There are 31 species of true dolphins. About 21 of these species are found in Australian waters. Most dolphins have the "typical" streamlined shape, with a pointed beak and prominent dorsal fin. The largest of the dolphins is the bottlenose dolphin, which grows to over 4 meters and can weigh over 650 kg. The smallest is Hector's dolphin at about 1.5 meters and 60 kg. All dolphins are "homodonts", which means that all their teeth are the same, not having molars and incisors like most mammals. Their teeth are sharp and peg-like in shape and function for grabbing prey, not for biting out chunks or chewing. You can tell a dolphins age by counting the growth rings in their teeth.
The dorsal fin (on the back) stabilizes the dolphin, flippers assist in balance and direction, and the tail flukes provide driving power. Calves receive assistance by swimming in their mother's slipstream. Dolphins have adapted to swimming in the marine environment by streamlining their bodies, through the loss of hair and hind limbs, and with the movement of their nostril (blowhole) to the top of the skull.
Dolphins can see well both in and out of the water. Touch is well developed all over the skin, with highly sensitive regions around the blowhole and on the tongue. A high level of physical contact between dolphins suggests touch is important in communication. Dolphins have a good sense of taste but lack the sense of smell.
Dolphins live in world of sound. They use sounds to communicate and each animal has its own signature whistle which acts like a name. They can hear sounds as high as 150 kilohertz (bats hear up to 120), which are picked up through their jaw.
Dolphins use sonar or echo-location to navigate and find their prey in dark and murky water. A series of "clicks" are made with the blowhole and emitted through their forehead. The echoes are heard through their jaw and enable the animal to form "sound images" of their surroundings. Dolphins may even be able to use super strong bursts of clicks to stun fish, making them easier to catch.
Dolphins do have a large brain and are superbly adapted to their environment. In captivity dolphins have been taught a type of language and can understand complex instructions, just like a dog or horse.