Killer whale moms struggle to regain body fat with more offspring

One and a half years of pregnancy, up to two years of nursing, then hunting and sharing food with their adult children who never leave.

Orca moms go hard. 

“There’s a long, long cost of taking care of offspring because they are spending so much time with them throughout their entire lives,” said Sharon Kay, a biologist and author of new research in the journal Scientific Reports that shows a physical toll of that motherhood. 

The study looked at body fat — a vital sign linked to survival in these ocean mammals — and how quickly moms regain it after pregnancy. Turns out, it’s now how many births — it’s how many kids that are around after. 

Experts say these findings help unpack the societal pressures these animals face, even as human pressures persist in their environment. 

Pixel-perfect patches 

The study focused on northern resident killer whales, whose range extends from Alaska, down the coastal waters of British Columbia, to northern Washington. They are also listed as threatened under Canada’s Species at Risk Act. 

“After they give birth and they start nursing, they have a really steep decline in their fat levels,” said Amy Rowley, a co-author and biologist with the Raincoast Conservation Foundation. This decline lasts around three years, but some moms struggle to recover back to pre-pregnancy condition. 

To figure out why, the team needed to measure that body fat, but unlike other terrestrial creatures, capturing and measuring wild orcas isn’t feasible. So they used drones, flying 30 metres above the animals and taking pictures. 

“The body condition of these killer whales is related mostly to the fatness behind their head because that’s where they lose their fat first,” said Kay, who led this research as a graduate student at the University of Victoria. 

Those images would be combed over, pixel by pixel, to measure changes in the white patches above the eyes. 

“If the eye patch is more kind of angled outwards, that’s a fatter, healthier whale,” said Rowley. “Versus if they’re kind of parallel or even slightly inwards, that’s the sign that the whale doesn’t have a lot of fat and is potentially in poor condition.” 

Number of living offspring 

Studying 66 adult females, the researchers found the strongest factor affecting body fat levels was the number of living offspring. (Up to half of all killer whale calves don’t survive past the first year.)

“It’s not just nursing and reproducing these offspring that have a toll on the mothers, it’s actually the lifelong care of finding fish and sharing that food,” Kay said. 

Hannah Myers, a marine ecologist at the University of Alaska Fairbanks, said this work adds another piece to the puzzle of these socially complex animals. 

“It’s really exciting to see some of these questions that you really could never answer without this type of long-term monitoring program and some of the new technologies that have come online,” Myers said. 

WATCH | Cooperation instead of competition while hunting:

Orcas and dolphins teaming up to hunt salmon

New research published in Scientific Reports says orcas and dolphins have been caught on video collaborating to hunt for salmon. Researchers say dolphins dive deep for salmon and orcas eavesdrop on their echolocation to hunt. The dolphins feed on scrap.

“It’s got some quite striking results in it, some of which align really closely with what we might hypothesize about how killer whales work,” said Michael Weiss, research director at the Centre for Whale Research.

Interestingly, the new study in northern resident killer whales didn’t find evidence of what he and his team found in the more endangered, southern resident killer whales: That sons are more costly to their mother than daughters. But Weiss explains they weren’t looking for the same things. 

“Whereas our previous research looked at the kind of end results — how do they reproduce? — this research is looking at the middle step, which is how does their body condition change?” Weiss told CBC News from the San Juan Islands in Washington. 

One theory to explain that discrepancy, Rowley suggests, might be that daughters stick around — meaning their offspring become another mouth to feed — adding a pressure on the mom, who’s now a working grandmother. Weiss points out that in the southern population, the reproductive rate is lower so there might be fewer mouths in general. 

Human and societal pressures

This impact on body condition is important to understand, experts say, because it’s one of multiple pressures that orcas face. 

“This study just really points out what a big deal that maternal investment is for any given animal,” Myers said, adding that human influences add to what they have to deal with. 

These include chemical contaminants in their environment, noise pollution from shipping traffic and climate change shifting the location of their prey. Ultimately, it could mean further body fat decline — a precursor to death. 

“They have a lot to maintain, and so when we go and cause further stress through these anthropogenic conditions … they end up getting stressed even further,” Kay said.