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Marine mammals are a class of animals (Mammalia) that are warm blooded and live most of their lives in the sea
Most people think of whales, dolphins and seals as the primary marine mammals. However, polar bears and sea otters, among others, also are classified as marine mammals
To be classified as a mammal an animal must posses all five of these characteristics
They have lungs and breath air
They do not use or have gills
They are warm blooded and maintain a constant body temperature.
Most bear live young. One primitive mammal group, the Monotremes (which includes the duckbill platypus and the echidna) lays eggs.
They produce milk and nurse their young.
They have hair at some point in their lives.
Even dolphins, whales and porpoises posses hair around their mouths which is usually lost soon after birth. If you look closely at some of these adult animals you can still see the raised follicles were the hair used to be.
All mammals are members of the "class" Mammalia. Within this class are smaller subgroups called "orders". Three of these orders are made up entirely of marine mammals. They are the orders of Cetacea, Pinnipedia, and Sirenia. A fourth order Carnivora contains both marine and land animals.
Cetaceans - whales, dolphins and porpoises
Pinnipeds - seals, sea lions and walruses
Sirenians - dugongs and manatees
Carnivores - sea otters and polar bears
This group also includes dogs and their relatives, bears, raccoons, civets, hyenas, and cats
Similar origin, different environment, different features
Different origin, similar environment, similar features