This is an interesting article about recent evidence of deep thinking by dolphins:
Social tagging: Biology > Consciousness > Environment > Interspecies Communication
At the Institute for Marine Mammal Studies in Mississippi, Kelly the
dolphin has built up quite a reputation. All the dolphins at the
institute are trained to hold onto any litter that falls into their
pools until they see a trainer, when they can trade the litter for
fish. In this way, the dolphins help to keep their pools clean.
has taken this task one step further. When people drop paper into the
water she hides it under a rock at the bottom of the pool. The next
time a trainer passes, she goes down to the rock and tears off a piece
of paper to give to the trainer. After a fish reward, she goes back
down, tears off another piece of paper, gets another fish, and so on.
This behaviour is interesting because it shows that Kelly has a sense
of the future and delays gratification. She has realised that a big
piece of paper gets the same reward as a small piece and so delivers
only small pieces to keep the extra food coming. She has, in effect,
trained the humans.
cunning has not stopped there. One day, when a gull flew into her pool,
she grabbed it, waited for the trainers and then gave it to them. It
was a large bird and so the trainers gave her lots of fish. This seemed
to give Kelly a new idea. The next time she was fed, instead of eating
the last fish, she took it to the bottom of the pool and hid it under
the rock where she had been hiding the paper. When no trainers were
present, she brought the fish to the surface and used it to lure the
gulls, which she would catch to get even more fish. After mastering
this lucrative strategy, she taught her calf, who taught other calves,
and so gull-baiting has become a hot game among the dolphins.