on 185 female chimpanzees living in Uganda’s Kibale National Park. Over 21 years, they closely observed these wild chimpanzees to understand their mortality and fertility rates. One key measure they looked at was PrR (post-reproductive representation), which indicates how much of their adult life the chimpanzees spend in a post-reproductive state. To explore if these chimps experience something similar to human menopause, the scientists analyzed hormone levels in 560 urine samples from 66 females of different ages and reproductive statuses. This research sheds light on the fascinating world of chimpanzee reproductive biology and its potential similarities to our own experiences.

Researchers discovered that, similar to humans, female chimpanzees in the Ngogo population experienced a decline in fertility after age 30, with no births recorded after age 50. Surprisingly, unlike other chimpanzee groups, many Ngogo females lived beyond 50 years old. This longevity meant that approximately one-fifth of their adult lives were spent in a post-reproductive state, a phenomenon resembling human patterns but occurring for a shorter duration. Hormonal analyses revealed parallels between Ngogo females and humans during reproductive transitions, highlighting the intriguing similarities between our species.

While menopause typically ends reproductive capabilities around age 50 in both humans and chimpanzees, significant post-reproductive lifespan (PrR) had never been witnessed in wild primate populations, including chimpanzees. This unexpected finding raises intriguing questions about the evolutionary factors at play. One theory suggests that favorable ecological conditions at Ngogo, such as low predation rates and abundant food, may have triggered this unique response. Alternatively, PrR could be an inherent trait in chimpanzees, obscured by recent human impacts like disease epidemics. Theories such as the grandmother hypothesis, which posits that older females survive past reproductive age to support offspring, may not directly apply to chimpanzees due to their social dynamics. Instead, the reproductive conflict hypothesis, focusing on competition among females for breeding opportunities, may offer a more pertinent explanation. These theories, though not mutually exclusive, provide insight into the complexities of PrR and hint at broader implications for understanding human societies’ higher prevalence of post-reproductive lifespan.