Some scientists study intelligence by looking at the brain.
Dolphins and whales have quite large brains, also. The average weight of an Atlantic bottlenose dolphin brain is 1.53 kg and the sperm whale has the largest brain of any animal at 7.8 kg. Dolphins are much larger animals, on average, than humans. The human brain has a lot of grooves and ridges so the structure itself is much more complex than most animal brains. An animal's behavior is guided by instinct and learned behavior.