Well, their brains, of course. Dolphins have large brains for their bodies -- in fact, a bottlenose dolphin is second only to humans in the ratio of brain size to body size. Researchers have also pointed to the parallels in the organization of dolphin and primate brains as more evidence of high intelligence in dolphins. Some have gone so far as to suggest that dolphins actually have a language that humans simply cannot comprehend.

But others say that in our enthusiasm to anthropomorphize dolphins, we give them powers they just don't possess. A closer look suggests that much of the dolphin's large brain is taken up with echolocation and handling acoustical information -- processes at which they excel. But dolphins tend to rank at about the level of elephants in "intelligence" tests and haven't shown any unusual talent at problem solving.

They are excellent mimics of sounds and clearly communicate with one another, but does that mean they "talk?" No one less than Aristotle once wrote, "The voice of the dolphin in the air is like that of the human in that they can pronounce vowels and combinations of vowels, but have difficulties with the consonants." But a more scientific analysis of dolphin sounds suggests that for all their communication skills, dolphins lack the repertoire to have anything approaching language as we know it.

So what have researchers learned about dolphin intelligence?

Dolphin researcher Pieter Arend Folkens tells this story: "Since trash can be dangerous to dolphins if ingested, some of the animals at Marine World Africa USA were trained to retrieve the trash and return it to the trainer for a reinforcement reward.

"A trainer would come out onto the floating stage and a dolphin would perform a tail stand with a piece of trash in its mouth. The trainer would then reward the dolphin with a bit of fish.

"One day the lead trainer went through the routine only to notice that the dolphin kept coming back with a piece of trash even though the tank appeared clean. The trainer asked a colleague to go below to the engineer's port to observe what the dolphin was doing when a trainer came out on the float. The trainer came out on the float and sure enough, the dolphin quickly showed up with a piece of trash and got his reward.

"The scam was revealed! This dolphin had established a savings account of sorts. He collected all the trash and stuffed it in a bag wedged in a corner of the tank near the intake of the filtering system. In there was paper, rope, and all sorts of trash. The amazing thing is that when he went to the bank he did not simply take a piece, rather he would tear a bit off to maximize the return.

"This behavior is particularly interesting because it shows that the dolphin had a sense of the future and delayed gratification. He had enough presence to realize that a big piece of trash got the same reward as a small piece, so why not deliver only small pieces to keep the extra food coming? He in effect had trained the humans."

How complex is dolphin communication?

In a noted experiment Dr. Javis Bastian found that dolphins could communicate about abstract ideas. Two dolphins in captivity, Buzz and Doris, were trained to push different switches to earn a fish. When a light came on and stayed on, they were supposed to push the right switch. When it blinked on and off, they were to push the left switch. No problem. But then the test became more complicated. They were taught to follow a sequence -- Doris had to wait for Buzz to press the signal first. Then they were separated so they could only hear each other. Finally, Buzz was blocked from seeing the light.

When the light came on under those conditions and stayed on, Doris waited for Buzz to hit his signal. When he didn't, Doris eventually made a sound, and soon after Buzz pushed the right signal. Doris followed suit, and they got their fish. Almost every time the experiment was repeated Buzz pushed the correct switch, leading Dr. Bastian to conclude that dolphins can communicate abstract ideas, such as left and right.

More recently, researchers at the Sea Life Park in Hawaii have been testing dolphins through an underwater touchscreen attached to a computer. There are no food rewards, so the dolphins use the touchscreen solely for intellectual stimulation. The scientists found that the dolphins weren't particularly interested in abstractions, such as geometric patterns or artificial sounds. But they were very excited about touching the screen if it resulted in their seeing videos of other dolphins or hearing dolphin sounds. The next step will be to let dolphins choose video or audio sequences and then try to analyze why they're making those decisions.

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