What Is a Marine Mammal?

A marine mammal is a mammal that lives in the ocean and depends on marine environments for its life cycle. Like all mammals, marine mammals breathe air with lungs, are warm-blooded, have hair at some stage of life, and bear live young that they nourish with milk. What makes them “marine” is their unique suite of physiological and behavioral adaptations to life in seawater. Some species, such as manatees and seals, may also use fresh water or land, but all marine mammals depend primarily on marine resources for food and survival. These animals are found in all the world’s oceans, from the far polar reaches to the Equator. And although most marine mammals are coastal, some pelagic whales and dolphins dwell far offshore.

Importantly, marine mammals do not form one unified evolutionary group. They include whales and dolphins (cetaceans), seals and sea lions (pinnipeds), manatees and dugongs (sirenians), and sea otters. Each lineage descends from different terrestrial ancestors and entered the marine realm independently. Their similarities—streamlined bodies, insulating blubber (with some exceptions, such as sea otters, which rely instead on exceptionally dense fur), modified limbs (e.g., flippers or flukes), and the ability to store oxygen for deep diving—are examples of convergent evolution, shaped by the shared demands of life in the sea rather than by close common ancestry.

The following is a list of selected marine mammals, organized by taxonomic group and common name. (The polar bear is often included among marine mammals because of its reliance on sea ice and marine prey, though it is primarily a terrestrial animal.)

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List of marine mammals