Around 50 million years ago, relatives of modern day dolphins first entered the seas. Today, these aquatic mammals -- who are part of the order Cetacean -- can be found in oceans all across the globe, and even occasionally in rivers. Although there are 36 different species of dolphin, they all have certain traits in common.
All dolphin species have a similar physical appearance. They have elongated snouts and rounded heads that have no distinct separation from the rest of their bodies. The main part of their bodies taper down into their tails -- known as flukes -- at the end. Dorsal fins and pectoral flippers are also present. A dolphin's eyes, one on each side of its head, can see independently of one another. Dolphins breathe through blowholes positioned on top of their heads.
Sounds and Communication
Dolphins communicate with one another using a series of clicks, whistles, screams, grunts, moans, squeaks and other noises. Individual dolphins use a distinct signature whistle to identify themselves, almost like a name. Without outer ears they are able to hear through an area on their jaws that acts as a window to their inner ears. Not all sounds that dolphins make are used for communication. Clicking noises are used as echolocation, which enables them to sense objects. They use echolocation more than they use visual ability.
Extremely social animals, dolphins live together in groups or pods. These pods can contain up to 100 individuals, but size varies according to species. Members of a pod form strong bonds with each other and will help out sick or injured individuals, working together to protect the group. Pods of dolphins have even been known to kill sharks that are threatening them. These playful creatures will often chase one another or throw found items back and forth as a game.
Among the most intelligent species on the planet, dolphins have extremely large brains compared to their size, with only humans having comparatively larger brains. It has been observed that they enjoy complex problems and quickly lose interest in mundane activities or tasks. Dolphins have a large memory capacity, and they can follow complicated instructions and recognize the difference between shapes, materials and colors, as well as differentiate between solid and hollow items. Dolphins are one of very few species who can recognize their own reflection in a mirror.