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Southern resident orcas care for their sons much longer than their daughters.
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Scientists have observed them feeding their young well into adulthood, an expert said.
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With food in short supply for these whales, that strategy may now be threatening their ability to survive.
Southern resident orcas care for their young well into adulthood, and this may be threatening the population’s ability to survive, according to a study published Wednesday.
Daughters find their independence at an early age, but mothers continue to care for their children and share food with them throughout their lives.
Sons are much more likely to die without their mothers, Michael Weiss, director of research at the University of Exeter’s Center for Whale Research and an author of the study, said in an interview with Insider.
While this behavior likely gave mothers an evolutionary advantage in the past, it’s backfiring now that the whales have less food. That is how.
Orca offspring need their mother for their entire lives.
All orcas work in a matrilineal society, which means that families tend to stay with the mother.
Among the southern resident orcas, a small population that summers off the coast of Washington state, daughters will learn fairly quickly how to fend for themselves.
When they reach the age of 12, they will stop receiving food from their mothers. While they will remain in the herd, they will also become more independent, hunt, and have calves of their own.
Sons, however, never stop asking their mothers for help. Unlike other killer whale populations, where the bull may break away from the group and go hunting alone, the southern resident males mostly stay with the pod.
Moms will continue to share their food with their children throughout their lives, even if it costs them.
“Evolution has selected for this brain in the female killer whale that is so intent on keeping her child alive, that she potentially won’t get the food she needs, but instead will go hungry to keep her child happy and healthy,” Weiss said.
“There’s a really strong social bond between mother and child. They spend a lot of time floating on the surface together, rubbing against each other and swimming in tandem and in sync,” he said.
To take care of their children, mothers have fewer babies
That sacrifice comes at a reproductive cost, according to the study, which was published Wednesday in the peer-reviewed journal Current Biology.
By tracking pods of southern resident orcas, the researchers found that mothers who care for their young are much less likely to have another calf.
A 21-year-old female has a one in five chance of having a calf next year. If she has a child, that drops to one in 10, the study found.
Scientists believe it may be because mothers are feeding their children.
“They really don’t have enough resources to take on that extra burden of carrying and nursing a calf,” Weiss said.
The males are huge and clumsy, so they need help to feed.
It can be reduced to the size of the males. They are huge, about 50% bigger than their mothers, which means they need more food than females.
Their size also makes them poor hunters.
The southern resident killer whales only feed on Chinook salmon, which is tiny prey for a large, lumbering male.
“We know they catch some of their own fish, but they may be less efficient and at the same time need more,” Weiss said.
That means that sons are completely dependent on their mothers.
Men are eight times more likely to die after their mother’s death, while women are unscathed, Weiss said.
Keeping offspring alive makes evolutionary sense, if there’s enough food
Because the young tend to stay with the pack, a child or grandchild means another mouth to feed and more competition for reproduction. If the pod becomes too large, there may not be enough resources to move.
But if the male breeds with a female in another pride, “you have grandchildren that 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 evolution selected programmed mothers to care for their children.
But that strategy may now backfire as Chinook salmon become rarer.
The southern resident orcas are now critically endangered. Only three family herds are known in the world J, K and L, for a total of 73 individuals.
The reward may no longer be worth it, as mothers who have sons have far fewer babies.
This behavior is uncommon among mothers with multiple children.
Not many animals will continue to care for their young for most of their lives, and if they do, it is usually at no cost to the parents.
“They actually derive some benefits from their young. For example, mother chimpanzees continue to help their sons and daughters, but the older chimpanzees help care for their younger siblings,” he said.
This is the first documented example of an itroparous mother, that is, a mother who has the capacity to have multiple children in her lifetime, sacrificing her own well-being throughout the lives of her offspring, according to the study.
“It’s such an extreme and bizarre strategy,” Weiss said.
It’s not clear if other killer whale populations have the same behavior, but Weiss suspects they might, at least to some degree.
“We’re pretty sure that similar behaviors will be present in other resident killer whale populations where mothers keep their sons and daughters for their entire lives,” Weiss said.
Read the original article on Business Insider
