From Wikipedia, the free encyclopedia - View original article
Temporal range: 55–0Ma Early Eocene – Present
|Humpback whale breaching|
|Around 88 species.|
|This article needs additional citations for verification. (January 2010)|
Temporal range: 55–0Ma Early Eocene – Present
|Humpback whale breaching|
|Around 88 species.|
The order Cetacea // 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.
Fossil evidence suggests that the cetaceans share a common ancestor with land-dwelling mammals that began living in marine environments around 50 million years ago. Today, they are the mammals best adapted to aquatic life. The body of a cetacean 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. At the 2012 meeting of the American Association for the Advancement of Science, support was reiterated for a cetacean bill of rights, listing cetaceans as non-human persons.
Cetaceans breathe air, and 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 comes to 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, (about seven to 120 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 sides rather than the front of the head. This means 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.
The external ear of cetaceans has lost the pinna (visible ear), but still retains an extremely narrow external auditory meatus. To register sounds, instead, the posterior part of the mandible has a thin lateral wall (the pan bone) behind which a concavity houses a large fat pad. The fat pad passes anteriorly into the greatly enlarged mandibular foramen to reach in under the teeth, and posteriorly to reach the thin lateral wall of the ectotympanic. The ectotympanic only offers a reduced attachment area for the tympanic membrane and the connection between this auditory complex and the rest of the skull is reduced in cetaceans — to a single, small cartilage in oceanic dolphins. In odontocetes, the complex is surrounded by spongy tissue filled with air spaces, while in mysticetes it is integrated into the skull similar to land mammals. In odontocetes, the tympanic membrane (or ligament) has the shape of a folded-in umbrella that stretches from the ectotympanic ring and narrows off to the malleus (quite unlike the flat, circular membrane found in land mammals.) In mysticetes, it also forms a large protrusion (known as the "glove finger"), which stretches into the external meatus, and the stapes are larger than in odontocetes. In some small sperm whales, the malleus is fused with the ectotympanic. The ear ossicles are pachyosteosclerotic (dense and compact) in cetaceans and very different in shape compared to land mammals (other aquatic mammals, such as sirenians and earless seals, have also lost their pinnae). In modern cetaceans, the semicircular canals are much smaller relative to body size than in other mammals.
In modern cetaceans, the auditory bulla is separated from the skull and composed of two compact and dense bones (the periotic and tympanic) referred to as the tympano-periotic complex. This complex is located in a cavity in the middle ear, which, in Mysticeti, is divided by a bony projection and compressed between the exoccipital and squamosal but, in Odontoceti, is large and completely surrounds the bulla (hence called "peribullar"), which is therefore not connected to the skull except in physeterids. In odontoceti, the cavity is filled with a dense foam in which the bulla hangs suspended in five or more sets of ligaments. The pterygoid and peribullar sinuses that form the cavity tend to be more developed in shallow water and riverine species than in pelagic mysticeti. In odontoceti, the composite auditory structure is thought to serve as an acoustic isolator, analogous to the lamellar construction found in the temporal bone in bats.
Odontoceti (toothed whales, which includes dolphins and porpoises) are generally capable of echolocation. 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, they can distinguish between prey and nonprey (such as humans or boats); captive Odontoceti can be trained to distinguish between, for example, balls of different sizes or shapes. Mysticeti (baleen whales) have exceptionally thin, wide basilar membranes in their cochleae without stiffening agents, making their ears adapted for processing low to infrasonic frequencies.
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.
The toothed whales, such as the sperm whale, beluga, dolphins, and porpoises, have teeth 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 killer whale (Orcinus orca) catches a seal, they bite off and swallow one chunk at a time.
Instead of teeth, the Mysticeti 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 sardines, called micronecton. The gray whale (Eschrichtius robustus), is a benthic feeder, primarily eating sea-floor crustaceans.
The order Cetacea contains about 90 species, all marine except for four 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.
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. A cetacean's tail — called a fluke — is horizontal and moves up and down, because cetacean spines bend in the same manner as a human spine.
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. The terrestrial origins of cetaceans are indicated first by their need to breathe air from the surface, the bones of their fins, which resemble the limbs of land mammals, and by the vestigial remains of hind legs inherited from their four-legged ancestors.
The question of how land animals became ocean-going was a mystery until discoveries starting in the late 1970s in Pakistan revealed several stages in the transition of cetaceans from land to sea:
This image does not capture the true phylogenetic evolution of a particular species, but it is an 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, hydrodynamic 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, before evolving baleen, the Mysticeti also had teeth, so defining the Odontoceti by teeth alone is problematic. Instead, paleontologists have identified other features uniting fossil and modern odontocetes that are not shared by Mysticeti. It was also assumed that toothed whales evolved their asymmetrical skulls as an adaptation to their echolocation, but newer discoveries indicate 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. This likely means, while the asymmetry in the Odontoceti skull has increased over time, the Mysticeti skull has evolved from asymmetrical to symmetrical.
|Feeding||Echolocation, fast||Filter feeder, not fast|
|Size||Smaller (except Sperm whale and beaked whale)||Larger (except pygmy right whale)|
|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||Vestigial|
|Periotic bone||External to skull, fused with tympanic bulla||Fused with skull|
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" and "MSW3" respectively. Further differences due to recent discoveries are also noted.
Discussion of synonyms and subspecies are relegated to the relevant genus and species articles.
|Wikispecies has information related to: Cetacea|
|Wikimedia Commons has media related to Cetacea.|
|The Wikibook Dichotomous Key has a page on the topic of: Cetacea|
|Look up Cetacea in Wiktionary, the free dictionary.|