Marine mammals are a diverse grouping of animals that all depend on the sea for their existence. Although they are all classified as mammals, they are not necessarily directly related by an immediate common ancestor and have come to be dependent on the sea through convergent evolution. The most important criterion for inclusion is that they depend on the sea for their food. Some have adapted to living their entire lives in the water, such as whales and dolphins, however, others can survive either on land or the sea, such as polar bears, seals, and otters.

Marine mammals inhabit waters in all the world’s oceans. The highest diversity of marine mammals is generally in cool, temperate regions (rather than in the tropics like most species). Some species, like the vaquita porpoise, have very limited ranges of only a few hundred miles. Others follow annual migration paths of thousands of miles across oceans. These long migrations can complicate efforts to make accurate biodiversity measurements and also to establish regulations protecting them.

There are 130 described species of marine mammals living in the earth’s oceans. They are divided into three orders: Cetacea (dolphins, whales, and porpoises), Sirenia (sea cows), and Carnivora (carnivores). Within those orders are several suborders and families. Definitions of species can sometimes be complicated and are based on morphologic characteristics, behavioral patterns, and habitat use. Recent advancements in genetic analysis have enabled scientists to define sub-species within certain marine mammal species, such as killer whales.

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