Scientists studying “tooth rake marks” – the scarring left when one whale scrapes their teeth across the skin of another – found males had fewer marks if their mother was present and had stopped breeding.
Only six species – humans and five species of toothed whales – are known to experience menopause, and scientists have long been puzzled about why it occurs.
Female killer whales live for up to 90 years in the wild, and most live an average of 22 years after menopause.
Scientists have wondered why humans and some whale species spend a significant portion of their life not reproducing.
Previous research has shown that, even after having their last calf, killer whale mothers take care of their families by sharing the fish they catch.
The new study, by researchers from the universities of Exeter and York and the Centre for Whale Research in the US, adds to growing evidence that post-menopause females boost the life chances of their offspring, especially males.
Study lead author Charli Grimes, from the University of Exeter, said: “We were fascinated to find this specific benefit for males with their post-reproductive mother.
“These males had 35 percent fewer tooth marks than other males.
“For males whose mother was still breeding, we found no evidence that her presence reduced tooth rake injuries.
The study, published in the journal Current Biology, is part of long-term research on “southern resident” killer whales, which live off the Pacific coast of North America.
The body of evidence suggests that – instead of competing with their daughters to breed – female killer whales have evolved to pass on their genes by helping their children and grandchildren.
The researchers speculated on why female orcas focus efforts on their sons.
Grimes, an animal behavior scientist, said: “Males can breed with multiple females, so they have more potential to pass on their mother’s genes.
He added: “It’s fascinating to see this post-menopausal mother-son relationship deepening our understanding of both the intricate social structures in killer whale societies and the evolution of menopause in species beyond humans.”
Grimes said: “It’s possible that with age comes advanced social knowledge.
Croft added: “We’ve got hypotheses, but we need to test them by seeing what’s happening underwater when these different groups interact.
“We’ve learned so much from this population, but we’ve still got so much to learn from them.”
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