Researchers are just beginning to uncover the extent of their intelligence. Dolphin-usually bottlenose dolphin (Tursiops truncatus) here-use flexible behavior to adjust to their physical and social environment. They can navigate the rules of their complex and hierarchical society, and they can also improvise and adapt-and teach new behavior to other dolphin. Like humans, dolphin learn through play. Long-term studies in Sarasota Bay have shown that juvenile male bottlenose dolphin gang together and behave like boisterous teenage boys, building lifelong friendships.

To keep track of these complex social relationships, dolphin use clicks and whistles, unique to each pod. But they also use touch and posture to communicate. We can't understand their languages, but they can learn ours-sign language, anyway. Studies in Hawaii have shown that dolphin understand words and the significance of word order. One dolphin verbal genius has a repertoire of 60 words and 2,000 sentences.

Dolphin, like chimpanzees and three-year-old humans, recognize themselves in the mirror, and dolphin understand TV. They know it is representational. (You may or may not conclude that this is intelligence.)

In Florida Bay, bottlenose dolphin have invented mud-net fishing. They work cooperatively. One zooms around in a circle creating a ring of rising mud. Others position themselves around the ring. As mullet jump to escape mud and dolphin, the fish become lunch.