David Ellifrit/Center for Whale Research
Many human grandmothers love to spoil their grandchildren with attention and treats, and for good reason: Studies have shown that having a living grandmother increases a child’s chance of survival. Now, new research shows the same may be true for killer whales. By providing young animals with some freshly caught salmon now and then—or perhaps with knowledge on where to find it—grannies increase their grand-offspring’s chance of survival.
The new study is the first direct evidence in nonhuman animals of the “grandmother hypothesis.” The idea posits that females of some species live long after they stop reproducing to provide extra care for their grandchildren.
“It’s very cool that these long-lived cetaceans have what looks like a postfertile life stage,” says Kristen Hawkes, an anthropologist at the University of Utah in Salt Lake City who has dedicated much of her career to studying the grandmother effect; she was not involved in the new study.
Women usually go through menopause between ages 45 and 55, even though they may live to age 80, 90, or older. Studies in modern-day hunter-gatherer communities as well as in populations in Finland and Canada show that older women can help increase the number of children their daughters have, and boost the survival rates of their grandchildren. Dan Franks, a computer scientist and biologist at the University of York in the United Kingdom, wanted to know whether this grandmother effect occurs in other species as well.
Enter killer whales, or orcas, one of the few animal species—along with narwhals, belugas, and short-finned pilot whales—in which females outlive their fertility. Female killer whales stop having calves in their late 30s or early 40s, but they can live past 100. (Males generally have shorter life spans and keep reproducing until the very end.)
Franks and his colleagues analyzed survival data from two groups of killer whales off the coast of Washington state and British Columbia in Canada that have been surveyed for decades. They used underwater cameras that allow scientists to identify individual whales by their distinctive markings and follow them as they age.
Among the more than 700 whales in the two groups, Franks was able to find 378 with known maternal grandmothers. His team analyzed their survival rates, making sure to control for the available supply of food. When a grandmother died, her grandchildren were 4.5 times more likely to die during the following 2 years than other whales in the community, the researchers report today in the Proceedings of the National Academy of Sciences.
Grandmothers provided an extra boost to their grandoffspring in “times of need,” Franks says. This included when populations of Chinook salmon—the meal of choice for hungry orcas—were low. That’s likely because they either shared their catch with the kids or helped the community find resources based on their memories of past low-salmon year, or both.
Past studies have shown a similar effect for postreproductive mother orcas: Middle-aged male orcas whose mothers are still around—scientists call them “mommy’s boys,” Franks says—also have significantly better chances of survival.
In the new study, male and female grand-offspring benefited equally from the presence of a grandmother—a surprising finding, because young male orcas are thought to be worse at catching salmon than their female counterparts. Perhaps what matters is not just the salmon itself but what Franks calls the grandmoms’ “leadership”: “They have more wisdom, and they have more information about foraging for salmon,” that they can pass on to grandchildren of both genders, he says.
“In killer whales, what granny knows is really, really important,” says Hal Whitehead, a biologist at Dalhousie University in Halifax, Canada, who has studied social structure among killer whales but was not involved in the new study.
Still, the benefits to grand-offspring can’t fully explain why the females stop reproducing at a relatively young age, giving up the chance to continue to pass along their genes. Other factors likely play a role as well, Franks says. A study published in 2017, for instance, suggested that when female orcas of multiple generations breed simultaneously, the offspring of older mothers are less likely to survive than those of younger moms because of competition over resources. That, too, could be a reason why older females stop reproducing and focus on their daughters’ offspring instead.
The new study is based on “particularly fantastic data,” Whitehead says. It’s “not only revealing about killer whales,” he says, “but it also tells us a bit about how we should think about [the evolution of menopause] in humans.”
In humans, Hawkes adds, researchers are still working out whether creatures that outlive their fertility simply evolve longer life spans, or whether natural selection causes them to experience menopause earlier. “We don’t have a time machine,” she says. “You can’t go back and say, ‘What was it like then?’” But with more data on other species, she says, we can make better guesses.
Source: Science Mag