Anything warm blooded...as long as it gives birth to life young & obviously lives on planet earth.

Mammalogy is the study of mammals - vertebrates with characteristics such as homeothermic metabolism, fur, 4-chambered hearts, & complex nervous systems. It branches off into other taxonomically-oriented disciplines such as primatology (study of primates), cetology (study of cetaceans), equinology (study of horses), cynology (study of dogs), and many other fields.