ROBERT SIEGEL, HOST:
A mystery now of the reproductive kind. In most animals, females keep having babies until the end of life. There are only three known exceptions - humans, short-finned pilot whales and killer whales. We have some clues in the case of why killer whales experience menopause, as NPR's Nell Greenfieldboyce reports.
NELL GREENFIELDBOYCE, BYLINE: Female killer whales start having babies when they're around 15 years old, and these whales can live a long time.
DARREN CROFT: They certainly get to 80 years old - possibly 90 years old - but the incredible thing is that they stopped reproducing in the 30s to 40s.
GREENFIELDBOYCE: That's Darren Croft, who studies animal behavior at the University of Exeter in the United Kingdom. He says menopause seems to make no sense. As far as evolution is concerned, the whole point of life is to churn out as many little babies as possible.
CROFT: So it seems unusual that a female should give up the opportunity to have the direct offspring to transfer her genes directly part way through life. And certainly most mammals - most species - don't do that.
GREENFIELDBOYCE: Killer whales do, and luckily scientists in the Pacific Northwest have been recording the births and deaths of these whales since the 1970s.
CROFT: This data spans over four decades.
GREENFIELDBOYCE: Now, killer whales live in family groups. An older female lives with both her sons and her daughters. And the researchers noticed something interesting. If an older female whale has a baby at the same time that her daughter is having babies, the calf of the older mother is almost twice as likely to die.
CROFT: It's the calves of the older mothers that have higher risk of mortality.
GREENFIELDBOYCE: But when older moms have babies by themselves, they do just fine.
CROFT: So it's not the older mothers are bad mothers - that they're not able to raise their calves as well as younger mothers. It's that when they enter into this competition with their daughters, they lose out, and their calves are more likely to die.
GREENFIELDBOYCE: So what are they competing for? Probably fish. Whales societies are complicated, but the bottom line is as a female gets older, her genetic relationship with her group grows stronger. That means she probably becomes ever more willing to share fish, leaving less for herself and her babies. Croft says it's this urge to help out your relatives later in life...
CROFT: ...Combined with competition between the generations that is really key to unlocking this mystery of menopause.
GREENFIELDBOYCE: The findings appear in the journal Current Biology. And needless to say, they'll leave people wondering if something similar was going on in human ancestors. Croft knows not everyone will see a connection.
CROFT: There is going to be mixed feelings about it because there are competing hypotheses...
GREENFIELDBOYCE: ...Including the so-called grandmother hypothesis. That's the more warm and fuzzy idea that grandmotherly, post-menopausal women succeed by helping their children and doting on their grandchildren. Kristen Hawkes at the University of Utah is one of the anthropologists who proposed that after watching grandmothers in hunter-gathering cultures. She thinks the whales are super cool, though hard to study...
KRISTEN HAWKES: ...'Cause they're doing all kinds of stuff where you can't see it. And even to get demographic data on them is just so tricky because they're all underwater, and they're long-lived.
GREENFIELDBOYCE: She's not ready to buy the idea that conflict between mothers and daughters may have helped produce menopause. She thinks we need to know a lot more about these whales. Nell Greenfieldboyce, NPR News.