What would cause natural selection to select traits for smarter animals?

Question

Wouldn't other animals be better off being smarter?

Answer 1

My guess is that intelligence is like a snowball effect...once you are intelligent enough, it becomes very beneficial to be even a little smarter.  

But below a certain level, it just isn't that benefical to be a little smarter.

So I'd say that only a small number of populations have ever reach that minimal snowball intelilgence level.  And only one such population now survives, homo sapiens sapiens.

Answer 2

Having a bigger brain makes more demands on food requirements, so how much it is worth it depends on the animal's lifestyle.  Selection is always a matter of trade-offs, and different species arise through selection for different niches.