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CTR: New study reveals Southern Resident orca mothers face consequences for bearing male offspring

New research by the Center for Whale Research (CWR) has shown that “providing care to weaned sons reduces female killer whales’ reproductive output.” In other words, caring for a son lessens a mother’s annual breeding success (a calf surviving to one year old) by about half.

 LEFT: L22 with son L89 in 2012 (Photograph by Dave Ellifrit). RIGHT: Aerial footage of L54 and her sons L108 and L117 in 2021 (Video footage by Dr. Michael Weiss). Research permit number: NMFS 21238

And this cost persists as male orcas age. The mothers pay more attention to their sons than their daughters, particularly once females achieve adulthood. The study’s authors found that the lifelong assistance afforded their sons came at a future price—moms dramatically reduced childbearing success.

“Our analysis demonstrates that the long-term survival benefits that Southern Resident killer whale females provide to their sons come at a significant cost to their own reproductive success,” said the Center for Whale Research’s Research Director Dr. Michael Weiss, lead author of the study by Washington State’s Center for Whale Research and the United Kingdom universities of Exeter, York, and Cambridge.

“Our previous research has shown that sons have a higher chance of survival if their mother is around,” said Dr. Weiss. “In this study, we wanted to determine if this help comes at a price. The answer is YES. Killer whale mothers pay a high cost in terms of their future reproduction to keep their sons alive.”

The study used the Center for Whale Research’s ORCA SURVEY data from 1982-2021, concentrating on 40 females in the Pacific Northwest’s Southern Resident killer whale (orca) population.

The strategy uncovered in the study—mothers surrendering the majority of their future reproduction to benefit their sons across their lifespan—is unusual in nature and may even be unique. Dr. Weiss said: “The evolutionary benefit of helping their male offspring survive into maturity is that the sons will mate, passing their genes to future generations. It’s a tactic that has been effective in the animals’ evolutionary past: Mothers support their sons, so they survive to old age and produce a large number of offspring.

L22s Photo by CWR’s Dave Ellifrit Research permit number: NMFS 21238

But this Southern Resident orca reproduction strategy may backfire for the population’s long-term survival. The current Southern Resident population census is critically low—a total of 73 whales in three pods (i.e., J, K, and L pods). Males and females remain in their birth pod, led by an experienced female.

“The endangered Southern Residents’ population recovery will, of course, be determined by the females in the community, by the number of females and those females’ reproductive output,” said Dr. Weiss. “A strategy of females reducing reproduction to increase male offspring longevity only works if a sufficient number of female calves are born, survive, and reproduce.”

The Southern Resident orcas are fish-eating whales, preying predominantly on nutritionally rich Chinook salmon, now scarce in areas of the whales’ range. Several Chinook salmon stocks are threatened or endangered. Orca mothers regularly share their fish catch with their young sons and daughters, eating half and giving half to their offspring until their daughters reach reproductive age when food sharing is inclined to cease. But mom continues to feed her sons into adulthood.

The study, Costly lifetime maternal investment in killer whales, published in the journal Current Biology, was funded by the Natural Environment Research Council (United Kingdom) and National Marine Fisheries Service (United States).


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