Previous research indicated that extended post-reproductive life was a unique trait observed only in humans and certain toothed whale species. However, a recent study reveals that female chimpanzees in Uganda exhibit signs of menopause, surviving well beyond their reproductive years. These observations of menopause in wild chimpanzees offer valuable insights into the evolutionary origins of this unusual characteristic in humans.
The majority of mammals typically remain fertile throughout their entire lives, making humans and a few toothed whale species exceptions by experiencing menopause. In humans, menopause generally occurs between the ages of 45 and 55, marked by a natural decline in reproductive hormones and the permanent cessation of ovarian function. Understanding the evolutionary advantages of menopause has been challenging due to its seemingly ambiguous benefits, and the reasons behind its emergence in humans, while not in other long-lived primates, remain uncertain.
In this study conducted by Brian Wood and his colleagues, they present demographic and hormonal evidence of menopause in wild chimpanzees. By analyzing behavioral and demographic data from a well-studied Ngogo community of wild chimpanzees in Uganda’s Kibale National Park, Wood and his team introduced a metric known as post-reproductive representation (PrR), which measures the average proportion of an adult chimpanzee’s life spent in a post-reproductive state. In contrast to most mammals, including other chimpanzee populations, which exhibit a PrR close to zero, the Ngogo chimpanzees displayed a PrR of 0.2. This indicates that, on average, female Ngogo chimpanzees live 20% of their adult years in a post-reproductive state. Furthermore, analysis of urine samples from 66 females of varying reproductive statuses and ages revealed hormonal changes akin to those seen in human menopause, such as variations in gonadotrophins, estrogens, and progestins.
However, in contrast to humans, post-reproductive female chimpanzees in the Ngogo population were not involved in caring for their grandchildren, challenging the widely accepted “grandmother hypothesis” that has been used to explain the adaptive evolution of extended post-reproductive lifespans in humans.
In a related Perspective, Michael Cant notes that the study by Wood and his team sheds light on the evolution of menopause while also raising new questions. It underscores the significant contributions that long-term field studies, often conducted with limited budgets and at the risk of being discontinued, can make to our understanding of fundamental aspects of human biology and behavior.
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