Killer whale mothers pay a high price for raising sons

While it is not unusual for parents – and especially mothers – to sacrifice their own future success for the sake of their offspring, a new study published in the journal Current Biology has found that killer whale mothers take this to a surprising extreme. Killer whale moms sacrifice their own reproductive success in order to care for their sons, even after they are full-fledged adults. 

“We’ve known for over a decade that adult male killer whales relied on their mothers to keep them alive, but it had never been clear whether mothers pay a cost to do so,” said study lead author Michael N. Weiss, an expert in Behavioral Ecology at the University of Exeter.

The scientists studied a group of killer whales known as the “southern resident” population of the coastal waters of the Washington State and British Columbia to find out whether the care adult whales (particularly males) receive from their mothers comes at a significant cost. 

“The southern resident killer whale community presents an incredible opportunity to investigate these kinds of questions. Along with their bizarre social system, where both males and females stay with their mom for life, they are also one of the best studied wild populations of mammal anywhere in the world,” Weiss explained.

The investigation revealed a marked negative correlation between females’ number of surviving weaned sons and their annual probability of producing another viable calf. Such costs did not appear to decline even as their sons grew older. These findings – which cannot be explained by lactation or group composition effects – provide the first clear evidence of lifetime maternal investment in any animal.

“The magnitude of the cost that females take on to care for their weaned sons was really surprising,” Weiss said. “While there’s some uncertainty, our best estimate is that each additional surviving son cuts a female’s chances of having a new calf in a given year by more than 50 percent. This is a huge cost to taking care of [adult] sons!”

According to the scientists, there seem to be major benefits to caring for adult sons, such as the latter’s reproductive success, which is enough to outweigh the large direct costs their mothers have to face.

“One big take-away is further evidence for how special (and maybe unique) the mother-son bond in killer whales is. Maybe more importantly, our study adds to the growing body of work showing the importance of animals’ social systems in determining demographic patterns. This is of central importance both for an understanding of our world, and to effectively conserve endangered species,” Weiss added.

Since the southern resident killer whales are critically endangered, these findings have important conservation implications, revealing a major and previously unknown factor in female’s reproductive success, which may help to inform future population viability analyses.

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By Andrei Ionescu, Earth.com Staff Writer

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