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:

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 Sea Otter and Marine Otter
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 mammal.

[edit]
Research
Considerable research has been conducted on the incidence of diseases that afflict marine mammals in the marine environment. this work has addressed leptospirosis, phocine herpesvirus, neurological diseases and other pathologies affecting marine mammals. Entire research organizations have developed such as the Marine Mammal Center to focus upon the rehabilitation and research functions of marine mammals.