Among the adaptations of dolphins are hydrodynamic bodies, blowholes on top of their heads, flippers and flukes and echolocation. Some scientists believe that dolphins are able to enjoy the benefits of sleep even while they're in the water by having one half of their brains alert and the other shut down.
As with other whales, the ancestors of dolphins were terrestrial animals that returned to the sea for good. They are entirely pelagic, but they are still mammals. This means they are warm-blooded, need to breathe air and nurse their young with milk from mammary organs.
To help the animals move through the water, their bodies became streamlined. Their forelimbs evolved into flippers and their tails evolved into flukes. They lost their hind limbs.
The unique shape of a dolphin's pupils allow it to see well in both air and water.
The dolphin's nostrils moved up to the top of its head and became blowholes. This allows it to breathe without raising its head out of the water.
Dolphin babies are born flukes first and need help to the surface for their first breath. They don't have lips to suckle milk from their mothers, so the milk is injected into their mouths.