This means among other things, that they breathe air.
Yet, they live all the time in water, so how did this come to be?
How dolphins adapted over time
The ancestors of dolphins were animals that once lived on land.
No one is quite sure why they turned to the sea, but they did so about 50 million years ago and never returned to land.
When this happened, the dolphin’s ancestor had to slowly adapt to its new environment.
Its front legs became flippers and its tail became flukes.
Its back legs diminished till they were absorbed into the body.
Even now, if a person looks at a dolphin’s skeleton, vestigial, floating pelvic bones can still be seen.
Now and then there’s a baby dolphin or whale that’s born with tiny hind legs, but this is rare.
Mammals also have fur or hair and dolphins are sometimes born with hair around their beaks, but they lose this hair not long after birth.
Nature wants the dolphin to be as hydrodynamic as possible, so the animal sacrificed its hair, though it still retains hair follicles.
What dolphins have to keep them warm is a layer of blubber beneath their skin.
The dolphin’s body became streamlined in order to allow it to move more efficiently through water.
The nostrils moved from the front of its snout to the top of the head, which is very convenient.
This means that the dolphin only needs to just break the surface of the water in order to breathe.
This is probably a reason why dolphin babies are born tail first.
When they’re fully out of the womb, the mother, sometimes with the help of a sister or a friend, helps the baby to the surface so it can have its first lungful of air.
Even through all of this evolution dolphins still have lungs and not gills as fish do.
A dolphin must hold its breath when it’s swimming under water and can do this for a long time.
But it can’t hold its breath forever and needs to surface.
Some biologists believe that when a dolphin sleeps only one half of its brain is asleep and the other half is awake or at least awake enough to make it swim up to the surface of the water to take a breath.
Though it doesn’t seem that way to a human swimmer, sounds travel much better under water than it does through air and can travel hundreds of miles underwater.
Though their outer ears have also been reduced to holes on either side of their heads, dolphins can hear much better than humans.
When they’re under water dolphins pick up sounds through their lower mandible.
Dolphins have developed high-pitched clicks and whistles to communicate with each other.
Biologists believe that this may be an actual language and not just the series of sounds other animals make that indicate the presence of food or danger or to declare the animal’s territory.
Dolphins also use echolocation through an organ in their head called the melon.
Echolocation is so accurate that the dolphin not only knows exactly where the object is but how big it is and what shape it is.
All in all, the dolphin, the descendant of a dog-sized land animal, has adapted beautifully to its aquatic life.