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A Humpback whale (Megaptera novaeangliae), a member of Order Cetacea

A Leopard seal (Hydrurga leptonyx), a member of infrafamily Pinnipedia

A West Indian Manatee (Trichechus manatus), a member of Order Sirenia

A Sea Otter (Enhydra lutris), a member of family Mustelidae

A Polar bear (Ursus maritimus), a member of family Ursidae

Marine mammal refers to a diverse group of roughly 120 species of mammal that are primarily ocean-dwelling or depend on the ocean for its food. They include the cetaceans (whales, dolphins, and porpoises), the sirenians (manatees and dugong), the pinnipeds (true seals, eared seals and walrus), and several otters (the sea otter and marine otter). The polar bear is also usually grouped with the marine mammals.

Marine mammals evolved from land dwelling ancestors and share several adaptive features for life at sea such as generally large size, hydrodynamic body shapes, modified appendages and various thermoregulatory adaptations. Different species are, however, adapted to marine life to varying degrees. The most fully adapted are the cetaceans and the sirenians, whose entire life cycle takes place under water, whereas the other groups spend at least some time on land.

Despite the fact that marine mammals are highly recognizable charismatic megafauna, many populations are vulnerable or endangered due to a history of commercial exploitation for blubber, meat, ivory and fur. Most species are currently protected from commercial exploitation.

There are some 120 extant species of marine mammals, generally subdivided into the five groups bold-faced below.^[1]

• Order Sirenia: Sirenians
  • family Trichechidae: manatees (3 species)
  • family Dugongidae: dugong (1 species)
• Order Cetacea: Cetaceans
  • Suborder Mysticeti: Baleen whales (14 or 15 species)
  • Suborder Odontoceti: Toothed whales (around 73 species)
• Order Carnivora,
  • superfamily Pinnipedia
    • family Phocidae: true seals (around 20 species)
    • family Otariidae: eared seals (around 16 species)
    • family Odobenidae: walrus (1 species)
  • family Mustelidae
    • sea otter (Enhydra lutris)
    • marine otter (Lontra felina)
  • family Ursidae
    • polar bear (Ursus maritimus)

The sirenians and cetaceans are thought to be descendent from an ungulate ancestor, while the pinnipeds, otters and polar bear are descended from a caniform ancestor. The morphological similarities between these diverse groups are a result of convergent and parallel 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, bubbles 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 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 74 km in a day. For these reasons, some scientists regard it as a marine mammal.

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.

• Read Congressional Research Service (CRS) Reports regarding Marine mammals
• A 2005 Report by the National Academy of Sciences entitled Marine Mammal Populations and Ocean Noise, is available for free online reading and research
• University of Washington Libraries Digital Collections -- Freshwater and Marine Image Bank -- Aquatic Mammals An ongoing digital collection of images related to marine and aquatic mammals.
• Marine Mammal Resource