Southern Resident killer whales take care of their sons much longer than their daughters.
Scientists have observed them feeding their sons well into adulthood, an expert said.
As food is scarce for these whales, that strategy may now be threatening their ability to survive.
Southern Resident killer whales look after their sons well into adulthood, and it may be threatening the population's ability to survive, according to a study published Wednesday.
Daughters find their independence early on in life, but moms continue caring for their sons and sharing food with them throughout their lives.
Sons are much more likely to die without their moms, said Michael Weiss, research director at the Center for Whale Research of Exeter University and an author on the study, in an interview with Insider.
While this behavior probably gave the moms an evolutionary advantage in the past, it's backfiring now that the whales have less food. Here's how.
Killer whale sons need their mom throughout their lives
All killer whales work in a matrilineal society, which means families tend to stick with the mother.
Among Southern Resident killer whales - a small population that spend the summer and fall off the coast of Washington state - daughters will learn pretty quickly how to fend for themselves.
When they reach about age 12, they will stop taking food from their moms. While they will stay in the pod, they will also become more independent, hunt, and have calves of their own.
The sons, however, never really stop asking their moms for help. Unlike some other killer whale populations, where the bull can peel off from the group and go hunting on his own, Southern Resident males stay mainly with the pod.
Moms will keep sharing their food with sons throughout their lives, even if it costs them.
"Evolution has selected for this brain in the female killer whale that is so determined to keep her son alive, that she will potentially not get the food she herself needs, but go hungry to keep her son happy and healthy," said Weiss.
"There's just a really strong social bond between the mom and the kid. They spend a lot of time floating at the surface together, rubbing on each other, and swimming in tandem and in synchrony," he said.
To care for their sons, moms have fewer babies
That sacrifice comes at a reproductive cost, according to the study, which was published in the peer-reviewed journal Current Biology on Wednesday.
By following pods of Southern Resident killer whales, researchers found that mothers that care for sons are much less likely to have another calf themselves.
A 21-year-old female has a one in five chance of having a calf in the coming year. If she has a son, that drops to one in ten, the study found.
Scientists think that may be because the mothers are feeding their sons.
"They actually don't have enough resources to take on that extra burden of gestating and nursing a calf," said Weiss.
The males are huge and awkward, so they need help feeding themselves
It may come down to the size of the males. They are enormous, about 50% bigger than their moms, which means they need more food than females.
Their size also makes them poor hunters.
Southern Resident killer whales only feed on Chinook salmon, which is a tiny prey for a big lumbering male.
"We know they do catch some of their own fish, but they might be less efficient at it, and at the same time they need more of it," said Weiss.
That means the sons are completely dependent on their mothers.
Males are eight times more likely to die after their mom's death, while females carry on unscathed, Weiss said.
Keeping sons alive makes evolutionary sense - if there's enough food
Because offsprings tend to stay with the pod, a child or a grandchild means another mouth to feed and more competition for reproduction. If the pod gets too big, there may not be enough resources to get around.
But if the male reproduces with a female in another pod, "you have grandchildren who have a lot of your DNA, but they're someone else's problem," said Weiss.
It may be that when food was plentiful, this was a great strategy, which is why moms hardwired to care for their sons were selected by evolution.
But that strategy may now be backfiring, as Chinook salmon becomes scarcer.
Resident Southern killer whales are now critically endangered. There are only three family pods known in the world J, K, and L, for a total of 73 individuals.
The payoff may no longer be worth the effort as mothers who have sons have much fewer babies.
This behavior is uncommon among moms that have several kids
Not many animals will keep caring for their offspring for most of their lives, and if they do, it usually doesn't come at a cost for the parents.
"Actually they get some benefits back from their offsprings. For instance, chimpanzees mothers 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 - meaning a mother that has the ability to have several offspring in her life - sacrificing her own well-being throughout the lifespan of her offspring, per the study.
"It's such an extreme, weird strategy," said Weiss.
It's not clear whether other populations of killer whales have the same behavior, but Weiss suspects they might, at least to some extent.
"We're fairly confident similar behaviors will be present in other resident killer whale populations where the mothers keep their sons and daughters around for their whole life," said Weiss.