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.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 conjecture there is overwhelming evidence that the menopause is an evolved trait deep rooted in our past.
While not discounting the granny effect, Croft’s team – working with scientists at York 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.
“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.”