Marine mammals are a diverse group of 120 species of mammal that are primarily ocean-dwelling or depend on the ocean for food. They include the cetaceans (whales, dolphins, and porpoises), the sirenians (manatees and dugong), the pinnipeds (true seals, eared seals and walrus), and several otters (the sea otter and marine otter). The polar bear, while not fully aquatic, is also usually considered a marine mammal because it lives on sea ice for most or all of the year.
Marine mammals evolved from land dwelling ancestors and share several adaptive features for life at sea such as generally large size, hydrodynamic body shapes, modified appendages and various thermoregulatory adaptations. Whales are the largest mammals ever. Different species are, however, adapted to marine life to varying degrees. The most fully adapted are the cetaceans and the sirenians, which cannot live on land.
Despite the fact that marine mammals are highly recognizable charismatic megafauna, many populations are vulnerable or endangered due to a history of commercial use for blubber, meat, ivory and fur. Most species are currently in protection from commercial use.
There are some 120 extant species of marine mammals, generally sub-divided into the five groups bold-faced below. Each group descended from a different land-based ancestor. The morphological similarities between these diverse groups are a result of convergent and parallel evolution. For example, although whales and seals have some similarities in shape, whales are more closely related to deer than they are to seals.
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