| Cetaceans[1] Temporal range: 55–0 Ma Early Eocene – Present |
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| Humpback whale breaching | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Superorder: | Laurasiatheria |
| (unranked): | Ferungulata |
| (unranked): | Cetartiodactyla |
| Order: | Cetacea Brisson, 1762 |
| Suborders | |
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Mysticeti |
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| Diversity | |
| Around 88 species; see list of cetaceans or below. | |
The order Cetacea ( /sɨˈteɪʃⁱə/) includes the marine mammals commonly known as whales, dolphins, and porpoises. Cetus is Latin and is used in biological names to mean "whale"; its original meaning, "large sea animal", was more general. It comes from Ancient Greek κῆτος (kētos), meaning "whale" or "any huge fish or sea monster". In Greek mythology the monster Perseus defeated was called Ceto, which is depicted by the constellation of Cetus. Cetology is the branch of marine science associated with the study of cetaceans.
Cetaceans are the mammals best 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 from the cooler water they inhabit by a thick layer of blubber. Some species are noted for their high intelligence.
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Cetaceans breathe air. They surface periodically to exhale carbon dioxide and inhale a fresh supply of oxygen. During diving, a muscular action closes the blowholes (nostrils), which remain closed until the cetacean next breaks the surface; when it surfaces, the muscles open the blowholes and warm air is exhaled.
Cetaceans' blowholes have evolved to a position at the top of the head, simplifying breathing in sometimes rough seas. When the stale air, warmed from the lungs, is exhaled, it condenses as it meets colder external air. As with a terrestrial mammal breathing out on a cold day, a small cloud of 'steam' appears. This is called the 'blow' or 'spout' and varies by species in terms of shape, angle and height. Species can be identified at a distance using this characteristic.
Cetaceans can remain under water for much longer periods than most other mammals, (approximately 7–30 minutes, varying by species) due to large physiological differences. Two studied advantages of cetacean physiology let this order (and other marine mammals) forage underwater for extended periods without breathing:
Cetacean eyes are set on the side rather than the front of the head. This means that only cetaceans with pointed 'beaks' (such as dolphins) have good binocular vision forward and downward. Tear glands secrete greasy tears, which protect the eyes from the salt in the water. The lens is almost spherical, which is most efficient at focusing the minimal light that reaches deep water. Cetaceans make up for their generally poor vision (with the exception of the dolphin) with excellent hearing.
As with the eyes, cetacean ears are also small. Life in the sea accounts for the cetacean's loss of its external ears, whose function is to collect and focus airborne sound waves. However, water conducts sound better than air, so the external ear is unneeded: it is a tiny hole in the skin, just behind the eye. The highly developed inner ear can detect sounds from dozens of miles away and discern from which direction the sound comes.
Odontoceti are generally capable of echolocation.[4] From this, Odontoceti can discern the size, shape, surface characteristics, distance and movement of an object. With this ability cetaceans can search for, chase and catch fast-swimming prey in total darkness. Echolocation is so advanced in most Odontoceti that they can distinguish between prey and non-prey (such as humans or boats); captive Odontoceti can be trained to distinguish between, for example, balls of different sizes or shapes. Mysticeti have exceptionally thin, wide basilar membranes in their cochleae without stiffening agents - making their ears adapted for processing low to infrasonic frequencies.[5]
Cetaceans also use sound to communicate, whether it be groans, moans, whistles, clicks or the complex 'singing' of the humpback whale.
Besides hearing and vision, at least one species, the Tucuxi or Guiana Dolphin, is able to use electroreception to sense prey.[6]
The toothed whales such as the sperm whale, beluga, dolphins and porpoises, have teeth that they use for catching fish, squid or other marine life. They do not chew but swallow prey whole. When they catch large prey, such as when the orca (Orcinus orca) catches a seal, they bite off and swallow one chunk at a time.
Mysticeti instead have baleen plates made of keratin (the same substance as human fingernails) which hang from the upper jaw. These plates filter small animals (such as krill and fish) from the seawater. Cetaceans included in this group include the Blue, Humpback, Bowhead and Minke whales.
Not all Mysticeti feed on plankton: the larger species eat small shoaling fish, such as herring and sardine, called micronecton. The gray whale (Eschrichtius robustus), is a benthic feeder, primarily eating sea floor crustaceans.
