Female humans, pilot whales and killer whales are unique among animals: they are the only three mammals known to undergo menopause, National Geographic reports. Killer whales were just added to this list.

Rather than stay fertile until they die, these females lose their ability to have babies well before the end of their lives. This allows them to help their children survive into adulthood and—if they’re lucky—to act as grandmothers to the next generation.

Female killer whales, researchers from the University of Exeter found, can have babies from sexual maturity until their 30s, but may live an additional 40 or 50 years as infertile caretakers for those around them. The researchers monitored around 500 killer whales for more than 30 years.

This life history pattern likely evolved as a way for the female whales to ensure their genetic mark in the world does indeed get passed on. “For menopause to evolve, the benefits of stopping reproduction in late life have to outweigh the costs,” the researchers explained to NatGeo. Evidence for this abounds in killer whales. In the year after their mother died, for example, male killer whales’ chance of dying themselves was 14 times greater than that of whales whose mothers were still around to protect them, NatGeo writes.

There may be other species whose female members go through menopause, the researchers told NatGeo. But for now the only ones scientists are sure of are those special three. Whether female pilot whales and killer whales also experience typical human symptoms of menopause, such as hot flashes, is also an open-ended question, they say.

More from Smithsonian.com:

New Evidence that Grandmothers Were Crucial for Human Evolution

Can We Blame Men for Menopause?