Menopause is an evolutionary anomaly. Only in three species – humans, orcas and short-finned pilot whales – does female reproduction grind to a halt part-way through life.
For orcas (Orcinus orca), a new study published in Current Biology shows that menopause isn’t just a quirk of nature, but a reproductive strategy that specifically evolved to match their social structure.
The whales’ unique social structure, combined with the benefits that older females provide the pod, have created a perfect crucible for menopause to evolve, says study lead author and University of Exeter behavioural ecologist Darren Croft.
For most animals, bearing as many offspring as possible over your lifetime is the best way to ensure your evolutionary legacy, or “fitness”. Biologists have long sought to explain why menopausal animals forgo this seemingly sensible strategy.
One explanation – dubbed the “grandmother hypothesis” – suggests that females stop reproducing to assist their children bear grandchildren.
This does provide some evolutionary benefit, but it’s not as potent as bearing your own offspring, nor does it explain why females sacrifice up to half their reproductive life. Female orcas, for instance, bear their last calf in their 30s or 40s, but can live well into their 90s.
The more recent “reproductive conflict hypothesis” suggests that competition between females might play a role.
To see if this was the case, Croft and colleagues examined an “incredible dataset” of more than 500 births over 43 years in salmon-eating orcas living off the west coast of North America.
Overall, a mother’s age had no bearing on whether or not a calf made it to adulthood. But this wasn’t the case when mothers and daughters bred at the same time, when calves born to older females were 1.7 times more likely to die.
Croft puts this down to social structure. Male and female orcas stay with their family – or “pod” – for life, but mate with whales from other pods. This means young females aren’t related to pod members as much as older females, who have already given birth to multiple calves.
So it makes sense for a young female to compete tooth and flipper for a share of resources that improve her calf’s chances of survival. An older female, in contrast, gains from providing for the whole pod – sharing food and leading them to fertile fishing grounds. This is especially so for her sons, who can father more grandchildren than her slower-reproducing daughters.
“It’s this tug of war between old and young females over resources,” Croft says.
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