Some species are well adapted for diving to great depths.
Although cetaceans are widespread, most species prefer the colder waters of the Northern and Southern Hemispheres.
Cetaceans range in size from the 1 m (3 ft 3 in) and 50 kg (110 lb) Maui's dolphin to the 29.9 m (98 ft) and 173,000 kg (381,000 lb) blue whale, which is also the largest animal ever known to have existed. They have streamlined bodies and two (external) limbs that are modified into flippers.
The two parvorders, baleen whales (Mysticeti) and toothed whales (Odontoceti), are thought to have diverged around thirty-four million years ago.
Baleen whales have bristles made of keratin instead of teeth.
) are a widely distributed and diverse clade of aquatic mammals that today consists of the whales, dolphins, and porpoises. There are around 89 extant species, which are divided into two groups or parvorders, the Odontoceti or toothed whales, a group of more than 70 species that includes the dolphins and porpoises, and the Mysticeti or baleen whales, of which there are now 15 species.
The name is derived from the Latin cetus "whale" and Greek ketos "huge fish".
Mothers of some species fast and nurse their young for a relatively short period of time, which is more typical of baleen whales as their main food source (invertebrates) aren't found in their breeding and calving grounds (tropics).