Image Credit - Center for Whale Research (permit number NMFS 21238)

Killer whale mums pay high price for raising sons

Raising sons is an exhausting experience that leaves killer whale mothers far less likely to produce more offspring, new research shows.

The study found that each living son cut a mother’s annual likelihood of successful breeding (a calf surviving to one year old) by about half.

And this effect continued as sons grew older – suggesting sons are a lifelong burden on their mums.

Killer whale mothers are known to provide more support to sons than daughters, especially after daughters reach adulthood, and the findings confirmed that this support comes at a considerable cost to the mothers.

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

“Our previous research has shown that sons have a higher chance of survival if their mother is around,” said Dr Michael Weiss, of the Centre for Research in Animal Behaviour at the University of Exeter.

“In this study, we wanted to find out 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 data from 1982-2021 on 40 females in the “southern resident” killer whale population, which live off the Pacific coast of North America.

Both male and female resident killer whales stay in the group they were born into, and each group is led by an experienced female. The southern resident killer whales are fish-eating whales and feed predominantly on salmon. Mothers commonly bite salmon in two, eating half and giving half to their sons.

They also feed their young daughters, but once the daughters reach reproductive age this tends to stop – whereas they continue to feed their sons into adulthood.

The strategy discovered by this study – in which mothers indefinitely sacrifice their future reproduction to keep their sons alive – is highly unusual in nature and may even be unique.

Explaining how this could have evolved, Professor Darren Croft said mothers gain an “indirect fitness” benefit: helping their sons survive and reproduce improves the chances of their genes passing to future generations.

This strategy has clearly been effective in the evolutionary past. Mothers pouring effort into their sons would be beneficial because their sons could mate with numerous females, producing a large number of grand offspring.

However, this strategy may now cause problems for the future viability of the population.

Southern resident killer whales specialise in eating Chinook salmon. These salmon have become scarce in many parts of the whales’ range, with many stocks threatened or endangered. With their food limited, the southern residents are also endangered.

Just 73 southern resident killer whales remain and – as they don’t inter-breed with other killer whale populations – this number is critically low.

“For this population that’s living on a knife’s edge, the potential for population recovery is going to be limited by the number of females and those females' reproductive output,” Professor Croft explained.

“A strategy of females reducing reproduction to increase the survival of male offspring may therefore have negative impacts on this population’s recovery.”

Professor Dan Franks, from the Departments of Biology and Computer Science at the University of York, said: "This strategy of indefinitely sacrificing future reproduction to keep their sons alive may have been beneficial in their evolutionary past, but it now potentially threatens the future viability of the southern resident killer whale population, which is critically endangered with just 73 individuals remaining."

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

The paper, published in the journal Current Biology, is entitled: “Costly lifetime maternal investment in killer whales.”

Date: 6 February 2023