Female humans aren't the only mammals known to go through menopause, then live for many years after—some whale species also undergo the process, which is when menstruation stops for good. Now, in what the Washington Post calls a "landmark discovery," researchers say chimpanzees have also joined the menopause ranks, living "well past the end of their ability to reproduce." In a study published Friday in the journal Science, scientists note that they looked at the fertility and mortality rates for 185 wild female chimps in the Ngogo community in Uganda's Kibale National Park, based on more than two decades of observation that ended in 2016.
Using those observations and newer urine samples, the researchers found that the chimps spent, on average, about 20% of their lives in a post-reproductive state. Older female chimps appeared to undergo a shift in various hormones, such as estrogen, just like their human counterparts. When this change happens is also similar to the human timeline, with a decline in fertility starting around age 30 and dropping off altogether by 50. Signs of menopause have been observed in chimps in captivity before, but it's been more difficult to spot it in wild populations. That may be because wild chimpanzees simply don't live long enough to reach that state.
The Ngogo chimps are an unusual case, however, in that they are indeed wild, but they're also left alone, more or less, by humans. There also aren't many other predators, and food is plentiful. This means that resident chimpanzees there tend to live longer than they might in other wild habitats. One, a 67-year-old female named Garbo, is currently the oldest in the pack, and others have lived to be almost 70. But why do only a few mammal species go through menopause, then live for years after, as the "genetic priority" for animals is to reproduce, per the Post?
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One theory on why human females typically see reproductive cycles cease way before they die involves the "grandmother hypothesis." That theory suggests older females stop giving birth to their own babies so that they can help with their children's offspring. However, STAT notes that the Ngogo chimps "spend pretty much no time grandmothering," as daughters typically leave the group they were born into once they become adults; sons similarly don't seem to benefit from their mom's assistance once they have young of their own. The Post notes that even though that part isn't clear for chimps, the scientists' findings on "the uncanny similarity" between humans and chimps "could shed light on how menopause evolved in humans." (More menopause stories.)
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