Why would any organism ever choose to cooperate?

All organisms are innately competitive with others of their own kind, This is the core message of Darwin's theory of evolution. But he was never able to explain why we see so many creatures in the wild cooperating.

Here's a mind experiment.

If 100 monkeys descend on a tree which has only 90 fruits, then only 90 monkeys get to eat. 10 monkeys will go without food today and be severely weakened when looking for food tomorrow.

But if those 10 monkeys were to share fruit, getting only 9/10ths of a fruit each, then they would survive another day and have the energy to look for more food tomorrow.

The simply answer to Darwin's quandary is to treat food in the wild as quantised.

This cooperative strategy may not make much difference on a one-off basis. But, if there continues to be a scarcity of fruit, then cooperation becomes a good strategy to smooth the food intake for individuals, even though they are inherently competitive creatures.