What whales tell us about the evolution of menopause

Scientists have been hard pressed to explain why menopause happens so early in humans there's no obvious evolutionary advantage to having your reproductive system shut down decades before the rest of your body.

Most other long-lived animals keep reproducing until the end. Female turtles can lay fertile eggs at 100. Our primate relatives, too, keep pumping out young until they are near death.

Now, scientists are finding clues to our unusual life pattern in killer whales - one of the few other species in which females get decades of so-called post-reproductive life. What they found was a surprising connection between longevity of mothers and their sons.

Biologist Emma Foster of Exeter University in England said that females become fertile around 12, have a calf every 3 to 5 years, and then stop reproducing in their late 30s and early 40s. After that they can live many years, sometimes to 90 and beyond. "No other animals have such long post-reproductive lives," she said, except for pilot whales and humans.

And while there's a small difference between the sexes in human longevity, it's extreme for whales, with females living to 90 and males rarely getting past 40. Scientists have little understanding of why this would be.

In puzzling over this anthropologists have proposed what they call the grandmother hypothesis - the possibility that post-menopausal women gain an evolutionary edge by helping their existing children and grandchildren.

In Darwinian terms, after all, no matter how many offspring you have, you'll still be a dead end unless your offspring have surviving offspring.

Ultimately, according to the hypothesis, grandmotherly support could allow women to pass on more copies of their genes than by investing the same energy in continuing to have their own babies.

The other possibility is that menopause is some sort of evolutionary byproduct that can occur in long-lived animals.

Scientists have tried to test the grandmother hypothesis by studying hunter-gatherer populations - people representing the way humans lived through most of our evolutionary past. Results over the years have been mixed, but work in the last decade has supported the idea, said Foster.

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What whales tell us about the evolution of menopause