Killer whales and humans would seem to have little in common. We inhabit very different ecosystems, after all. Yet the two species share one unexpected biological attribute. Females of Orcinus orca and Homo sapiens both go through the menopause.

It an extraordinary aspect of our development. In contrast to the vast majority of animals on our planet, women and female killer whales stop reproducing halfway through their lives. Only one other species – the short-finned pilot whale (Globicephala macrorhynchus) – behaves this way.

The question is: why? For what reason do females of these three different species give up the critically important process of reproduction in middle age? According to Darren Croft of Exeter University, whose team has been studying killer whales for several years, there are many different theories. “Some have argued that it is an artefact that has appeared during our recent evolution and has simply persisted in our lineage,” he said. In other words, there is no specific reason for the menopause in humans. It is simply an evolutionary accident. However, Croft believes there is overwhelming evidence that the menopause is an evolved trait deep rooted in our past.

One idea to account for the deep-rooted evolution of this trait uses the concept of the “granny effect”: older females are programmed to close down their reproduction so they can devote themselves exclusively to the rearing of grandchildren. In doing so, they lose the ability to pass on their genes directly to one generation but gain because they can help the following generation to reach adulthood, thus promoting their genotype for the future, it is argued.

While not discounting the granny effect, Croft’s team – working with scientists at York University, Cambridge University, the US Center for Whale Research, and Fisheries and Oceans Canada – suggests the real explanation is more complex. “Our previous work shows older, post-reproductive females do help their offspring survive but that, on its own, does not explain why they stop reproducing,” said Croft. “Females of many species act as leaders in late life but still breed – elephants for example.”

Croft and his colleagues – whose recent research has just been published in the journal Current Biology – have been studying two populations of killer whales, which live off the north-west Pacific coast of Canada and the US. The populations included several pods, made up a several family groups. One of these pods – known as J pod, which currently consists of more than 20 individuals – was led by J2, or Granny, the killer whale matriarch. Granny was thought to be more than 100 years old – until her death, reported a few days ago, made headlines across the world. She had stopped reproducing more than 40 years ago, it is thought.

Killer whales normally start breeding around the age of 15 and then stop between the ages of 30 and 40 – although females often live until they are more than 80. In addition, it has been discovered that older female orcas play a particularly important leadership role in their family group. “They appear to be particularly good at pinpointing places to hunt salmon, the main source of food for the resident killer whales,” said Croft.

This point was backed by Deborah Giles, of the Center for Whale Research, who has been observing Granny and the J2 pod for years. “Granny was the ‘wise elder’ of that killer whale clan. She had an amazing ability to call the other whales to her by vigorously slapping her tail on the water. Even from miles away the other whales would turn around and come immediately to J2’s side,” she said.

However, it was the study’s observations of middle-aged orca mothers – those approaching menopause – that provided the real insights. It was found that these mother whales suffer much higher costs when competing to reproduce with younger mothers. These older mothers’ offspring were 1.7 times more likely to die than those of younger ones. “This new research shows that old females go through the menopause because they lose out in reproductive competition with their own daughters,” said Croft.

This point was backed by Daniel Franks from the University of York, a co-author of the study. “It’s easy to think that an older female will pass on their genes better by continuing to give birth in late life but our new work shows that if an old female killer whale reproduces, her late-life offspring suffer from being out-competed by her grandchildren. This, together with her investment in helping her grandchildren, can explain the evolution of menopause.”

This reproductive conflict hypothesis was originally proposed by Mike Cant of Exeter University and Rufus Johnstone of Cambridge University, who argued that conflict between generations was critical in explaining the appearance of the menopause in some species.

The new study was the first test of this hypothesis in non-human animals, added Cant. “Our theoretical predictions have turned out to be correct. We have captured a key piece of the puzzle of postreproductive life. Now we can explain not just why older females live so long after reproduction, but why they stop breeding in the first place.”