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
There’s a couple of possible situations where this becomes an evolutionarily stable strategy (if I may use jargon to hide from the squeamishness).
One is if the animal doesn’t know, at the point of conception, what resources will be available later. In that case, an animal that holds back and fertilises only a few eggs will miss out if conditions are good later; on the other hand, the one that fertilises a lot of eggs at least gets some of the calories back.
Another is that if the animal lays and hatches more eggs than they can support, then they can pick and choose the most promising ones to survive. If they lay just enough, they are leaving that to the genetic lottery, and might end up with fewer adult offspring than would be optimal.
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.
In the case of birds, it is natural selection getting an early start. The parents kick the weakest or least likely to survive out of the nest. The initial egg production is a fraction of the energy cost of keeping a chick alive through fledging, so they are cutting their losses by focusing on the ones likely to survive. By having more eggs initially, they raise the chances of getting at least a couple strong chicks. The crab thing may be similar, I have no clue.
It exists because it IS very efficient. Mother mammals will often consume their young when in high stress situations around food and/or predation. Some mammals will actually abort their litter when under stress and the mother will then consume the litter. In this way they recoup as many calories as they can from the breeding attempt and are now fueled for a fight or flight to live and breed again another day.
I have little to add here aside from the following: This is one of the better questions I’ve seen in this sub and nature is f’in metal.
They’re not falling for the sunk-cost fallacy. Sure it took a lot of time and resources to hatch all those eggs, but it also takes a lot of time and resources to actually raise those chicks to adulthood. If they have limited resources, they’re going to save themselves the effort of feeding all of their chicks in order to make sure the strongest ones get the best chance of surviving and reproducing.
A lot of animal behavior comes down to passing on their genes, and raising a bunch of weak chicks who will either die or fail to get a mate if they do survive isn’t conductive to that goal.
so there are a few reasons. the big 2 are ‘my offspring’ essentially means if they aren’t mine they don’t matter (lions) where it isn’t ‘for the good of us all’ so nothing else matters outside of my descendants
the other big one is essentially the kangaroo one. “I can survive without the child, it cannot survive without me” approach. The adult can die to save the child but even if it works (It won’t) but the child essentially dies to the elements (at best) but the adult can ‘throw away’ the child and survive itself. Quit applying human traits to this, one option is objectively better from a species standpoint
The parents need to produce enough young that *some* survive. The wild is a very unforgiving place and so mass production of young is required for the species to survive.
There are all sorts of stories about this. The two that come to mind is snakes that slip into unattended nests and eat everything in the nest. So that bird’s entire mating season is done in about ten minutes.
And then there is the time where a parent bird is trying to get all of its chicks fed and eventually one of them fails to thrive and dies. The parent does the utterly pragmatic thing: it breaks down the dead chick and feeds it to the remaining living ones.
It’s not just nature… up until about a 100 years ago it was super common in humanity as well.
Listen to Dan Carlin’s Hardcore History episode “Suffer the Children” podcast
Some animals have lots of babies often, like rabbits. Some animals usually have only one baby and it takes them a long time to have that baby, like elephants.
For animals that have lots of babies, they may stop raising the babies when it gets too hard to raise them, or only raise some of the babies. For animals that only have one baby, it makes more sense to keep trying to raise that one baby. Animals don’t think about this, it’s not a choice they make, but over time, it just worked out this way.
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