Like us, dolphins are masters of their habitats. Both humans and dolphins have "adapted almost perfectly to the world we live in," former "Flipper" trainer Ric O'Barry writes. Dolphins aren't just sleek, strong swimmers -- they've got hefty brains (when compared to body size, dolphin brains place second after ours) and are capable of complex thought, as these studies show.

Dolphins are great at communicating

The distinctive whistles and clicks of a dolphin helps the marine mammal locate food in murky water -- and they can make these calls for a staggering 15 days. But research suggests there's much more going on with these sounds than echolocation -- bottlenose dolphins use vocal labels to identify each other and can remember other dolphins' whistles for decades.

[Listen to a whistling dolphin here.]

Dolphins are also adept at communicating with another species: humans. Researchers at the University of Honolulu's Dolphin Institute, for example, trained a dolphin named Akeakami to understand approximately 40 words of a rough sign language, including grammar. If a scientist said, "Left basket right ball," Akeakami knew to carry a ball from her right to a basket on the left.

Dolphins recognize their reflections

Dolphins are one of the few animal species that can recognize themselves in a mirrored surface, according to dolphins researcher Diana Reiss and Lori Moreno. "Dolphins may pay less attention to marks on the bodies of companions because, unlike primates, they do not groom each other," the researchers write. "This difference makes our findings even more interesting because dolphins clearly are interested in marks on their own body despite the fact that they do not have a natural tendency toward social grooming."

Dolphins show creativity

Akeakamai -- the dolphin who understood sign language -- and another dolphin, named Phoenix, demonstrated creative answers to a problem posed by researchers at the University of Hawaii. When the scientists withheld fish snack rewards, usually given for the dolphins' "normal repertoire of behaviors," according to Psychology Today, "the dolphins began to find new combinations of soaring leaps with twists, underwater rolls with fin slaps, and pairings of vocalizations with gestural behaviors."

Dolphins use tools -- and hang out in cliques

By tearing off pieces of sponge in with their mouths, a dolphin can probe for nutritious bottom-dwelling fish that hide among stones and broken coral. The sponge covers the dolphin's snout like a mouth guard, keeping the animal's beak safe from rocky edges.