Mammals
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Mammals are characterised by a number of features not shared by other animals: females have modified sweat glands which produce milk to feed their offspring; hair (although cetaceans only have hair in the foetal stage) and three tiny bones that transmit sound waves across the middle ear.
All mammals except for monotremes give birth to live young. Monotremes, which include echidnas and duck-billed platypuses, lay eggs. Mammals are warm-blooded, meaning that they are able to regulate their internal body temperature.
There are about 5,000 species of mammals and 2.5 per cent of these are marine. At least three orders include species which spend the majority, if not all, of their lives in water: cetaceans (whales, dolphins and porpoises), carnivores (pinnipeds - seals, sea lions and fur seals - and otters) and sirenians (mantatees and dugongs).
There are about 78 species of cetaceans. They are wholly aquatic, and are superbly adapted to an environment usually hostile to mammals and thus are able to breathe air, maintain a constant body temperature and bear live young which are then nursed. They are divided into two main subgroups, Mysticeti and Odontoceti, based on their feeding apparatus. Mysticeti are the filter-feeding baleen whales. Odontoceti have teeth and include some whales and all dolphins and porpoises.
Pinnipeds are aquatic mammals that spend some of their time on land, particularly for breeding. There are 33 pinniped species which include Phocoidea, the true seals, and Otarioidea, amongst which are the fur seals, sea lions and walruses. They have characteristic webbed feet which enables them to move on land and in the water.
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