A marine mammal is a mammal that is primarily ocean-dwelling or
depends on the ocean for its food. Mammals originally evolved on
land, but later marine mammals evolved to live back in the ocean.
There are five groups of marine mammals:
1. Order Sirenia: the manatee, dugong, and sea cow
2. Order Carnivora, family Ursidae: the polar bear
3. Order Carnivora, infrafamily Pinnipedia: the seal, sea lion,
4. Order Carnivora, family Mustelidae: the Sea Otter and Marine
5. Order Cetacea: the whale, dolphin, and porpoise
Since different groups of marine mammals originate from different
ancestors, this is a case of convergent evolution.
Since mammals originally evolved on land, their spines are optimized
for running, allowing for up-and-down but only little sideways motion.
Therefore, 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. All members
of the Carnivora have a coat of fur or hair, but it is far thicker
and more important for thermoregulation in Sea Otters and Polar
Bears than in seals or sea lions. Thick layers of fur contribute
to drag while swimming, and slow down a swimming mammal, giving
it a disadvantage in speed.
Marine mammals have thick layers of blubber used to insulate their
bodies and prevent heat loss. Sea Otters and Polar Bears are exceptions,
relying more on fur and behavior to stave off hypothermia.
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. The milk from the mammary glands of
marine mammals often exceeds 40-50% fat content to support the development
of blubber in the young.
Marine mammals maintain a high internal body temperature. Unlike
most other marine life, marine mammals carefully maintain a core
temperature much higher than their environment. Blubber, thick coats
of fur, blubbles of air between skin and water, countercurrent exchange,
and behaviors such as hauling out, are all adaptations that aid
marine mammals in retention of body heat.
The polar bear spends a large proportion 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 74 km
in a day. For these reasons, some scientists regard it as a marine
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