Marine mammals are a diverse group of 120 species of mammal that are primarily ocean-dwelling or depend on the ocean for 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, while not aquatic, is also usually considered a marine mammal because it lives on sea ice for most or all of the year.
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. Whales are the largest mammals in the world. Different species are, however, adapted to marine life to varying degrees. The most fully adapted are the cetaceans and the sirenians, which cannot live 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 use for blubber, meat, ivory and fur. Most species are currently protected from commercial use.
There are some 120 extant species of marine mammals, generally sub-divided into the five groups bold-faced below. Each group descended from a different land-based ancestor. The morphological similarities between these diverse groups are a result of convergent and parallel evolution. For example, although whales and seals have some similarities in shape, whales are more closely related to deer than they are to seals.
Several groups of marine mammals existed in the past that are not alive today. In addition to the ancestors of the modern day whales, seals, and manatees, there existed desmostylians, cousins of the manatees, and Kolponomos, a species of clam-eating marine bears not related to the modern polar bear. A Polar Bear weighs upto 1 tonne.
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:
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.