Cetacea
Cetaceans (from Latin: cetus, lit. 'whale', from Ancient Greek: κῆτος, romanized: kētos, lit. 'huge fish', sea monster)[1] are aquatic mammals constituting the infraorder Cetacea (/sɪˈteɪʃə/). Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel themselves through the water with powerful up-and-down movement of their tail which ends in a paddle-like fluke, using their flipper-shaped forelimbs to maneuver.[2]
While the majority of Cetaceans live in marine environments, a small number exclusively reside in brackish water or freshwater. Having a cosmopolitan distribution, they can be found in some rivers and all of earth's oceans and many species inhabit vast ranges where they migrate with the changing of the seasons.
Cetaceans are famous for their high intelligence and complex social behaviour as well as the enormous size of some of its members, like the blue whale reaching a maximum confirmed length of 29.9 meters (98 feet) and weight of 173 tonnes (190 short tons), making it the largest animal known to have ever existed.[3][4][5]
There are approximately 86[6] living species split into two parvorders: Odontoceti or toothed whales (containing porpoises, dolphins, other predatory whales like the beluga and the sperm whale, and the poorly understood beaked whales) and the filter feeding Mysticeti or baleen whales (which includes species like the blue whale, the humpback whale and the bowhead whale).
Cetaceans have been extensively hunted for their meat, blubber and oil by both indigenous peoples and commercial operations. Although the International Whaling Commission has agreed on putting a halt to commercial whaling, some nations continue to do so. They also face environmental hazards such as underwater noise pollution, plastic buildup and ongoing climate change,[7][8] but how much they are affected varies widely from species to species, from minimally in the case of the southern bottlenose whale to the baiji (or Chinese river dolphin) which is considered to be functionally extinct due to human activity.[9]
The two parvorders, baleen whales (Mysticeti) and toothed whales (Odontoceti), are thought to have diverged around thirty-four million years ago.[10]
