I was watching a nature documentary where a crab produced several dozen babies, and then turned around and started eating them. If she needed the nutrients so badly, why not just have fewer kids? From a thermodynamic standpoint that would preserve more calories.
I’ve also seen footage of birds brooding, laying, and then hatching multiple eggs, only to push half of the chicks out of the nest. That’s such a huge investment of time and energy. Why not just lay fewer eggs?
In other situations it is more understandable: A male lion might kill another male’s offspring to make room for his own. Cuckoos push other baby birds out of the nest so they can be adopted by the parents. But many cases of infanticide in the wild just seem time-consuming and wasteful.
In: Biology
First of all, I don’t think it’s as common as all that for animals to eat their young, unless there’s some kind of crisis that happened in between conception and birth. You get a false sense of the frequency of events when every observed instance is made into a documentary because it’s shocking.
The causes for it vary. [Minute Earth did a video about it.](https://youtu.be/8xVgAULDwNE)
Also, having fewer babies isn’t always an easy option. If your body produces five eggs at a time, that’s how many you get. Adaptive behavior can be faster than evolution. Maybe those parents are culling their brood so that they have fewer babies competing with each other for resources. Maybe they’re selecting the strongest ones to survive, and having extra babies gives them the opportunity to make that choice.
Ultimately, it’s hard to know what’s going on in the animal’s mind at the time.
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