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Women go through menopause. So do some toothed whales. Now, a new study has found that some chimpanzees do, too.

Even so, researchers still haven’t figured out what the evolutionary advantage of menopause is, or why it only impacts certain species, Popular Science reported.

Menopause usually occurs in human females between the ages of 45 and 55, and means a decline in reproductive hormones and an end to ovarian functions.

That’s what a research team, conducting a 25-year study of chimps at Ngogo in Kibale National Park, also found: They noticed many older females had not reproduced in decades.

They then calculated the post-reproductive representation (PrR), which measures the proportion of adult lifespan that an animal spends in its post-reproductive state.

While most mammals are close to zero, the Ngogo primates had a PrR of 0.2 – meaning that females live 20 percent of their adult years in a post-reproductive state.

Urine samples also showed that female chimpanzees transitioning to a post-reproductive state experienced hormonal changes.

But entering that new state didn’t mean the older chimps were involved in raising their grandchildren. This behavior challenges the “common grandmother hypothesis,” which sees menopausal females taking care of future generations – something observed in orca whales.

The team suggested the extended post-reproductive lifespans in the Ngogo chimpanzees could be attributed to two factors: One possibility is that, similar to captive mammals, these primates benefit from reduced threats like natural predators and pathogens.

Additionally, their relatively isolated habitat, less impacted by human activities such as hunting, may resemble their evolutionary past more closely than other primate populations.