The cetaceans are one of the most derived mammal groups, and have adapted to live their entire lives in the water. All cetaceans have a streamlined body shape, fore limbs adapted into flippers, tails enlarged and highly developed, and hind limbs reduced dramatically. They have also developed internal reproductive organs, and have lost the hair and sweat glands found on other mammals. Porpoises are the smallest cetaceans, with some species reaching only 1-2 meters. The largest species is the blue whale, which can measure around 30 meters long.
Species of cetacean can be found in all of earth’s oceans, with some species living in freshwater lakes and rivers. They are a very diverse order, with species varying in habitat from open ocean to coastal areas. While some species have huge ranges spanning multiple oceans (eg. Orcas), others can only be found in one area of ocean in the world (eg. Vaquitas).
Cetaceans have adapted lungs and circulatory systems to allow a high level of efficiency, meaning cetacea species are able to dive for long periods of time. Compared to terrestrial mammals, they are able to use a much higher percentage of the oxygen they breathe, and, when diving, their heart rates slow down dramatically, allowing them to use less oxygen.
Like other mammals, cetaceans give birth to live young and nurse them with milk. Females can nurse their calves from anywhere between six month and over two years. Most cetaceans give birth to just one calf about every 6 years. They do not reach sexual maturity for at least 2 years, a process which can take up to 10 years for some species.
Many cetaceans have brains which are disproportionately large compared to their body size, with some species considered to be the most intelligent non-primates. Many species are known to communicate with various sounds and clicks, and toothed whales also use clicks for echolocation purposes. This group is also known to live in packs which are able to live and hunt cooperatively.