If you were to step out into the street and ask the first ten people you see whether or not they believed dolphins were creatures of intelligence, 8 of 10 would probably say yes. If you ask two scientists the exact same question, you just may be opening up a debate that could last for hours.
Most scientists to-date have refused to take a firm stand on the intelligence level of the dolphin, and although many will admit that dolphins seem to be intelligent creatures, it is not a proven fact. The biggest problem researchers are running into seems to be the actual definition of intelligence.
Darwin's theory bases intelligence on the ability of a species to recognize what it needs to survive, while other schools of thought believe the the size and architecture of the brain, the ability to communicate, or the ability to exhibit playfulness are the indicators.
Another reason that the research on dolphin intelligence is limited is because it's difficult and expensive to do in the wild. Dolphins in captivity, may respond differently based on their surroundings and are therefore may not be representative of the wild dolphin. While the U.S. Navy has carried out extensive research on the dolphin and have even trained them to search and even tag mines, that information has not been released to the public.
While most of us outside the scientific realm know that dolphins do communicate with each other and they're one of the most playful animals in the universe, we've never been exposed to the intricacies of the dolphin's brain. Consequently, you may surprised to know that the dolphin brain is actually much larger than the human brain. Dolphins have two hemispheres just like humans however, theirs are split into four lobes instead of three. The fourth lobe in the dolphin's brain actually hosts all of the senses, whereas in a human, the senses are split. Some believe that having all of the senses in one lobe allow the dolphin to make immediate and often complicated judgments that are well beyond the scope of a human ability.
When studying the neo-cortex, which is the outside surface of the brain that is responsible for forming perceptions, memories and thoughts, dolphins have more convolution than the most intelligent humans. It is thought that dolphins may also be able to use the hemispheres of their brain separately as they have separate blood supplies which is something that is exclusive only to the dolphin. To add more weight to this supposition, dolphins are also able to move their eyes independently which has lead some researchers to suggest that the dolphin may actually be able to sleep with one side of it's brain at a time.
Some researchers have suggested that the size and complexity of the brain at birth is a better measure of intelligence. If that research holds up, however, once more the dolphin comes out on top. The bottle-nose dolphin has a brain mass at birth that is 42.5% of that of an adult humans brain mass. Humans at birth have 25% of their adult counterparts. At 18 months, the brain mass of the bottle-nose dolphin is 80% of the adult human, who doesn't usually achieve this level until the age of three or four.
While the research that has been obtained certainly makes it look like the dolphin may be the closest to our own level of intelligence, it may be quite some time before it's actually prove. Until then, they're true capabilities will remain a mystery.