Toothed whale porpoise as a fish. Toothed whales, as well as baleen whales, are descendants of land-dwelling mammals of the artiodactyl order (even-toed ungulates). They are closely related to the hippopotamus, sharing a common ancestor that lived around 54 million years ago (mya). The primitive cetaceans, or archaeocetes, first took to the sea approximately 49 mya and became fully aquatic by 5–10 million years later. The adaptation of echolocation and enhanced fat synthesis in blubber occurred when toothed whales split apart from baleen whales, and distinguishes modern toothed whales from fully aquatic archaeocetes. This happened around 34 mya. Unlike toothed whales, baleen
Captive killer whales twice that length or and a minimum depth of , giving a minimum volume of for two killer whales. Each additional killer whale requires a pool with an additional of volume. 9 CFR 3.104 also requires a minimum of surface area per killer whale in Table IV (the example with a cylindrical tank in diameter for two whales provides of surface area per killer whale). Swiss regulations require a larger minimum volume: × deep for two killer whales, or . The Alliance of Marine Mammal Parks and Aquariums (AMMPA) goes further, and recommends for two killer whales. The US exhibitors