Orca mothers ‘pay a high price’ for raising sons, researchers say

Raising sons is an exhausting experience that makes killer whale mothers less likely to produce more offspring, a new study suggests.

According to the study, each living son reduced a mother’s annual chance of successfully breeding — a calf surviving past one year of age — by about half.

In addition, the effect persisted as the sons grew older, indicating that they were a lifelong burden on their mothers, the scientists said.

Killer whales, or killer whales, are known to be more supportive of sons than daughters, especially after daughters reach adulthood.

Researchers suggest the findings confirm that this support comes at a significant cost to the mothers.

The research was carried out by the universities of Exeter, York and Cambridge, and the Center for Whale Research.

Dr. Michael Weiss, from the Center for Research in Animal Behavior at the University of Exeter, said: “Our previous research has shown that sons are more likely to survive when their mothers are nearby.

“In this study we wanted to find out whether 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.”

In the study, researchers used data from 1982 to 2021 on 40 females in the southern-dwelling killer whale population, which live off the Pacific coast of North America.

An orca pod

Previous research showed that sons had a greater chance of survival when their mother was nearby (Hebridean Whale and Dolphin Trust/PA)

Both male and female killer whales remain in the group they were born into, and each group is led by an experienced female.

The southern killer whales are fish-eating whales and feed mainly on salmon.

Mothers usually bite salmon in half, eat half, and give half to their sons.

Although the mothers also feed their daughters, this stops when they reach childbearing age.

But the sons are still fed into adulthood.

The scientists suggest the strategy is unusual and may even be unique.

Explaining how this could have evolved, Professor Darren Croft, professor of animal behavior at the University of Exeter, said mothers gain an “indirect fitness” benefit: helping their sons survive and reproduce increases the likelihood that their genes are passed on to future generations.

But the approach now could pose problems for the future viability of the population.

Killer whales living in the south specialize in eating Chinook salmon, but these fish have become scarce in many parts of the whale’s range, and many stocks are threatened or endangered.

With limited food, the southern inhabitants are also at risk.

There are only 73 southern-dwelling killer whales left and – since they don’t interbreed with other orca populations – this number is critically low.

Prof Croft said: “For this population living on the cutting edge, the potential for population recovery is limited by the number of females and the reproductive output of those females.

“A strategy in which females reduce reproduction to increase the survival of male offspring may therefore have a negative effect on the recovery of this population.”

The study, published in Current Biology, was funded by the Natural Environment Research Council (UK) and the National Marine Fisheries Service (US).

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