By Katharine Gammon, Inside Science
(Inside Science) — In the chilly Pacific waters off Canada, orca whales glide through the dark seas. The majestic creatures may look like they have no cares in the world, but whales now face an ocean with fewer fish.
In tough times, they turn to those who know best: grandmothers. Orca whale females can live to be well over 100. Their longevity isn’t tied to their ability to reproduce – most females give birth to their final calf in their thirties. But the knowledge that the old ladies have helps keep the group alive.
The orcas feed nearly exclusively on Chinook salmon, but the fish aren’t always in the same place during the same season.
“There’s variation in salmon abundance across years, due to natural differences,” said Darren Croft, a director of research at the University of Exeter in the United Kingdom. “One hypothesis is that these old females gain knowledge through experience about the salmon.”
Croft and a team of researchers set out to dive into video data showing which whales were leading the pod at which times. This data was combined with information on where salmon were at different times.
The scientists found that females were more likely to lead the groups than males, and the older females were more likely to lead when fish were hard to find.
“These whales really live on a knife edge,” said Croft. “In years of low salmon, it’s a driver of mortality — these animals have to know where and when to find them.” The research was published in March in the journal Current Biology.
This new finding supports earlier work by Croft and colleagues, which showed that if a male orca’s mother died before his thirtieth birthday, he was three times more likely to die the next year. And as the mom got older, she was even more helpful to him: if she was already past reproductive age, he was fourteen times more likely to die in the year following his mom’s death. The effect was smaller with female offspring than with male, but it was clear that older females did have an impact on the group.
Scientists have questioned why this might be happening. Whales tend to stay with their moms most of their lives.
“As a female ages, her relatedness in her group increases,” explained Croft. “She gains offspring around her as she ages, and there’s a tipping point where she should stop reproducing and start helping.”
The idea that grandmothers have important roles to play in the evolutionary fitness of human groups dates back decades – and perhaps centuries – in anthropology and has been debated widely. Did menopause evolve as an adaptive trait, because older women could pass knowledge down to their children?
Kristen Hawkes, an anthropologist at the University of Utah in Salt Lake City, has been researching the so-called grandmother hypothesis since the 1980s in groups like the Ache of Paraguay or the Hadza of Tanzania, where around half of women survive past their child-bearing years.
Hawkes said that part of the reason that human grandmothers are so vital is that we have a long lifespan and mature more slowly compared with our primate cousins – and that comes at a cost.
“If you’re maturing later, then your rate of baby production is slower,” she said. “But when grandmothers are around to subsidize the feeding of those kids, then moms are not stuck with that kid all the way to independence.” That way, mothers can have another baby.
Hawkes said that while grandmothering in whales is probably not like that in humans, it’s still a fascinating look into the evolution of menopause.
Croft said that whales have the opposite of modern medicine working on their side – in fact, their lives have gotten harder and more dangerous over the past few decades, as fish stocks have declined precipitously. Still, there continue to be female whales leading the group well past their reproductively useful time. And whales seem to be apart from other large mammals, like elephants.
“We’ve known that elephants have matriarchal society and older elephants have social knowledge – but they continue to reproduce for their entire lifespan,” said Croft. “That leaves a big question: Why do whales stop reproducing?”
In the future, Croft hopes to study so-called transient orca whale populations, which eat seals, don’t live in large groups and don’t have a set migratory pattern. Unlike the resident groups that have been catalogued and studied for decades, little is known about transient group structure.
On land or in the ocean, there’s always a time to have gratitude for grandmother’s knowledge – especially when it comes to food.
Katharine Gammon is a freelance science writer based in Santa Monica, California, and writes for a wide range of magazines covering technology, society, and animal science. She tweets at @kategammon.
Reprinted with permission from Inside Science, an editorially independent news product of the American Institute of Physics, a nonprofit organization dedicated to advancing, promoting and serving the physical sciences.