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Parents, at least the human sort, know the benefits of having a grandmother close by: the extra help with child care, the reassuring advice borne from years of experience. In evolutionary biology, scientists call this the “grandmother effect,” and have hypothesized it’s one of the reasons humans live so long.
Now, a new study suggests that the effect isn’t limited to humans, and that killer whales also benefit from having grandmothers around. The study, published Monday in the Proceedings of the National Academy of Sciences, found that grandmother killer whales helped improve their grandcalves’ chances of survival, particularly when food was scarce.
The findings may shed light on an enduring mystery: why some whale species live for years after they go through menopause and stop reproducing. The study showed that, by stopping reproduction, grandmother killer whales avoided conflict with their reproducing offspring and helped their grandcalves find enough to eat when salmon stocks dwindled.
“Having a living grandmother improves your survival; you’re less likely to die when she’s alive than in the years following her death,” Stuart Nattrass, the study’s lead author, and a researcher at the University of Hull, in Britain, wrote in an email.
“We’ve known this is true of humans and a few other animals that don’t have menopause, like African elephants, for a while, and had a strong inkling that it was also true in these resident killer whales,” he wrote.
He and other researchers said the findings could be important for orca conservation, suggesting it’s just as vital to protect older, postmenopausal females as it is younger females of breeding age and their offspring.
Worldwide, there an estimated 50,000 killer whales, or orcas. But several populations have declined in recent decades, and some have become endangered, according to the National Oceanic and Atmospheric Administration.
To assess the survival rates of killer whales in lean years, the study examined census data for two populations, off Washington state and British Columbia, as well as annual catch numbers from chinook salmon fisheries in the Pacific Northwest.
The population off Washington state has been endangered since 2005 and is now critically endangered. It has just 73 orcas and four grandmothers, according to Deborah Giles, a killer whale researcher at the University of Washington’s Center for Conservation Biology. Giles, who was not involved in the study, called that scarcity of grandmothers “startling” and “scary.”
Orca matriarchs, she said, play a critical role in the species’ survival by guiding their families to fish when stocks are low and caring for the young while the mothers of breeding age hunt.
“It’s part of what makes killer whales amazing animals,” Giles said. “They are these large-bodied, long-lived apex predators, and a lot of the life span for post-reproductive females is spent caring for, and being there with, their families. We don’t see that as readily in other species.”
Female killer whales typically start reproducing in their teens and stop in their 30s or 40s. Yet they can live well into their 80s and 90s, posing the question for scientists of why this postmenopausal stage of life has evolved. Why stop breeding if the goal is to pass on your genes?
Nattrass said the study could point to some answers. It found that postmenopausal killer whales provided the biggest boost to their grandcalves’ chances of survival, beyond that provided by grandmother killer whales that were still breeding.
“This is a particularly striking example of a case where there might be a fitness benefit to not breeding yourself — you can better help your grandkids if you’re not preoccupied with a baby of your own,” he wrote.
Nattrass said postmenopausal killer whales are able to guide their families to salmon when it’s scarce, using stores of “ecological knowledge” gained over decades of life experience. Without that knowledge, Nattrass said, the grandcalves could die.
“As salmon stocks continue to fall, the presence of these grandmothers becomes more and more important,” Nattrass wrote. “But there is going to be a point where that knowledge isn’t enough. We really need to boost salmon stocks if these grandmothers are going to be able to help their families.”
Giles recalled one striking instance of this grandmotherly help: an aerial photo, taken in 2016, that showed a killer whale known as J2, estimated to be at least 75 and possibly older than 100, catching and sharing salmon with a recently orphaned youngster, presumed to be her granddaughter. The grandmother was feeding the youngster even as she was getting thinner and thinner toward the end of her life, Giles said.
“Here’s this old, old female still trying to make sure her family members have enough food to eat,” Giles said. “She could have eaten that fish in one bite. But she didn’t.”