pilot whale vs sperm whale

pilot whale

noun
  • Either of two species of whale in the genus Globicephala, dark grey in colour. 

sperm whale

noun
  • A whale (Physeter catodon, syn. Physeter macrocephalus), the largest of the toothed whales. 

  • Any whale in the superfamily Physeteroidea, which also includes the small whales of genus Kogia. 

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