What are mammals?

Mammals are vertebrate animals with pulmonary respiration and constant temperature.
Mammals have two occipital condyles in the skull, a body covered in hair, and mammary glands in females.
The appearance of mammals is very variable, and they can have a certain resemblance to birds or fish.
The skin of mammals is generally covered in hair, which can vary in shape and length.
Mammals have four locomotor limbs, although the second pair can be absent, and a more or less developed tail.