Menopause in Wild Chimpanzees Discovered in Uganda

To date, research has suggested that only humans and some species of toothed whales live many years of active life after the loss of reproductive ability. But now, a new study shows female chimpanzees in Uganda show signs of menopause – surviving long past the end of their ability to reproduce. Signs of menopause in wild chimpanzees may provide insights into the evolution of this rare trait in humans. The vast majority of mammals stay fertile until the very ends of their lives, with humans and several species of toothed whales as the outliers; they experience menopause. In humans, menopause typically occurs between the ages of 45 and 55, characterized by a natural decline in reproductive hormones and a permanent cessation of ovarian function. Explaining how menopause evolved is challenging because the evolutionary benefits aren't obvious. Why menopause evolved in humans but seemingly no other long-lived primates also remains uncertain. Here, Brian Wood and colleagues report demographic and hormonal evidence of menopause in wild chimpanzees. Using behavioral and demographic observations from a long-studied Ngogo community of wild chimpanzees in Uganda's Kibale National Park, Wood et al. calculated a metric called the post-reproductive representation (PrR), which is the average proportion of the adult life span that is spent in a post-reproductive state. While most mammals, including other chimpanzee populations, have a PrR close to zero, the authors discovered that the Ngogo chimpanzees had a PrR of 0.2, meaning that, on average, females live 20% of their adult years in a post-reproductive state. Moreover, urine samples from 66 females that differed in reproductive status and age showed that the transition to this post-reproductive state was marked by changes in hormones like gonadotrophins, estrogens, and progestins. According to the authors, similar hormonal variations are diagnostic for human menopause. However, unlike humans, post-reproductive chimpanzees in the Ngogo population were not involved with raising their children's children, suggesting that the popular "grandmother hypothesis," which has been used to explain the adaptive evolution of long post-reproductive lifespans, does not apply. "The study of Wood et al. both illuminates and raises questions about the evolution of menopause," writes Michael Cant in a related Perspective. "It also highlights the power of difficult long-term field studies – often run on small budgets and at constant risk of closure – to transform fundamental understanding of human biology and behavior."