Cetacea

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Cetaceans1
Temporal range: 55–0 Ma
Early Eocene – Present
Humpback stellwagen edit.jpg
Humpback whale breaching
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Clade: Cetancodontamorpha
Suborder: Whippomorpha
Infraorder: Cetacea
Brisson, 1762
Parvorders

Mysticeti
Odontoceti
Archaeoceti
(see text for families)

Diversity
Around 88 species.
Whale watching in Valdes Peninsula, Argentina

The infraorder2 Cetacea /sɨˈtʃə/ 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), used for whales and huge fish or sea monsters. Cetology is the branch of marine science associated with the study of cetaceans. An ancient ancestor of the whale, Basilosaurus was thought to be a reptile until vestigial parts were recognized.3 Traditionally Cetacea was treated as an order, but it has become increasingly known based on physiological data that cetaceans are not only a clade of even-toed ungulates, but that Cetacea might be recognized as an infraorder.2

Fossil evidence suggests that the cetaceans share a common ancestor with hippopotamuses that began living in marine environments around 50 million years ago.4567 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 attributed with high levels of 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.8

The collective noun "pod"9 is often used for a herd or school of marine mammals, especially a small herd or family group of whales or dolphins.

Physical characteristics

Further information: Dolphin#Anatomy and Whale#Anatomy

Respiration

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 returns 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:

  • Mammalian myoglobin concentrations in skeletal muscle have much variation. New Zealand white rabbits have 0.08 to 0.60% myoglobin by weight in wet muscle,10 whereas a northern bottlenose whale has 6.34 grams (0.224 oz) myoglobin per 100 grams wet muscle.11 Myoglobin, by nature, has a higher oxygen affinity than hemoglobin. The higher the myoglobin concentration in skeletal muscle, the longer the animal can stay underwater.
  • Increased body size also increases maximum dive duration. Greater body size implies increased muscle mass and increased oxygen stores. Cetaceans also obey Kleiber's law, which states that mass and metabolic rate are inversely related, i.e., larger animals consume less oxygen than smaller animals per unit mass.

Vision, hearing and echolocation

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.12

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.13

Odontoceti (toothed whales, which includes dolphins and porpoises) are generally capable of echolocation.14 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.15 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.16 Echolocation clicks also contain characteristic details unique to each animal, which may suggest that toothed whales can discern between their own click and that of others.17

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.18

Tails

All cetaceans have tails containing (or known as) flukes. These lobes are horizontally oriented and the tail moves up and down, unlike fish tails. The feature evolved by the time of Basilosauridae and Dorudontinae. Porpoises lack a notch in their tail fluke, which makes distinguishing them from dolphins (which have a very noticeable notch in their tail fluke) very easy.

Feeding

The toothed whales, such as the sperm whale, beluga, dolphins, and porpoises, have teeth for catching fish, squid or other marine life. When killer whales catch a seal, they bite off and swallow one chunk at a time.

Instead of teeth, the Mysticeti (baleen whales) 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. Not all Mysticeti feed on plankton.

The larger cetaceans, like the blue, humpback, bowhead, and minke whales, 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. Instead of preying on few species, beluga whales are opportunistic feeders. They prey on about 100 different kinds of primarily bottom-dwelling animals. They eat octopus, squid, crabs, snails, sandworms, and fishes such as capelin, cod, herring, smelt, and flounder.

In terms of food intake, a blue whale eats up to 3,600 kg (8,000 lb) of krill each day for about 120 days while it is estimated to take 1,000 kg (2,200 lb) of food to fill a blue whale's stomach. Gray whales eat about 150,000 kg (340,000 lb) of food during a 130 to 140 day feeding period – a daily average intake of about 1,089 kg (2,400 lb.). It is estimated to take 300 kg (660 lb.) of food to fill a gray whale's stomach. Gray whales gain about 16% to 30% of their total body weight during a feeding season.19

Reproduction

Cetaceans evolved from land-dwelling ancestors and lost external hind limbs. They evolved significantly reduced pelvic bones that seemed to serve no other function except to anchor muscles that maneuver the penis. Cetacean pelvic bones are unique because they are no longer constrained by sacral and hind limb attachments or by hind limb locomotion.20

Cetacean pelvic bones were often thought of as “useless vestiges” of their land-dwelling ancestors.21 However, they are a critical component of male reproductive fitness.

