Dolphins, as well as whales and porpoises, are marine mammals commonly called cetaceans, as they belong to the order Cetacea. They inhabit all oceans and although, classified as marine mammals, some species of dolphin also inhabit freshwater rivers and they are even found as far from the ocean as in Nepal. They are fully adapted to spend their entire life in water. Being marine mammals they are warm blooded like us. Their body is insulated by a thick layer of blubber (fat) that allows them to keep a constant body temperature. Furthermore their circulatory system is modified in order to minimise the loss of heat.
As all mammals, they breathe air through lungs and therefore need to surface at regular intervals to breathe through their blowhole (the equivalent of the nose in terrestrial mammals). Some cetacean species like the sperm or beaked whales, can dive to a depht to 2000 metres for up to two hours and therefore have adapted their skeletal and oxygen storage systems to enable them to sustain the extreme high pressure and lack of oxygen. Most of the species have a dorsal fin that can range from a small bump to a very tall fin, like in the killer whale. They all have two side pectoral fins and a fluke or tail that they use a main source of propulsion.
Cetaceans are subdivided into two sub groups:
Odontecetes: comprise toothed whales and dolphins. They all have teeth and a single hole blowhole. About 70 species of cetaceans belong to this group. Odontocetes also use sound to echolocate and catch their prey.
Mysticetes: include whales that have baleens instead of teeth. Baleen are a filter system form of plates and hair made of keratin (the same substance of our nails and hair). During feeding the whale fills its wide mouth with water and then pushes it out using its tongue, filtering the nutrients through the baleen, and once the water has been disposed of, they swallow all the nutrients. Mysticetes do not use echolocation and have a two holes blowhole.
As light does not travel in water as fast and efficiently as in the air, sound plays a fundamental role in the life of all cetacean. All cetaceans are extremely social animals and sound is used to communicate among pairs. Whales can produce low frequency sounds that can travel across oceans and able them to keep in touch with other individuals even across thousand of kilometres. Odontocetes utilize sound to echolocate as well as for communication. Echolocation implies that a high frequency sound is emitted by the animal through its larynx, which travels through the environment, and reflects an echo back from any object. The animal hears this sound and transforms the information into an acoustic image. Odontocetes use echolocation to orientate, explore the surrounding, and catch their prey. River dolphins rely totally on sound as they mostly live in very murky waters where sight is basically useless and their eyes have atrophied.
All cetaceans live in social groups and they collaborate throughout their life to support their community and the survival of their progeny. They are extremely intelligent animals and we are only now discovering the complexity of their social structure and their languages. Dolphins for example are known to be among those mammals that can use tools and dolphins in captivity are renown to be able to repeat human sounds.