Raising sons is draining killer whale mothers, study finds

Raising sons is draining killer whale mothers, study finds

Imagine, if you will, that you have a daughter and a son. You feed and nurture the pair as they grow and mature. They rarely leave home, and you get little time to yourself. Eventually, your daughter becomes of age and begins to feed herself.

Your son, on the other hand? He only leaves to hang out with other girls and have sex. He makes his way home after his amorous adventures, and while he does get his own food sometimes, he pretty much hoards it, and you’re still forced to provide for him day in and day out.

If this sounds exhausting, it is. Welcome to the life of southern resident killer whale mothers.

These endangered whales are mainly found off the coast of British Columbia and Washington state. There are only 73 alive today.

It’s unknown why this population is struggling, but there have been suggestions that it could be due to reduced food sources, boats or even pollutants in the water that could reduce reproductive success.

Now, a new study has found something else that could be a contributing factor: the neediness of male offspring.

There only 73 remaining southern resident killer whales. While there is no definitive cause for their low numbers, this new study’s finding could account for one such reason. (Center for Whale Research/David K. Ellifrit/NMFS 21238)

According to the study published today in Current Biology— which used data from 1982 to 2021 on 40 females — this need to constantly provide comes at a great cost to the mothers, specifically when it comes to reproduction.

Michael Weiss of the Centre for Research in Animal Behaviour at the University of Exeter, and lead author of the study, used an example of a 21-year-old female orca who hasn’t had a calf in the last year (they can’t have a calf until they’re no longer nursing).

“Our best estimate for that female is that she has about a one in five chance each year of having a calf, a 20 per cent chance in a given year,” Weiss said.

“Same female, same situation, but [she] has a single son that she’s taking care of: That drops to less than a one in 10 chance. So a 10 per cent chance. So you’re actually about halving or even a little bit more than halving the chance in a given year that you can successfully have a calf just by taking care of one son.”

‘They need more food’

Weiss noted that previous studies have more closely examined the behaviours of the orcas, finding that, unlike with female offspring where the mothers stop sharing food, they continue to share with the males into adulthood.

“This kind of makes some sense, given that males are bigger, they need more food,” Weiss said. “Because they’re bigger, they might be less manoeuvrable, and it might be harder for them to catch their own food. There’s also some good evolutionary reasons why you might want to support your son or your daughter.”

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Various species are known to sacrifice their future reproductive success in order to boost the chances of survival for their offspring, which is known as parental investment. But this is the first time maternal investment has been examined in this particular group of orcas.

Weiss said he was surprised at the extent to which there was a maternal investment.

“You know, this wasn’t some subtle thing of, oh, you know, across your life, on average, you might lose out on one extra offspring. This is like, our best estimate is more than a 50 per cent drop in your reproductive output, because you’re taking care of a son.”

The data showed there was a negligible change if the mothers had daughters.

‘It just shows how much we don’t know’

Weiss said that back when these orcas were evolving this sort of behaviour — when there may have been more food around — may not have hurt their reproduction as much. More abundant food means mothers can have enough to keep them both alive and to be able to reproduce. But now, that may not be the case.

“I think this kind of study just highlights how much we don’t know about southern resident killer whales. We’re trying to … understand why they’re so endangered,” said Fanny Couture, a marine ecologist and PhD candidate at the University of British Columbia’s Institute for the Ocean and Fisheries, who was not involved in this study.

Couture recently published a study that found these orcas have not been getting enough food since 2018. She said that these new findings indicate there may be other factors threatening this population.

“It’s the first example of long-term maternal investment in [these’] species. So it just shows how … many other factors we need to consider when we talk about southern residents,” she said.

Weiss noted that southern resident killer whales have been studied for decades, with exact counts being provided annually. But there are other populations that are faring better — such as the northern resident killer whales and the Bigg’s (transient) killer whales — that he’d like to study as well.

“We are really interested going forward to see if we can utilize some of that data and see if these same effects happen in those other populations,” he said. “And specifically for me, I’m interested to see if first of all if there’s the same kind of effect. And if potentially, that effect is lessened because these populations are less food stressed.”

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