Cetaceans are mammals, that is, members of the class Mammalia. Their closest living relatives are the even-toed ungulates, such as the hippopotamus and deer.[7][8]
| Cetartiodactyla |
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Mammalian characteristics include warm-bloodedness, breathing air through their lungs, and suckling their young, and growing hair, although very little of it.
Another way of distinguishing a cetacean from a fish is the shape of the tail. Fish tails are vertical and move from side to side when the fish swims. Cetacea tails—called a fluke—are horizontal and move up and down, because cetacea spines bend in the same manner as a human spine.
The order Cetacea contains about 90 species, all marine except for 4 species of freshwater dolphins. The order contains two suborders, Mysticeti (baleen whales) and Odontoceti (toothed whales, which includes dolphins and porpoises). The species range in size from Commerson's dolphin, smaller than a human, to the Blue Whale, the largest animal ever known to have lived.
The cetaceans (whales, dolphins and porpoises) are marine mammal descendants of the artiodactyl family Raoellidae, a group of land mammals characterized by an even-toed ungulate skull, slim limbs, and an ear with significant similarities to that of early whales.[9] The terrestrial origins of cetaceans are indicated by their need to breathe air from the surface or the bones of their fins, which resemble the limbs of land mammals, including the vestigial remains of hind legs inherited from the four-legged ancestors.
The question of how land animals evolved into ocean-going leviathans was a mystery until discoveries starting in the late 1970s in Pakistan revealed several stages in the transition of cetaceans from land to sea:
| Pakicetus | Ambulocetus | Kutchicetus | Protocetus | Janjucetus (Mysticeti) | Squalodon (Odontoceti) |
This series of images does not capture the true phylogenetic evolution of a particular species, but they are illustrative representation of the evolution of cetaceans from terrestrial four-legged mammals, from their probable ancestor, through different stages of adaptation to aquatic life to modern cetaceans type; aerodynamic body shape, fully developed caudal fin and vestigial hind legs. The separation of cetaceans in suborder baleen whales and suborder toothed whales, occurred during the Oligocene (Janjucetus and Squalodon represent the early forms of their suborders).
Fossils indicate that before evolving baleen, Mysticeti also had teeth, so defining the Odontoceti via teeth alone is problematic. Instead, paleontologists have identified other features uniting fossil and modern odontocetes that are not shared by Mysticetes. It was also assumed that toothed whales evolved their asymmetrical skulls as an adaptation to their echolocation, but newer discoveries indicate that the common ancestor of the present whales actually had a contorted skull as well. Cranial asymmetry is now known to have evolved in ancient whales as part of a set of traits linked to directional hearing, including pan-bone thinning of the lower jaws, the development of mandibular fat pads, and the isolation of the ear region.[10] This likely means that while the asymmetry in the Odontoceti skull has increased over time, the Mysticeti skull has evolved from asymmetrical to a symmetrical.[11]
| Characteristic | Odontoceti | Mysticeti |
| Feeding | Echolocation, fast | Filter feeder, not fast |
| Size | Smaller (except Sperm whale and Beaked Whale) | Larger (except pygmy right whale) |
| Blowhole | One | Two |
| Dentition | Teeth | Baleen plates |
| Melon | Ovoid, in anterior facial region | Vestigial or none |
| Skull and facial tissue | Dorsally asymmetric | Symmetric |
| Sexual dimorphism | Some species have larger males | Females always larger |
| Mandible | Symphyseal (fused anteriorly) | Nonsymphyseal |
| Pan bone of lower jaw | Yes | No |
| Maxillae projection | Outward over expanded supraorbital processes | Under eye orbit, with bony protuberance anterior to eye orbit |
| Olfactory nerve and bulb | Absent[12] | Vestigial[13] |
| Periotic bone | External to skull, fused with tympanic bulla | Fused with skull[14] |
The classification here closely follows Dale W. Rice, Marine Mammals of the World: Systematics and Distribution (1998), which has become the standard taxonomy reference in the field. There is very close agreement between this classification and that of Mammal Species of the World: 3rd Edition (Wilson and Reeder eds., 2005). Any differences are noted using the abbreviations "Rice"[15] and "MSW3"[1] respectively. Further differences due to recent discoveries are also noted.
Discussion of synonyms and subspecies are relegated to the relevant genus and species articles.
†Recently extinct
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