Killer whale mothers take care of their sons much longer than their daughters. This could be why

Helping kids well into their adulthood is something many parents are familiar with.

It turns out that killer whales have that in common with humans. But only for their sons, not their daughters.

For female killer whales, the sacrifices they continue to make to care for their sons well after they are weaned is negatively affecting their rate of reproduction, according to new scientific research.

And the effects of that behaviour have implications on the southern resident killer whale population, which has been in decline since the early 1990s. Only 73 members of this specific and endangered group of orcas were known to be alive at the time of writing, authors state.

'EXTREME EXAMPLE’ OF MOTHERING

If a large portion of female whales of reproductive age end up with male offspring, researchers expect their reproductive capacity to be reduced, based on what they've observed.

The study, published in the journal Current Biology on Wednesday, looked at fish-eating killer whales in the inland waters of Washington State and B.C. Known as southern resident killer whales, this population is one of two that receives special protection under the law, due to its decline.

Researchers used demographic data from the Center for Whale Research in Washington State on 40 female whales from 1982 to 2021. They also conducted photographic surveys of the population.

A statistical analysis model was used to determine whether a female whale had successfully reproduced in a given year, and if that calf survived its first year of life.

The study found that giving birth to sons reduces the annual odds of subsequent reproduction by 70 per cent.

What was observed represents an “extreme example of extended maternal care,” researchers said.

For all calves, the presence of their mothers increases the chances of survival across their lifespan, especially for males. The survival benefits the mothers provide include food sharing and their “leadership during foraging” as the calves learn to find food themselves.

But the researchers found that sons do not become less costly to mothers as they age. Mother whales share food with their sons even into their adulthood, when they do not provide the same help to their daughters.

One reason for this is the relative size of male whales, which means they need more food to grow, according to other research.

“Males were consistently costly regardless of age,” those behind the more recent study state. This is the first direct evidence of “lifetime maternal investment” in an iteroparous animal – meaning an animal that can reproduce multiple times.

The authors also theorize that another factor in the difference in treatment may be that female whales recognize that their daughters reproduce in the same group, which can lead to “reproductive conflict.” A son’s offspring are more likely to be born in another matriline, meaning those offspring are less likely to compete with the mother.

As a result, there are benefits to “boosting” an adult son’s survival as “late-life reproductive conflict with daughters” creates pressure against an extended reproductive lifespan, the authors state.

Those benefits could outweigh the additional burden that sons are to mothers and the reproductive impact of needing to provide extended care to them, they say.

More work is needed around this “extreme life history strategy” to understand its evolution and to see if other species rely on their mothers well into adulthood, at a cost to reproductive health, the researchers said.