Marine mammals, which include seals, whales, dolphins, porpoises, manatees, dugongs, marine otters, walruses, and polar bears, form a diverse group of 129 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 aquatic environment for feeding. The level of dependence on the aquatic 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.