Different dolphin species vary a lot in size. The smallest one, tucuxi dolphin, is about 4 feet (1.2m) long and weighs about 110 pounds (50kg), whereas the enormous killer whales is over 30 feet (9-10m) long and weighs 12,000 pounds (5 500 kg).

You've seen the gray ones, but dolphins can also be black or brown, and can have patterns of white or light colors.

In the middle of their back, many dolphins have what is called a dorsal fin. It helps them to keep their balance. Furthermore, each dolphin's dorsal fin is shaped differently from the others. It's like an identification mark. In some ocean dolphins, the dorsal fin is hooked-shape instead of like a triangle. The male killer whales have very tall dorsal fins whereas in river dolphins it scarcely forms a ridge on the back.

Besides using eyes, dolphins can find their way by making clicking sounds, and listening to the sounds that echo or bounce back to them from their surroundings. This is called echolocation.

Under their skin, dolphins have a layer of fat called blubber. It insulates the dolphin so it can keep warm even in cold water. The colder the water is where the dolphin lives, the thicker the layer of blubber.

Behavior

Dolphins are social animals that live in schools of varying size; some only have 2-5 dolphins while enormous schools of 1,000 or more have been found as well. A typical school of bottlenose dolphins might be 10-20 individuals.

For food, dolphins hunt fish, squid and other invertebrates; killer whales also feed on other marine mammals, sea turtles, and birds. Dolphins often hunt together, trying to catch schools of fish. They can use echolocation to find prey in deep, dark waters.