WHO ARE THE WHALES?

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Abstract
Whales, dolphins and porpoises, 80 species of entirely aquatic mammals, constitute the order Cetacea. In the early Eocene, about 55 to 60 million years ago according to paleontologists, distant ancestors of modern cetaceans left land for aquatic life. Cetaceans are diverse; average adult size of cetacean species varies by 1000 to 2000 times. Small and large species occupy all oceans from the equator to the polar seas, some forms inhabit rivers and four species live only in fresh water. Cetaceans are born in water and spend their entire lives in the aquatic medium. There is a great gap in knowledge about hearing in most cetacean species and especially about how noise and high-intensity sound may affect all cetaceans and other mammals underwater. Studies of temporary threshold shift (TTS) and occupational noise exposure in human divers suggest a cautious approach to cetacean noise exposure until data on cetacean TTS can give us some idea of the dynamic range of cetacean ears.