This article is pretty interesting. It points to research done by Dr. Tadeusz Kawecki which indicates a negative correlation between intelligence and evolutionary fitness. They basically bred smarter flies by putting 15 successive generations through intelligence tests that determined their fate. It turns out these smarter flies die 20% faster than normal flies in typical fly conditions.

It's really no surprise then that not many animals are nearly as smart as humans. From the article, "Dr. Kawecki suspects that each species evolves until it reaches an equilibrium between the costs and benefits of learning... Humans' oversize brains require 20 percent of all the calories burned at rest. A newborn's brain is so big that it can create serious risks for mother and child at birth. Yet newborns know so little that they are entirely helpless. It takes many years for humans to learn enough to live on their own." So what is the great benefit to being so smart?