Menopause is a mystery to evolutionary biologists, but new insights could come from a long-term study of killer whales.
In these whales, the explanation may lie in a combination of conflict and cooperation between older and younger females, according to a report published Thursday in the journal Current Biology.
Killer whales are one of only three species known to have menopause — the others are pilot whales and humans. Researchers have long wondered why it was that these few species evolved to have females that spend so much of their lives unable to have babies.
Killer whales start reproducing around age 15, but stop having calves in their 30s or 40s, even though they can live for around a century.
A team led by behavioral ecologist Darren Croft of the University of Exeter decided to search for answers with the help of an unusual long-term study of killer whales in the Pacific Northwest. There, since the 1970s, researchers have carefully collected information on the births and deaths of individual whales that live in family groups.
Contained within the data is an intriguing clue about why female whales may stop reproducing later in life.
When older females reproduce at the same time as their daughters, who live alongside them, the calves of the older mothers are nearly twice as likely to die in the first 15 years of life. But when older mothers had calves in the absence of a reproducing daughter, their calves did just fine.
"It's not that older mothers are bad mothers, that they're not able to raise their calves as younger mothers," says Croft. "It's that when they enter into this competition with their daughters, they lose out and their calves are more likely to die."
The competition may center on access to food, says Croft, because there's good reason to believe older females feel more pressure to share their precious fish with the others around them.
That's because, in killer whales, females mate with males from other groups but then rejoin their families. That means when a new calf is born, its father is not around, and females start their lives in a situation where their relatedness to the group is rather low.
As a female grows older and starts having calves that stay with her, however, she develops more kinship ties to those around her. "It may be that older females are more likely to share, and younger females are less likely to share food," says Croft. That would mean younger females would have more resources to lavish on their own calves.
It's clear that in these whales, older females play an important role in the survival of not just their own calves, but all of the family members they live with. "If an old female dies, her son's risk of dying in the year following her death is over eight times higher than if his mother was still alive," says Croft, "and these are adult sons, these are not juveniles, these are 30-year-old, fully grown males."
The idea that older females safeguard and enhance their genetic legacy by protecting and providing for their children and grandchildren has been an influential explanation for why menopause evolved. It's known as the Grandmother hypothesis, and was developed by anthropologists who studied hunter-gatherer cultures.
But Croft thinks that alone isn't enough to account for menopause, because other long-lived, social species, like elephants, have older females that help their group but continue to bear young until the end of life. "Just the fact that these old females can store information and share that with the group and increase their survival doesn't explain why they stop reproducing," says Croft.
Proponents of the Grandmother hypothesis, however, may not be so convinced that intrafamilial conflict plays an important role.
Anthropologist Kristen Hawkes, at the University of Utah, says the killer whales are fascinating, but that they're hard to study. "They're doing all kinds of stuff where you can't see it, and even to get demographic data is just so tricky, because they're all underwater and they're long-lived," she says.
She points to one recent study on food-sharing in killer whales that found older females share fish with their older adult sons, perhaps to maximize the males' ability to sire more babies.
If that's the case, she says, "it's not the older females and younger females in competition, it's the older females contributing to the enormous success of their sons, and then those baby whales are all born somewhere else. They're not competing, because their moms are elsewhere."
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. Transcript provided by NPR, Copyright NPR.