Take my kids, please

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Writing about penguins in Slate, David Edelstein says:
In the most upsetting scene, a bird of prey goes after a group of young 'uns and the elders make no motion to intervene. Not even the parents. Previously, we've seen a grieving mother attempt to steal a baby from another mother at which point the whole village jumps in to drive her away. I'm no naturalist, but that's obviously adaptive: You can't have a viable society in which mothers steal babies from other mothers. But letting a predator take something: That's a strategy for survival.

This sounds intuitively right, but it seems to me that it's more complicated than that. Penguin evolution doesn't have any interest in having a viable society and it's arguable that penguins don't even have one. Moreover, it's not at all clear that wanting to raise your own children is adaptive behavior.

Caring for children involves a very substantial investment of time and energy--energy that could have been invested in producing more children. In order for it to be a good deal, you have to believe that you caring for them will increase their chances of survival more than not caring for them. Obviously, that's true if we're talking about the choice is just leaving them to starve to death, but if someone else is willing to care for them, then it's quite possibly a good deal, even if they wouldn't do quite as good job as you would, since you can invest that energy in having other children. In this model, the non-adaptive behavior here isn't wanting to raise your own children, but wanting to raise someone else's children, since that doesn't improve the propagation of your genes (unless they're related to you, of course).

My intuition here is that both behaviors are a result of the inherent imprecision of evolutionary mechanisms. A strong drive to raise children is simpler than a drive to raise only your own children, since the latter requires a whole suite of mechanisms for detecting which children are yours. A generic drive is "good enough" so that's what we get Now, obviously this drive is less efficient because you might end up caring for the children of others, so one simple mechanism to reduce false positives is to have it activate just after you've bred. It's easy to see how a mechanism like this could evolve. Of course, if your own children die you might end up stealing someone else's children and that's what we're seeing here.

Similarly, a generic drive to care for your own children makes you resist attempts by others to steal them, even if they would actually do a perfectly good job of raising them. This case is even harder to detect, as it requires divining the thief's intentions. For instance, if they're taking your child to kill it, that's obviously something you don't want to allow. And since you have no good way of determining their intentions, it's best to not allow it.

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7 Comments

Is it known that the attempted theft wasn't actually an attempted murder? Killing unrelated infants is a very common behavior in many species, for obvious evolutionary reasons. But I don't think I've ever heard of a species other than humans that's known to attempt to kidnap and care for other adults' young.

I certainly don't know it, indeed as the next-to-last paragraph suggests, I think it's worth considering. OTOH, it's not clear why a parent who had recently lost a child would be more likely to commit murder, since it's not especially likely to improve their reproductive fitness. The case that I'm aware of where murdering non-related infants is especially attractive is the one where the new alpha male wants to free up the mother for breeding, which isn't the case here.

In primates, the related phenomenon of "allomothering" is well known and well studied. I believe it's usually temporary, though. Sometimes females even kidnap other mothers' infants for a short time.


I think your analysis misses one huge benefit of giving up your children -- it allows the mother to go on to produce more children sooner, and the interbirth interval is a critical component of lifetime reproductive success.

Whoops -- I think you did cover that.
However, I think this sentence is imprecise:

"In order for it to be a good deal, you have to believe that you caring for them will increase their chances of survival more than not caring for them."

The important thing is that each infant's chance of survival should not be so badly diminished that it offsets the benefits of faster reproduction. This may mean that individually each child has a lower chance of survival, or even that the whole group could have a diminished lifespan, as long as there are more of them by a sufficient number to offset these costs. I think you were getting at that.

Yes, that's what I meant.

This article discusses attacks on infant primates--lemurs in particular, although others are mentioned--and suggests that they are nearly as often perpetrated by females as by males. Although no definitive explanation is given, resource competition is offered as one hypothesis. Perhaps the penguin case is analogous.

I suspect that how willing an animal is to raise kids it finds (or steals, though that seems like a side-effect of a more sensible mechanism) probably has a lot to do with how closely related random kids are likely to be. If lost kids you adopt are reasonably likely to be (say) your nieces and nephews, and it doesn't cost too much to adopt them, you probably end up with mechanisms to make you want to adopt them.

-John

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