Menopause, typically seen as a normal occurrence in humans, has been found to have a peculiar aspect when observed in wild animals. Researchers have identified menopausal symptoms in only six mammal species, most of which live in aquatic environments.
Human beings exhibit unique behaviors compared to other species. We have developed social structures and achieved remarkable feats that set us apart from the rest of the animal kingdom. Interestingly, we have even found a way to turn post-fertility into an advantage.
While menopause is considered normal for humans, it raises questions about its evolutionary implications, as it inhibits the passing on of maternal genes to offspring. This is where the “grandmother hypothesis” comes into play. It suggests that as females age, it becomes more energy-efficient to support the survival of their grandchildren rather than having more children of their own, which also serves their genetic interests.
The true reason behind menopause is still debated and does not seem to be a widely adopted evolutionary strategy. Most species can reproduce until the end of their lives. However, wild chimpanzees from Uganda now join the list of menopausal animals, which includes killer whales, pilot whales, belugas, narwhals, and humans.
A comprehensive two-decade study on the Ngogo community of wild chimpanzees in Uganda’s Kibale National Park has revealed that these close relatives of humans begin to exhibit menopause-like symptoms around the age of 30. After two decades of declining fertility, no births have been observed after the age of 50, indicating a loss of reproductive ability similar to humans.
However, reaching menopause does not signify the end of their lives. Like humans, many female chimpanzees live beyond the age of 50, and they do not appear particularly interested in caring for their grandchildren, which challenges the grandmother hypothesis.
The evolutionary purpose behind this mechanism is still a subject of speculation. One possibility is that it might be a temporary response, given that post-reproductive lifespans have primarily been observed in captive chimpanzees rather than their wild counterparts, potentially indicating a response to favorable ecological conditions.
Alternatively, menopause could be a natural evolutionary trait for these chimpanzees that we have not extensively observed due to human activities and diseases that have negatively impacted their populations, causing them to die at a younger age.
According to study author Brian M. Wood, “We now know that menopause and post-fertile survival arise across a broader range of species and socio-ecological conditions than formerly appreciated, providing a solid basis for considering the roles that improved diets and lowered risks of predation would have played in human life history evolution.”
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