Orca mothers spend more time with their sons than daughters, and it could threaten their survival

Two adult killer whales break through the water.  One rubs against the other.

Two Southern Resident orcas.David K. Ellifrit/Center for Whale Research, Permit Number: NMFS 21238.

  • Orcas from the South take care of their sons much longer than their daughters.

  • Scientists have observed them nurturing their sons well into adulthood, one expert said.

  • Since food is scarce for these whales, that strategy could now threaten their survival.

Southern killer whales care for their sons well into adulthood, and it could threaten the population’s ability to survive, according to a study published Wednesday.

Daughters find their independence early in life, but mothers continue to care for and share food with their sons throughout their lives.

Sons are far more likely to die without their mothers, Michael Weiss, director of research at Exeter University’s Center for Whale Research and author of the study, said in an interview with Insider.

While this behavior likely gave the mothers an evolutionary advantage in the past, it backfires now that the whales have less food. Here’s how.

Orca sons need their mother all their lives

A killer whale and a calf breach over gray water.

A southern resident killer whale and a calf.Kenneth C. Balcomb/Center for Whale Research, permit number: NMFS 21238.

All killer whales work in a matrilineal society, meaning families tend to stay with the mother.

Among southern killer whales — a small population that spends the summer and falls off the coast of Washington state — daughters will learn how to fend for themselves fairly quickly.

When they are about 12 years old, they will stop taking food from their mother. While they stay in the group they will also become more independent, hunt and have their own calves.

However, the sons never really stop asking their mothers for help. Unlike some other orca populations, where the bull can break away from the group and hunt on its own, the males of the southern inhabitants mainly stay with the group.

Mothers will continue to share their food with their sons throughout their lives, even if it costs them.

“Evolution has chosen this brain in the female orca that is so determined to keep her son alive that she may not get the food she needs herself, but starves to keep her son happy and healthy,” Weiss said.

“There’s just a really strong social bond between the mother and the child. They spend a lot of time floating on the surface together, rubbing each other and swimming in sync and in tandem,” he said.

To take care of their sons, mothers have fewer babies

Two Southern Resident orcas float above the water.

Two Southern Resident orcas.David K. Ellifrit/Center for Whale Research, Permit Number: NMFS 21238.

That sacrifice comes at a reproductive cost, according to the study, which was published Wednesday in the peer-reviewed journal Current Biology.

By tracking groups of killer whales from the south, researchers found that mothers who care for sons are much less likely to have a calf of their own.

A 21-year-old woman has a one in five chance of having a calf in the next year. If she has a son, that drops to one in ten, the study found.

Scientists think this is because the mothers feed their sons.

“They don’t actually have enough resources to take on that extra burden of carrying and nursing a calf,” says Weiss.

The males are huge and clumsy, so they need help feeding themselves

A southern killer whale breaks through the water, in front of a rocky surface.

Orcas from the South.Kenneth C. Balcomb/Center for Whale Research, permit number: NMFS 21238.

It may have to do with the size of the males. They are huge, about 50% larger than their mothers, which means they require more food than females.

Their size also makes them poor hunters.

Orcas from the south feed only on Chinook salmon, small prey for a large, bulky man.

“We know they catch some of their own fish, but they may be less efficient at it, and at the same time they need more of it,” Weiss said.

That means that the sons are completely dependent on their mother.

Men are eight times more likely to die after their mother’s death, while women go through unscathed, Weiss said.

Keeping sons alive makes sense evolutionarily – if there’s enough food

A chinook salmon is seen jumping out of a tank.

A Chinook salmon on May 27, 2021 in Oroville, California.PATRICK T. FALLON/AFP via Getty Images

Because offspring tend to stick with the group, having a child or grandchild means another mouth to feed and more competition for reproduction. If the pod gets too big, there may not be enough resources to move around.

But if the male mates with a female in another group, “you have grandchildren who have a lot of your DNA, but they’re someone else’s problem,” Weiss said.

It may be that when food was plentiful, this was a great strategy, which is why mothers who were determined to care for their sons were selected by evolution.

But that strategy may now backfire as Chinook salmon become scarcer.

Resident southern killer whales are now critically endangered. Only three family pods are known in the world J, K and L, for a total of 73 individuals.

The payoff may no longer be worth it, as mothers who have sons are having far fewer babies.

This behavior is uncommon in mothers who have multiple children

A close-up of a baby chimpanzee's face wrapped in a blanket is shown here.

Shown here is a baby chimpanzee.iStock/Getty Images Plus

Not many animals will care for their offspring for most of their lives, and if they do, it usually comes at no cost to the parents.

“Actually, they get some benefits back from their offspring. For example, mothers of chimpanzees continue to help their sons and daughters, but those older offspring help care for their younger siblings,” he said.

This is the first documented example of an iteroparous mother — that is, a mother who has the ability to have multiple offspring in her lifetime — sacrificing her own well-being over the lifetime of her offspring, according to the study.

“It’s such an extreme, weird strategy,” Weiss said.

It’s not clear if other killer whale populations display the same behavior, but Weiss suspects they do, at least to some extent.

“We are quite confident that similar behaviors will be present in other orca populations where the mothers keep their sons and daughters with them throughout their lives,” Weiss said.

Read the original article on Business Insider

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