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Killing newborns of one’s own species, known as conspecific infanticide, is rare among mammals, but it happens, mostly among primates, carnivores and rodents, and possibly dolphins. But it can be hard to tease out cause from effect when these events are so rarely observed.

The motivation is mysterious. Are they doing it for cannibalism, or for sport, or because of stress, or some kind of orca mental illness? It is particularly strange because orcas are generally so nice to each other. As the authors put it, “although killer whales (Orcinus orca) are one of the most studied and widespread species of cetacean, only a few observations of aggression between individuals of the same population have been reported.”

Photo by Scientific Reports

Menacingly, however, it is well documented that many individuals, males and females of all ages, have the teeth marks of other killer whales in their flesh.

So something wicked is going on under the surface of the ocean. But killing an infant seems particularly nasty, a crime of a different order than two adults fighting.

One key explanation is evolutionary, that a male who kills an infant also makes its mother fertile again. So in effect, the infanticidal male has done three very useful things for his odds of passing his own genes, and their role in this murderous behaviour, down the generations.

First, he boosts his own odds of procreating, and can do it in as little as a couple of months with the infant’s mother, as lactation stops and estrus resumes, rather than waiting far longer for the calf to wean.

Second, he gets rid of future competition for his own offspring.

Lastly, of course, he has made himself a tasty meal, which the authors coldly describe as a “secondary benefit of some infanticide events best explained by the sexual selection hypothesis.”

What is most striking in this B.C. case is the role of the accomplice mother. The theory is that it helps her son’s chances of procreating, which would seem to give the mother an evolutionary advantage too, in that her genes are more likely to live on through him.

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