Out of all the mammal species in the world, only three are known to experience menopause: humans, short-finned pilot whales and killer whales. But the exact reasons for it in killer whales (known formally as Orcinus orca) have remained up for debate.
"Why females of some species cease ovulation prior to the end of their natural lifespan is a long-standing evolutionary puzzle," the study authors wrote. "The fitness benefits of post-reproductive helping could in principle select for menopause, but the magnitude of these benefits appears insufficient to explain the timing of menopause."
Certainly, long beyond their most fertile years, older females play crucial roles in the lives of many social mammals. Researchers have argued that the presence of grandmothers in human populations helped improve the survival rates of their daughters' offspring. In a herd of elephants, typically run by a long-lived matriarch, older females help out with grandchildren and other relatives. Grandmothers help improve survival in these groups because they often find and share food resources communally. That's also true of killer whales, the researchers pointed out.