A Southern Resident killer whale male will stay with his mother for life. She catches and shares prey with him even when he’s fully grown. But at what cost? A new study led by the Center for Whale Research found an impact on future pregnancies. According to Dr. Michael Weiss, the Center’s Research Director, these moms are “less than half as likely to have a successful pregnancy” as those without sons.
Is this evolutionary strategy now putting the whole population at increased risk of extinction when prey is scarce?
Studies show that a maternal presence in the Southern Resident killer whale families “enhances survival across the lifespan, particularly for males.” This maternal investment is lifelong—and as this new study found—so is the impact on her future pregnancies. Why would she give up reproducing to raise one son
By food sharing, a mother ensures her son is big and healthy and more likely to be chosen by females for mating, thereby passing on his mother’s genes. The payoff is that her son’s offspring are born in other pods, “where they are less likely to compete” for resources with her family. Researchers believe these females have thus evolved with longer lifespans to support their sons.
Yet this longevity also creates a potential conflict between a mother and her daughters if they raise offspring simultaneously. This is the likely evolutionary driver for menopause in this species, where adopting the role of grandmother increases the survival chances of her daughters’ calves due to less competition.
J28, an adult reproductive male, travels alongside his mother, J22.
What was the study?
Researchers collected data on 40 reproductive-age females from 1982 until 2021, which included 67 births. “Of these, 54 calves survived their first year and were therefore considered cases of successful reproduction.”
They looked at the number of subsequent successful births of each female raising a son. What did the results show? “Sons, but not daughters, reduce their mother’s subsequent reproductive success.” The study found that it made no difference whether her son was 8 or 38, “suggesting that the indirect benefits of improving sons’ survival are significant enough to outweigh substantial costs to females’ reproductive success across their lifespan.”
What are the consequences of this costly evolutionary strategy today?
We know that good body condition is essential to sustain a healthy pregnancy, yet malnutrition drives a 70% miscarriage rate in this population. With the added burden of sharing food with one or more sons who may be twice the mother’s weight, the possibility of finding enough food to feed another calf seems almost impossible, given the current Chinook salmon crisis. As these authors noted, “if a large portion of reproductive-aged females have one or more surviving sons, we would expect the population’s reproductive capacity to be reduced.” The number of females with sons has been as high as 80% in the study years and remains worryingly high.
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What is the answer?
An immediate allocation of a share of the Chinook salmon catch with less commercial fishing in coastal waters. Allowing more salmon to migrate through the Southern Resident killer whales home waters would create more foraging opportunities for these starving families. Surviving salmon would return to their natal rivers to spawn and produce the next generation.
Until we undertake these urgent management interventions, we will continue to witness this population stagnation. It cannot and must not continue.
About this Study
This study was conducted and funded by our colleagues at the Center for Whale Research.
Learn more about them and support their important work here.
