water rat vs water vole

water rat

noun
  • The European water vole, Arvicola amphibius, formerly Arvicola terrestris. 

  • A member of the water police. 

  • A water mouse, especially, the rakali, Hydromys chrysogaster. 

  • Any of various aquatic or semi-aquatic rodents from Florida and southern Georgia, especially Neofiber alleni; the muskrat. 

  • A petty thief or waterfront ruffian. 

  • A person fond of water sports. 

water vole

noun
  • Any of three Old World vole species: the European water vole (Arvicola amphibius, formerly Arvicola terrestris), the montane water vole (Arvicola scherman), or the southwestern water vole (Arvicola sapidus). 

  • The North American vole (Microtus richardsoni). 

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