Aside from humans and some whales, most female mammals don’t live long past the end of their reproductive years.

A new study involving researchers from Arizona State University and University of Arizona reports a surprising exception to that rule — one that could either clarify or muddy our understanding of why menopause evolved in humans.

Research published in the journal Science reveals that females in the long-studied Ngogo community of wild chimpanzees in Uganda’s Kibale National Park spend up to one-fifth of their lives in a post-reproductive state.

That differs from most mammals, including other chimpanzee populations.

Urine samples from 66 females across various stages of life showed hormonal variations (gonadotrophins, estrogens and progestins) not unlike those used to diagnose human menopause.

But, unlike many humans, the chimps in question did not help raise their children’s children.

That fact could somewhat deflate the so-called “grandmother hypothesis” widely cited to explain the adaptive value of mammals living past their reproductive years.