critter vs mammal

critter

noun
  • A creature, an animal. 

mammal

noun
  • A vertebrate with three bones in the inner ear and one in the jaw. 

  • An animal of the class Mammalia, characterized by being warm-blooded, having hair and producing milk with which to feed its young. 

How often have the words critter and mammal occurred in a corpus of books? (source: Google Ngram Viewer )