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Surviving beyond the end of your reproductive life is a rare trait: only female humans, killer whales, and short-finned pilot whales are known to do it. The question is why?
If the purpose of life is to pass on your genes, as evolutionary biologists suggest, then an end to reproductive ability—menopause—really just makes no sense.
“From an evolutionary perspective, it’s just baffling,” says Darren Croft, who studies animal behavior at the University of Exeter. “Why would you give up reproducing when most other animals keep going until the end of their lives?”
Now Croft and his colleagues have found evidence that could explain the evolution of menopause in resident killer whales. They say that competition and conflict between aging mothers and their fertile daughters—rather than a sense of grandmotherly altruism—drove whales to stop giving birth later in life. The researchers suggest that a similar process could have led to the evolution of menopause in humans as well.
Previously, most attention has been paid to the advantages of menopause and to the so-called “grandmother effect,” in which older females help raise their children’s children. But Croft says that if the goal is to pass on and protect your own genes, that incidental effect is not enough to overcome the benefits of having more offspring of your own. Killer whales become fertile around age 15, hit menopause in their 30s or 40s, but can live into their 90s. “In order for menopause to work, something has to hold reproduction back,” says Croft.
The researchers hypothesized that the tight-knit social order of resident killer whale groups could have something to do with it. One of the key dynamics of resident killer whales—a subtype of killer whales that live in large kinship groups—is that a female whale’s offspring will stay with the pod for her entire life, and her daughters will bring their own offspring into the fold. As the composition of the pod changes over time, a female whale will find that more of the pod are her direct relatives.
When a female whale is young, she’s only somewhat related to the rest of the pod—her mother is there, but her father is in another pod. But as her own offspring come to make up more of the pod, the overall relatedness increases.
This means that while there is an evolutionary incentive for a young mother to act selfishly to help her own children—because they’re more related to her than to the rest of the pod—an older mother should be more inclined to cooperate with everyone equally.
In practice, this means that a young mother might share her food with her children, but not with the rest of the pod. This helps her children, even if it hurts the group as a whole. An older mother, on the other hand, will look out for the interests of the group. This lack of favoritism for her own newborn babies means that the offspring of older mothers get less preferential treatment than the offspring of young mothers.
Using a four-decades-long record of births and deaths in the northern and southern resident killer whale populations off the west coast of North America, Croft showed how this favoritism plays out in the real world. When older females have calves that overlap with calves from younger females, the older females’ calves are 1.7 times more likely to die before reaching sexual maturity at age 15.
“It’s a tug of war between young and old females, and the young pull harder,” says Croft.
John Jett, a biologist at Stetson University in Florida, says that it is not surprising that the offspring of older mothers suffer when they are born at the same time as their nieces and nephews, given how much time and energy older females devote to caring for all of their relatives. “Because female killer whales are such dedicated caregivers, the onset of relatively early menopause is a means of conferring a survival advantage to grandchildren while avoiding a dramatic erosion of survival [of their own offspring],” he says.
Though the population dynamics are different, it is believed that in ancestral human populations older women also grew more related to their group, says Croft. “It’s likely that very similar mechanisms explain the evolution of menopause in both whales and humans.”