Marine Mammals are those species of mammal that spend all or a major proportion of their life in the marine environment. Most mammals are characterised as air-breathing, warm-blooded vertebrates, that feed their young milk produced by the mammary glands of the female. Mammals on the whole have hair, although this is almost entirely lost in two groups of modern marine mammals.
Modern marine mammals either spend their entire life or part of their life in the marine environment (and occasionally rivers). There are two groups in the first category: the Cetacea, obligate aquatic carnivorous mammals, (whales, dolphins, and porpoises); and the Sirenia, obligate aquatic marine herbivores (manatees and dugongs). The marine mammals that also spend part of their lives ashore (to breed, for instance) include the Pinnipedia (true seals, eared seals, and walrus), and otters. The polar bear (Ursus maritimus) is also sometimes classified as a marine mammal, because it spends considerable time foraging in the marine environment. Altogether there are about 120 species of marine mammal.
Sub-order Mysticeti (baleen whales): 4 families, 6 genera and about 13 species.
Sub-order Odontoceti (echolocating toothed whales): 10 families, 40 genera and about 70 species.
Family Otariidae (eared seals): 7 genera, about 15 species.
Family Phocidae (true seals): 13 genera, about 19 species.
Family Odobenidae (walrus): one genus and one species.
Family Mustelidae: 2 genera, 2 species of marine otter, the sea otter and the marine otter.
Family Ursidae: one genus and one species of marine bear, the polar bear.