Here's why:
* They breathe air: Dolphins, unlike fish, have lungs and must surface regularly to breathe.
* They give birth to live young: They don't lay eggs. Dolphin babies are born live and are nourished by milk from their mothers.
* They have hair (at least during development): While dolphins don't have much hair as adults, they do have some hair follicles when they are born.
* They are warm-blooded: They maintain a stable body temperature regardless of the surrounding water.
What is a mammal?
A mammal is a warm-blooded animal that:
* Has hair or fur.
* Gives birth to live young.
* Nurses their young with milk produced by mammary glands.
* Has three middle ear bones.
* Has a neocortex, a part of the brain associated with higher cognitive functions.
Mammals are a diverse group, including everything from tiny shrews to massive blue whales. They are found all over the world and have adapted to a wide variety of habitats.