Menopause: Wild Female Chimps' Unexpectedly Prolonged Lifespan

Female chimpanzees in an East African forest have been observed to go through menopause and live for many years, sometimes decades, beyond their reproductive age.

This is the first documented instance of nonhuman primates in the wild undergoing menopausal changes and outliving their fertility. As such, it prompts fresh inquiries into the evolution of menopause.

To date, instances of females experiencing menopause and living for substantial periods afterwards have only been seen in humans and five species of whale. The reason why such longevity exists post-reproduction is currently unknown.

While the evolutionary reasons for menopause are still uncertain, this new discovery highlights the close genetic relationship between humans and chimpanzees. Both, according to anthropologist Brian Wood from UCLA, are more likely to survive post-reproduction than other great ape species.

It is believed that if chimpanzees live long enough, their fertility ends at a similar age to humans. Previous studies, however, have shown chimpanzees ageing rapidly and typically dying in their early 30s, still in their reproductive phase.

For their study, Wood’s team studied the mortality and fertility rates of 185 females in the Ngogo community of wild chimpanzees in Uganda's Kibale National Park from 1995 to 2016. The team discovered that chimps' fertility decreases after age 30, and usually stops by age 50. In the Ngogo community, females live about 20% of their adult lives post-reproduction.

The study challenges the theory that human menopause evolved because grandmothers provide essential care to their grandchildren. Since female chimps tend to leave their native communities during adolescence and don't have immediate families to help after menopause, other possibilities are that they may carry vital experience and knowledge needed for the group’s survival.

The researchers also suggested other factors that could promote longevity in chimps: a lack of predators, abundant food sources, successful competition with nearby chimpanzee groups, and protection from human activities.

However, the limited insights from studying captive animals may not reveal how menopause evolved in the wild. The researchers speculated that menopause might have evolved in wild chimpanzee populations with minimal human disturbances, due to the practice of young females migrating to new groups.

After these migrations, older females with kin in the community may stop reproducing to allow younger females to conceive new generations. However, this theory has been disputed by evolutionary biologist Susan Alberts from Duke University, who believes that chimpanzees, being slow reproducers, may not become genetically related to enough individuals to support this theory.

Instead, the unusually potent longevity-enhancing conditions for Ngogo chimps may enable females to display an evolved capacity for surviving well beyond the reproductive years, Alberts says.

Alberts and colleagues previously analyzed long-term reproductive data for seven nonhuman primate species — including chimps — and a population of African hunter-gatherers. Females in the nonhuman primate species could survive reproductive declines for a year or two, but lower death rates in hunter-gatherers resulted in extended lives after menopause, Alberts says. “Menopause may be a latent trait in primates that gets revealed as mortality rates decline,” she says.

Few wild chimp communities have been studied as thoroughly and for as long as the Ngogo crowd. That makes the new menopause findings tough to generalize to wild chimps living elsewhere. “We still don’t know much about most chimps’ lives,” Alberts says.

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