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Killer whale mommies are not good DarwinMoms, it turns out

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We’ll let ScienceDaily tell it (February 8, 2023):

“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 Michael N. Weiss (@CetaceanMike) of the University of Exeter, UK, and the Center for Whale Research in the US …

So whale researchers studied it.

Their analysis of the existing data found a strong negative correlation between females’ number of surviving weaned sons and their annual probability of producing a viable calf. Those costs didn’t get any smaller as their sons grew older, either.

The costs couldn’t be explained by lactation or group composition effects, which they say supports the hypothesis that caring for sons into adulthood is reproductively costly. They say that the findings offer the first direct evidence for lifetime maternal investment in any animal, revealing a previously unrecognized life history strategy.

“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!”

The findings suggest that there are significant benefits to keeping adult sons alive and well, he added.

“Females gain evolutionary benefits when their sons are able to successfully reproduce, and our results indicate that these benefits are enough to outweigh a large direct cost,” Weiss explained.

This seems like a clear example of Darwinthink getting in the way of understanding what is happening. First, no female gains any “evolutionary benefit” for doing anything. She can only gain a benefit in her own lifetime. Second, it’s not clear that the whales, as a group, gain an “evolutionary benefit” either:

The findings also may have important conservation implications, the researchers say. The southern residents are critically endangered, with one major concern being their low reproductive rates. The new findings reveal a major and previously unrecognized determining factor in a female’s reproductive success, which may help to inform future population viability analyses.

So no. The characteristic behavior isn’t promoting selfish gene survival. Any farmer knows that if you want the herd to grow rather than shrink in size, you encourage females, not males.

Okay, here’s a thought: Group dynamics like this may be one reason that a species becomes critically endangered or goes extinct. Yes, human activities drive many extirpations/extinctions.* But others may be due to the adoption of behaviors that result in fewer than the needed number of offspring. Not easy to change.

In future work, they hope to learn more about the nature of the costs to mother whales. They suspect mothers may not eat enough themselves as they continue sharing food with their full-grown sons. He noted that the southern resident killer whales are “very food-stressed.” As such, a primary conservation goal for the whales is to recover the population of Chinook salmon they rely on.

Here’s another thought: Mom needs to live in a group. She is likely dominant over her sons. If she kicked the sons out and took up with a strange guy whale, would she still be dominant?

Not that she is thinking it out of course – but the current arrangement may not be performing that badly for her, even though it isn’t good for overall whale numbers. So she is under no overwhelming pressure to alter her behavior.

This may be an opportunity to study how extirpations and extinctions can happen naturally.

The paper is open access.

*If human behavior were all that mattered, we wouldn’t be dealing with so many rats, feral cats, cockroaches, wild hogs, etc., that — far from going extinct — thrive on what humans do. It really depends on how human behavior affects a given life form.


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