A person who studies mammals is called a mammalogist.
A mammologist is a person that studies the life and biology of mammals.
a person that studies gems is called a gemologist.
A person who studies animals is called a zoologist.
A person who studies physics is called a physicist, a person who studies chemistry is called a chemist, and a person who studies biology is called a biologist.
A person who studies the mind and behavior is called a psychologist.
A person who studies or writes history is called a historian.
A person who studies alligators is called a herpetologist.
Yes, there is such a thing as a mammalologist, 'animal biologist' may be a more confusing but probably more apt term.
A person who studies a lot is often called a "bookworm" or a "knowledge enthusiast."
A person who studies optometry is called an optometry student or an optometry student.
A person who studies culture is called an anthropologist.