Humans, pilot whales and killer whales are the only known species in which females have a prolonged period of menopause — a time of life when they cannot reproduce. Now, a study in the journal Science reports the purpose that menopause serves in orcas: for females to care for their sons and make sure their genes are passed on to future generations.

“Females have a really unique life history,” said Emma Foster, a marine biologist at Exeter University in England. “They stop reproducing in their 30s and 40s, but they can live into their 90s.”

Using 36 years of data on orcase in the Pacific Northwest, the researchers found that for males over 30, the death of a mother meant an eightfold increase in the likelihood of death within a year.

Killer whales stick with their mothers their entire lives. Dr. Foster suspects that mothers help sons with foraging or offer protection in encounters with other males. Among female orcas over 30, there was only about a threefold increase in the likelihood of death in the year after a mother’s death. “It makes more sense for the mothers to invest more in their sons, because there is no increased burden on the family group,” Dr. Foster said. “Children of sons move on to new family groups.”