A marine mammal is a mammal that is primarily ocean-dwelling or depends on the ocean for its food. Marine mammals typically swim by moving their spine up and down. By contrast, fish normally swim by moving their spine sideways. For this reason, fish mostly have vertical caudal (tail) fins, while marine mammals have horizontal caudal fins.
Some of the primary differences between marine mammals and other marine life are:
- Marine mammals breathe air, while most other marine animals extract oxygen from water.
- Marine mammals have hair. Cetaceans have little or no hair, usually a very few bristles retained around the head or mouth.
- Marine mammals give live birth. Most marine mammals only give birth to one calf or pup at a time, and are never able to birth twins or larger litters.
- Marine mammals feed off milk as young. Maternal care is extremely important to the survival of offspring that need to develop a thick insulating layer of blubber.
- Marine mammals maintain a high internal body temperature, unlike most other marine life.
The polar bear spends a large portion of its time in a marine environment, albeit a frozen one. When it does swim in the open sea it is extremely proficient and has been shown to cover 46 miles in one day! For these reasons, some scientists regard it as a marine mammal.
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