How does a male orca stay out of trouble? With a lot of help from mom
Study finds post-menopausal orcas keep their adult sons from scrapping with other whales
What do female orcas do when they're finished having babies? Protect their adult sons from ill-advised fights, for one.
A new study has found that male southern resident killer whales are less likely to get scraped up by other whales when they have their post-menopausal mothers by their sides.
"This indicates that these post-menopause mothers are directing social support and protective behaviour towards their male offspring, and helping them avoid potentially dangerous interactions with other whales," lead researcher Charli Grimes told As It Happens guest host Robyn Bresnahan.
The findings — published this month in the journal Current Biology — add to a long list of ways that orca mothers take care of their sons, long after they've grown up into five-tonne adults.
"[Male orcas] have got a whole ocean to swim in, and we see them swimming really close with their moms. You know, they seem to be really dependent on them," Grimes, an animal behavioural scientist at the U.K.'s University of Exeter, said.
"If your mom's going to look after you, why look after yourself?"
The menopause mystery
Female orcas experience menopause and lose the ability to reproduce when they're around 40 years old. They can go on to live another 40 or 50 years after that.
"From an evolutionary perspective, animals should continue to reproduce until they die to pass on as many of their genes as possible to their future generations. So this is a really unusual strategy we're seeing in these whales," Grimes said.
In fact, other than orcas, only humans and four other species of toothed whales are known to experience menopause.
"A longstanding question in our own life history [of] evolution is how and why has menopause evolved?" Grimes said. "And now we've got this unique opportunity to explore it in killer whales that exist in really different physical and social environments."
The team looked at southern resident killer whales, a highly studied — and highly endangered — population of fish-eating orcas who live off the coasts of British Columbia, Washington and Oregon.
By studying photographs and census data on this population, the researchers determined that males with post-menopausal mothers in their pods were far less likely to have tooth rake marks than males whose mothers are still of breeding age, or those who don't have a mother at all.
The marks happen when another orca scrapes their teeth against the male's skin during a bout of fighting or roughhousing. Orcas don't have any natural predators, and the southern resident population only eats salmon and other fish, so the tooth rake marks almost always come from members of their own species.
The team hasn't observed the behaviour directly, but Grimes says it's possible the elder females use their knowledge and experience to "help her sons navigate the potentially risky interactions."
"Or it could be that she is directly involving herself when a fight looks likely," she said.
Killer whale biologist Deborah Giles, who wasn't involved in the study, says the findings shine a light on the important role of older female orcas.
"It helps us explain some of what we see in these whales — and you know, quite honestly, probably other species of whales, if not even humans — where the grandmothers, the post-menopausal females in the group, are so important," Giles, the scientist and research director at the non-profit Wild Orca, told CBC.
"And it makes sense as far as, like, the postmenopausal mothers have more time to dedicate to their older sons, which is important because these older males are the ones that are preferentially mating."
A healthy adult male orca has the potential to mate with a variety of females. So having strong, healthy, long-living sons is a mother's best chance of ensuring that her genes are passed on.
In fact, keeping them out of scrapes is just one way orca mommas look out for their boys. Mother orcas guide and feed their sons throughout their adult lives — often at the expense of their own well-being.
One 2012 study found that male orcas over 30 were eight times more likely to die in the year following their own mothers' deaths.
Orca mothers don't, however, adopt a similar protective stance with their daughters. That's in part, says Giles, because the daughters don't need it.
"I don't think the females roughhouse in the same way," she said.
But it's also a matter of getting more bang for your buck, says Grimes.
"Both males and females will stay with their moms for their whole lives. But mating males have the opportunity to meet with multiple females, and when they do, they do so outside of their own family group, and so the burden of that cost falls on another group," she said.
"Whereas when a female breeds, she has to carry the calf herself for up to 18 months and that calf is raised inside her own family group, which comes at a cost in terms of resources."
Struggling population
Southern resident killer whales are listed as endangered in both Canada and the U.S. There were only 73 alive as of January, according to the Department of Fisheries and Oceans.
The reasons for this are myriad, according to the U.S. National Oceanic and Atmospheric Administration, and include increased noise from boats and pollutants in the water that could reduce reproductive success.
But the biggest problem, says Giles, is the lack of salmon to eat.
"We're losing females in breeding age because there's not enough food. And so there's that these females aren't even getting old enough to go into menopause, let alone, you know, intervene on their adult sons' behalf," she said.
Orcas' complex behaviours and social structures — including the role of older females — likely took tens of thousands, if not hundreds of thousands of years, to evolve, she said.
"And in a very short amount of time ... we humans have decimated their prey base."
Clarifications
- An earlier version of this story stated that orcas only eat salmon. While some orcas hunt a wide variety of prey, the southern resident population feeds exclusively off salmon and some other fish.Jul 25, 2023 11:54 AM ET
Interview with Charli Grimes produced by Sarah Cooper.
