A study of killer whales in the North Pacific has revealed that mothers make a “lifelong sacrifice” for their sons.
Raising a son significantly reduced a female killer whale’s chances of reproducing in the future.
The energy they need to feed sons seems to compromise their health, reducing their ability to reproduce and raise other young.
“Mothers sacrifice their own food and their own energy,” says Professor Darren Croft from the University of Exeter.
Killer whales remain closely associated with their families throughout their lives. But while young female offspring become independent in adulthood, males are dependent on their mother – even demanding a share of the food their matriarchs catch.
Prof Croft described it as a “new insight into the complex social and family life of these amazing animals”.
The decades-long study, published in the journal Current Biology, is part of an ongoing mission to understand orca family life.
It was made possible by the Center for Whale Research (CWR), which has been tracking the lives of a population of killer whales known as the Southern Residents for more than 40 years.
Since 1976, the CWR has conducted a full census of the Southern Resident population, which has allowed biologists to conduct multi-generational studies like this one — disentangling critical social behaviors and family ties that directly affect the animals’ survival.
For this study, scientists examined the lives of 40 female killer whales between 1982 and 2021, and found that for every son alive, a mother’s annual chance of raising another calf until one year old was reduced by half.
“Our previous research has shown that sons have a greater chance of survival when their mothers are nearby,” says Dr Michael Weiss from the University of Exeter and the Center for Whale Research.
“We wanted to know if this help comes at a cost and the answer is yes. Killer whale mothers pay a high cost in terms of their future reproduction to keep their sons alive.”
The ongoing study of this endangered orca population, which lives in the coastal waters between Vancouver and Seattle, was initiated by Dr. Ken Balcomb. Initially he wanted to investigate the threats to their survival.
Subsequent work revealed insights into orca life that could only come to light through decades of research. Biologists have worked with the CWR to reveal, for example, the vital role of orca grandmothers and why females of this species, like humans, stop reproducing in mid-life.
Through years of studying killer whale interactions, scientists already knew that mothers and sons “hanged out” together well into male adulthood.
“They will even feed their sons any salmon they catch,” explains Professor Croft, while adult female offspring will hunt on their own.
This, the researchers think, could be a kind of evolutionary “bet hedging,” driven by the fact that the largest, oldest males have many offspring.
“If a mother can get their son to become that great man in the population, then he is the one who will beget [much of the next generation]’ explains Professor Croft.
It may seem paradoxical that such powerful, intelligent animals remain dependent on their mothers throughout their lives, but it seems that males simply do not need to become independent because their mothers stay by their side.
“If my mother cooked me dinner every night, maybe I just wouldn’t learn to cook my own dinner,” Professor Croft joked.
“But indirectly it seems to be in a mother’s best interest.”
There are currently only 73 of these killer whales left, so the scientists say they need to understand everything that can help make decisions about how to protect these marine mammals.
“These killer whales that live in the south are on the cutting edge and endangered,” said Professor Croft. “So anything that reduces female reproduction is a concern for this population.”
The Southern Residents were the subject of the award-winning BBC Radio 4 documentary, The Whale Menopause
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