Marine Mammals

Marine mammals possess all the characteristics of a mammal, which differentiate them from other zoological groups:
They have lungs and breath air; They are warm-blooded and maintain a constant body temperature; Most bear live young (the exception is the primitive mammal group, the Monotremes (the platypus and the echidna) which lays eggs); They produce milk and nurse their young; They have hair at some point in their lives.
They are known as marine mammals because they depend on the marine environment for food. Therefore they evolved specific adaptations throughout millions of years to adapt to their environment: They possess a layer of blubber underneath the skin, that provides thermoregulation and hydrodynamism; Their limbs have evolved to flippers and flukes; They possess special characteristics that enable them dive deep and for long periods of time, conserving the oxygen needed to breath.
All marine mammals descend originally from land mammals, which found in the oceans an opportunity to escape from predators and an additional source of food.
There are 5 main groups of marine mammals, composing a total of around 120 different species:
Pinnipeds (seals, sea lions, fur seals and the walrus); Cetaceans (whales and dolphins); Sirenians (manatees and dugongs); Carnivores (marine otters and the polar bear).