Marine Mammals

Wholly aquatic mammals (those that never normally emerge on to land) are confined to two orders, the Cetacea and the Sirenia. The Cetacea comprises some 78 species, all except five marine, distributed throughout the world 's seas. They include the largest living animals ' the rorquals in the family Balaenopteridae. All cetaceans are carnivorous; the baleen or whalebone whales filter feed on organisms (e.g. krill) several orders of magnitude smaller than they are. Only one species of Sirenia, (Dugong dugon) is exclusively marine and is herbivorous, feeding mainly on seagrasses.

The remaining wholly marine (those animals that do habitually frequent fresh water) mammals are all included in the order Carnivora and include two species of otter in the family Mustelidae, Enhydra lutris and Lutra felina. Members of the three pinniped families Odobenidae (the walrus), Otariidae (eared seals) and Phocidae (earless seals) are all largely aquatic, only emerging on land to breed and rest, particularly when molting. One member of the family Phocidae, the Caribbean monk seal Monachus tropicalis, has become extinct this century. All species are carnivorous.

In contrast to most terrestrial mammal families, pinnipeds are considerably more diverse and more abundant at higher rather than lower latitudes. Of the 32 extant or recently extant species, only five occur within the tropics (two marginally). Reasons for this might be the greater availability of suitable habitat at higher latitudes, the greater productivity of shelf waters at high latitudes and the more isolated character of many island breeding sites in temperate and sub-polar parts of the southern hemisphere.