For example, recent scans of dolphin brains has led to the discovery that a portion of their brain has extended far beyond what we humans have, This part, connected to the limbic system (which for all you people who don't spend their spare time reading about brains, largely controls emotion, learning and memory) is hypothesized to hold a place for more extreme, and more highly controlled emotions than we humans can even imagine. This means dolphins may very well have even deeper emotional lives than humans do, yet can control and rationalize their emotions better as well.
Dolphins are highly social animals. They work together like never before seen, acting as one unit at times. It is thought that maybe, just possibly, a dolphins sense of self is extended to the other members of its pod.
Another amazing thing about dolphins, especially Orcas (yes, Orcas are a dolphin) is that they have, at least to some extent, language. Orcas are particularly fascinating because each family has a different dialect than the other. They use this difference of language to prevent inbreeding. The language of an orca pod is passed down each generation, as their family units are extremely stable and the offspring stay with their mother their entire lives. Different pods who share some calls indicate that there was a common ancestor a few generations back.
In captivity, dolphins learn to read hand cues from their human trainers to preform tricks. They can understand complex hand signals, and even strings of sign language that would form a sentence. Such as ' Go touch the ball, swim around the hoop then jump through it.' Not only can they understand and execute multiple commands at a time, but they know the importance of the sequence of cues given and do them in order.
An experiment was done to test the level of creativity in dolphins. While I will not go into details here, the results went like this: it took about the same number of tries for a dolphin to understand that they needed to come up with a new behavior in order to get rewarded, as it did for the human to realize the same thing.
Another test showed that dolphins have the same self awareness as apes and humans do. So far it doesn't seem that humans are capable of many cognitive skills that dolphins are not.
With that being said, a dolphin in captivity is not much different than a human in captivity. So what justifies it?
On this subject is a movie that I recently watched and enjoyed, and would recommend to anyone who has ever visited Seaworld or is interested in the subject.