Female killer whales stop reproducing in their 30s-40s, but can survive into their 90s.
While different theories have been put forward for the evolution of menopause in humans, including the well established 'grandmother' hypothesis, there has been no definitive answer to why females of a small number of other species, including killer whales.
The research team, from the Universities of Exeter and York analysed records spanning 36 years, of the members of two populations of killer whales in the North Pacific ocean, off the coast of the USA and Canada.
They found that the presence of a female who was no longer reproducing significantly increased her older offspring's survival.
In the case of males over the age of 30, a mother's death meant a 14-fold increase in the likelihood of their death within a year.
Females also stay within their mother's group but for daughters of the same age, the difference is just under three-fold. For females under the age of 30, the death of their mothers had no effect on their survival rates.
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Killer whales live in unusual social groups, with sons and daughters staying with their mothers in a single group throughout their lives.
With this close association, older mothers have the opportunity to increase the transmission of their genes by helping their adult offspring survive and reproduce.
When sons mate, their offspring are cared for by females in another group, whereas when daughters reproduce the offspring stay in the group, which increases local competition for resources within the group.
Theory predicts that in order to have the best chance of spreading their genes, without carrying an additional burden, mothers should focus their efforts on their sons.
This research backs up this theory and demonstrates the extent to which older sons are dependent on their mothers for survival.
"Our analysis shows that male killer whales are pretty much mommy's boys and struggle to survive without their mother's help," said Dr Dan Franks, from the Department of Biology at the University of York, said.
"The need for mothers to care for their sons into adulthood explains why killer whales have evolved the longest post-reproductive lifespan of any non-human animal," Franks said in a statement.