Sperm whales are pelagic aquatic mammals that prefer deeper waters off the coast of tropical islands. They are migratory animals, though only the mature bulls will undertake the long trek to subarctic waters. This is perhaps the most famous species of marine mammal, owing its popularity to Herman Melville's novel Moby Dick, which featured an albino sperm whale. Sperm whales are gregarious by nature but their gams or schools are based on specific age and sex groups. These whales are one of the deepest divers found in the animal kingdom, and are frequently found entangled in electrical cables that are sitting on the ocean floor 1200 metres down. Sperm whales feed heavily on giant squid and much less so on fish; tests showed that 64% of sperm whales caught had nothing but squid in their stomachs, while only 9% had fish.