It's long been thought that, aside from a few species of toothed whales, humans are the only mammal that experiences menopause. But researchers have determined that the females of the Ngogo group of chimpanzees in Uganda do experience menopause, and become infertile at about the age of 50. Link to the article in Science here.
The researchers not only observed that females over the age of 50 stopped having offspring, menopause was confirmed by a hormonal analysis of the urine collected from those females.
Kevin Langergraber, a primatologist at Arizona State University, and one of the authors of the study, told The New York Times (link here ) that he leans toward the explanation that chimps inherently are menopausal, and it is only now being observed in this population because only chimps in this population, due to a favorable environment, have lifespans that typically exceed 50 years.
The common explanation for the uniqueness of human menopause has been the Grandmother Hypothesis. Humans are so intensely social and cultural that evolution favored older females who didn't have their own children at that frail age, but instead assisted in the unusually great effort and time required to raise grandchildren.
But menopause in this chimpanzee group (not to mention the toothed whales) undermines that neat theory, because female chimpanzees leave their birth groups at adulthood. So grandmothers don't live in the same groups as their adult child-bearing daughters. Further, grandmother chimps are not seen to provide extra food to their grandchildren.