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, porpoises and narwhal), 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.
Bottlenose dolphins, the genus Tursiops, are the most common and well-known members of the family Delphinidae, the family of oceanic dolphin. Recent molecular studies show the genus contains two species, the common bottlenose dolphin (Tursiops truncatus) and the Indo-Pacific bottlenose dolphin (Tursiops aduncus), instead of one. Research in 2011 revealed a third species, the Burrunan dolphin (Tursiops australis). They inhabit warm and temperate seas worldwide.
Bottlenose dolphins live in groups typically of 10–30 members, called pods, but group size varies from single individuals up to more than 1,000. Their diets consist mainly of forage fish. Dolphins often work as a team to harvest fish schools, but they also hunt individually. Dolphins search for prey primarily using echolocation, which is similar to sonar. They emit clicking sounds and listen for the return echos to determine the location and shape of nearby items, including potential prey. Bottlenose dolphins also use sound for communication, including squeaks and whistles emitted from the blowhole and sounds emitted through body language, such as leaping from the water and slapping their tails on the water surface.