Scientists have found the answer to why female killer whales have the longest menopause of any non-human species – so that they stick around long enough to care for their grown-up sons.
In a study published in the journal Science, researchers found that for a male whale over 30, the death of his mother means an almost 14-fold increase in the risk he will also die within the following year.
Yet males whose mothers live well beyond their reproductive years are more likely also to live to older age, they found.
"Male killer whales are pretty much mummy's boys and struggle to survive without their mother's help," said Dan Franks of the University of York, who worked on the study.
The reason for the menopause remains one of nature's great mysteries. Very few species have a prolonged period of their lifespan when they no longer reproduce, as humans do.
But women can look to female killer whales as kindred spirits. They stop reproducing in their 30s and 40s, but can survive into their 90s.
"Killer whales are extraordinary animals and their social groups are really unusual in that mothers and their sons are lifelong companions," said Emma Foster, a PhD student at Britain's University of Exeter, who led the study.
"Our research suggests they have developed the longest menopause of any non-human species so they can offer this level of commitment to their older offspring."
Ms. Foster's team, which included scientists at the United States Center for Whale Research and Pacific Biological Station in Canada, analyzed 36 years of records on the members of two populations of killer whales in the northern Pacific Ocean, off the U.S. and Canadian coasts.
They found that the presence of a mother killer whale who was not reproducing significantly increased her offspring's survival.
For males over the age of 30, a mother's death meant a 14-fold increase in the likelihood of their death within a year. But for daughters of the same age, the difference was just under three-fold. For female killer whales under the age of 30, their mothers' death had no effect on survival rates.
Ms. Foster said the study suggests that female killer whales who stop having offspring but increase the survival of their sons can maximize the transmission of their genes without increasing competition within their own group, as would be the case when a daughter reproduces.
The researchers said they had little data on the specific ways mothers may help their adult sons survive longer, but aiding them in the hunt for food and helping them out in dangerous situations may be two possible explanations.