Cetaceans are the mammals most fully adapted to aquatic life. Their body is fusiform (spindle-shaped). The forelimbs are modified into flippers. The tiny hindlimbs are vestigial, they do not attach to the backbone and are hidden within the body. The tail has horizontal flukes. Cetaceans are nearly hairless, and are insulated by a thick layer of blubber. Cetacea contains ten families, and about 80 species.
Cetaceans are descendants of land-living mammals, most likely of the Artiodactylorder. They entered the water roughly 50 million years ago. See evolution of cetaceans for the details.
Taxonomic listing
The classification here closely follows "Marine Mammals of the World: Systematics and Distribution" by Dale W. Rice (1998). The work has become the standard taxonomy reference in the field. Differences reflect usage of common names and further discoveries since the publication of that work.
Rice, Dale W. (1998). Marine mammals of the world: systematics and distribution. Society of Marine Mammalogy Special Publication Number 4. 231 pp. See the Society's website (http://www.marinemammalogy.org/publications.htm) for further details.
Cetaceans are nearly hairless, and are insulated by a thick layer of blubber.
Cetaceans evolved from land mammals (most likely from certain hoofed carnivores distantly related to hippopotamuses) that returned to the sea about 50 million years ago.
From this, cetaceans can discern the size, shape, surface characteristics and movement of the object, as well as how far away it is. This is called echolocation, and with it cetaceans can search for, chase and catch fast-swimming prey in total darkness.
However, it is the cetaceans that are the most specialised in the aquatic environment and for a long time even the most basic aspects of their biology remained a mystery.
Cetaceans sleep by floating at the surface of the sea with their blowholes exposed, some species rest during the day and others at night.
Cetaceans often live in environments where there is little light, such as the deep ocean or the turbid waters of rivers.
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