Dolphins are extremely intelligent mammals. There has been some research that suggests mothers give young dolphins a name from birth which the dolphin will then respond to.
Killer whales are actually dolphins. These "wolves of the sea" are one of the most wide ranging animals on the planet and can be seen off almost every coastline in the world.
Dolphins can "see" in murky water by using echolocation, in a similar way to bats and submarines. An organ called the melon focuses clicks, and by listening to the response they can interpret a 3D picture of the world around them.
Dolphins don't drink. They get all the water they need from what they eat, ensuring that they don't ingest too much salt.
Some dolphins have been found with vestigial limbs. These are artifacts from their land-based ancestry, held over in their genes which are sometimes mistakenly switched on during their fetal development.
Porpoising, the act of jumping out of the water while travelling fast, is actually a behaviour that saves energy by reducing friction.
In Brazil, a family of bottlenose dolphins have been recorded helping fishermen to catch mullet, which can be seen in Human Planet. They scare the fish into the shallows and signal to the fishermen where they are and their abundance.
Some dolphins are endangered. Irrawaddy, Indus river and Yangtze, to name a few, are in critical trouble and need our help. Common dangers are human encroachment, changes in habitat and climate, and dangerous fishing practices.