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Concept
Marine mammal
Summary
Marine mammals are aquatic mammals that rely on the ocean and other marine ecosystems for their existence. They include animals such as cetaceans (whales, dolphins and porpoises), pinnipeds (seals, sea lions and walruses), sirenians (manatees and dugongs), sea otters and polar bears. They are an informal group, unified only by their reliance on marine environments for feeding and survival.
Marine mammal adaptation to an aquatic lifestyle varies considerably between species. Both cetaceans and sirenians are fully aquatic and therefore are obligate water dwellers. Pinnipeds are semiaquatic; they spend the majority of their time in the water but need to return to land for important activities such as mating, breeding and molting. In contrast, both sea otters and the polar bear are mostly terrestrial and only go into the water on occasions of necessity, and are thus much less adapted to aquatic living. The diets of marine mammals vary considerably as well; some eat zooplankton, others eat fish, squid, shellfish, or seagrass, and a few eat other mammals. While the number of marine mammals is small compared to those found on land, their roles in various ecosystems are large, especially concerning the maintenance of marine ecosystems, through processes including the regulation of prey populations. This role in maintaining ecosystems makes them of particular concern as 23% of marine mammal species are currently threatened.
Marine mammals were first hunted by aboriginal peoples for food and other resources. Many were also the target for commercial industry, leading to a sharp decline in all populations of exploited species, such as whales and seals. Commercial hunting led to the extinction of the Steller's sea cow, sea mink, Japanese sea lion and Caribbean monk seal. After commercial hunting ended, some species, such as the gray whale and northern elephant seal, have rebounded in numbers; conversely, other species, such as the North Atlantic right whale, are critically endangered.
Official source
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