A marine mammal is a mammal that is primarily ocean-dwelling or depends on the ocean for its food. There are five groups of marine mammals:
- Order Sirenia: the manatee, dugong, and sea cow
- Order Carnivora, family Ursidae: the polar bear
- Order Carnivora, infrafamily Pinnipedia: the seal, sea lion, and walrus
- Order Carnivora, family Mustelidae: the otter
- Order Cetacea: the whale, dolphin, and porpoise
All mammals have hair. The marine mammals, with the exception of polar bears and otters, have lost most of their hair, thus decreasing water resistance. They rely on insulation by a layer of fat instead. Since the different groups of marine mammals originate from different ancestors, this is a case of convergent evolution.