* They breathe air: Unlike fish, dolphins need to come to the surface to breathe air through their lungs.
* They give birth to live young: Dolphins don't lay eggs. Instead, they give birth to live offspring.
* They nurse their young: Mothers produce milk to nourish their young.
* They are warm-blooded: Dolphins maintain a constant body temperature, unlike fish, which are cold-blooded.
* They have hair (at least as babies): Though most adult dolphins don't have visible hair, they do have hair follicles, and dolphin calves have a few wisps of hair around their snout and mouth.
* They have a streamlined body: While their shape resembles fish, their body is streamlined for swimming, not for the same reasons as a fish.
These are just some of the key characteristics that classify bottlenose dolphins as mammals. They share many of these features with other mammals, such as whales, seals, and humans.