Marine mammals, which include seals, whales, dolphins, and walruses, form a diverse group of 128 species that rely on the ocean for their existence. They do not represent a distinct biological grouping, but rather are unified by their reliance on the marine environment for feeding. The level of dependence on the marine environment for existence varies considerably with species. For example, dolphins and whales are completely dependent on the marine environment for all stages of their life, whereas seals feed in the ocean, but breed on land. Marine mammals can be subdivided into four recognised groups; cetaceans (whales, dolphins and porpoises), pinnipeds (seals, sea lions and walruses), sirenians (manatees and dugongs), and fissipeds, which are the group of carnivores with separate digits (the polar bear, and two species of otter). Both cetaceans and sirenians are fully aquatic and therefore are obligate ocean dwellers. Pinnipeds are semi-aquatic; 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 otters and the polar bear are much less adapted to ocean living. While the number of marine mammals is small compared to those found on land, their total biomass is large. They play important roles in maintaining marine ecosystems, especially through regulation of prey populations. These two factors make them an integral component of the marine environment. This is of particular concern considering 23% of marine mammal species are currently threatened.