Image caption Orca grandmothers develop close ties to infants in their pods

Scientists have discovered an evolutionary reason why humans and whales both have grandmothers.

As post-menopausal females age, the researchers say, they become increasingly interested and helpful in rearing their "grandchildren".

This could help explain why female great apes and toothed whales (cetaceans) have lifespans that extend long beyond their reproductive years.

They report the findings in the Royal Society journal Proceedings B.

It is specifically among great apes and toothed whales that menopause and post-reproductive helping have evolved Rufus Johnstone and Michael Cant, Evolutionary biologists

The "grandmother hypothesis" was first proposed in the 1950s. It stated that menopause, which stops a female's fertility well before the end of her lifespan, may have evolved to benefit a social group, because grandmothers went on to play such an important a role in caring for offspring that were already born.

Dr Michael Cant, from the University of Exeter in the UK, was one of the authors of this paper.

He explained that he and his colleague, Rufus Johnstone, looked at how humans and whales balanced "the costs and benefits of breeding with the costs and benefits of switching off breeding".

Dr Johnstone, who is an evolutionary biologist based at the University of Cambridge, told BBC News: "It's easy to forget about the cetaceans, but since they're the only other mammal apart from us [where females] have a comparable post-reproductive lifespan, it's important to study them in this context."

Previous studies have suggested that female chimpanzees and gorillas also go through menopause, but the conclusions are controversial.

The two scientists developed a mathematical model to study "kinship dynamics" in killer whales (orcas), short-finned pilot whales and humans.

This revealed that, as post-menopausal females aged, they developed closer ties to infants.

This showed, the scientists said, an "underlying similarity" between whales and great apes that might otherwise have been masked by the big differences in their social structures.

"Our analysis can help explain why, of all long-lived social mammals, it is specifically among great apes and toothed whales that menopause and post-reproductive helping have evolved," the researchers wrote in the paper.

Eric Ward, a scientist from the US National Oceanic and Atmospheric Administration in Washington, has carried out research into how post-reproductive females influence whale populations.

He told BBC News: "The model the authors propose is certainly interesting, and may explain the evolution of menopause in orcas."