The paired pelvic bones anchor the genitalia and the paired ischiocavernosus muscles that control the penis.2223 In male cetaceans, the paired ischiocavernosus muscles insert deeply toward the distal end of the penis, while proximally encapsulating the paired crus of the penis that anchor to each pelvic bone.2425 The ischiocavernosus muscles appear to maneuver the penis by pulling it to one side or the other26 and may also maintain erection by compressing the corpus cavernosum proximally.27 The ischiocavernosus muscles may work in coordination with other soft tissue innovacations to control the penis, which suggests the importance of ischiocavernosus muscles in penis movement.

Both size and shape of pelvic bones are evolutionary correlated to relative testes mass, which is a strong indication of postcopulatory sexual selection.2829 Male cetacean with relatively intense sexual selection tend to evolve larger penises and pelvic bones compared to their body length. The relative size of pelvic bones increases with relative testes mass. With PGLS (Phlogenetic generalized least squares) methodology, relative pelvic size was positively correlated with relative testes mass (phylogenetically controlled, P = 0.0006, r = 0.55), a pattern not observed in ribs.20 Sexual Selection also appears to favor divergence in shape. Pelvic bone shape diverges more rapidly among species that have diverged in inferred mating system. According to analysis on pelvic and rib bones, pelvic bone shape divergence (independent of size) was positively correlated (Pearson’s correlation P = 0.003, r = 0.82). However such a relationship did not help explaining rib bones (Pearson’s correlation P = 0.98, r = -0.009).20

Taxonomy

Cetacea contains about 90 species, all marine except for four species of freshwater dolphins. Cetaceans can be divided into two further groups: Mysticeti (baleen whales) and Odontoceti (toothed whales, which includes dolphins, porpoises, and sperm whales). 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.

Traditionally Cetacea was recognized as a unique order of mammals. However, molecular research and recent fossil discoveries show that cetaceans are nested within another order of mammals, Artiodactyla, with a close relationship with the family Hippopotamidae.4567303132333435 At least one author believes that Cetacea should be best recognized as an infraorder in the suborder Whippomorpha in Artiodactyla.2


   Artiodactyla   

 Tylopoda


   Artiofabula   

 Suina    


   Cetruminantia   

 Ruminantia


   Whippomorpha   

 Hippopotamidae



 Cetacea






Mammalian characteristics include warm-bloodedness, breathing air through their lungs, suckling their young, and growing hair, although in the case of cetaceans, very little of it.

Another way of distinguishing a cetacean from a fish is the shape and orientation 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.

Evolution

Cetacean (whales, dolphins and porpoises) evolution has one of the most complete fossil records, which makes tracking the intermediates between families easier. They are marine mammal relatives 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.36 One of these in particular, Indohyus, was characterized by a long snout and osteosclerosis, the second of which suggests that "Indohyus" was aquatic.37 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 vestigial remains of hind legs inherited from their four-legged ancestors. Loss of external hind limbs is particularly well documented and had been investigated to further explore the evolution of the families Pakicetidae, Ambulocetidae, Remingtonocetidae, Protocetidae, and Basilosauridae.

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:38

Illustrative representation of the cetacean evolution
Illustrative representation of the cetacean evolution (Pakicetus » Ambulocetus » Kutchicetus » Protocetus » Janjucetus / Squalodon).

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. Although Pakicetidae still had weight supporting hind limbs, they are considered to be the first cetaceans and are not included in Artiodactylia.39 This gave rise to Ambulocetidae whose strong tail suggested a life of more swimming.40 Remingtonocetidae remained able to support its own weight but also developed a strong base for foot-powered swimming.41 Protoceids were the first cetaceans to be found all over the world instead of simply Pakistan and India indicating more advanced aquatic locomotion. Two main genera are known: Artiocetus and Georgiacetus. The first had reduced hind limbs but was still able to bear weight, while Georgiacetus had a pelvis completely disconnected from the spine, meaning it could not support its own weight and thus spent all of its time in water.37 Basilosauridae were the first fully aquatic cetaceans. There were two main types, Basilosaurus and dorudontids. Both were characterized by tiny hind limbs, tail flukes, and an elongated vertebral column. This provided the foundation for the modern cetaceans mysticeti and odontoceti, respectively the suborders of baleen whales and toothed whales, a separation that occurred during the Oligocene (Janjucetus and Squalodon represent the early forms of their suborders).

Mysticeti vs Odontoceti

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.42 This likely means, while the asymmetry in the Odontoceti skull has increased over time, the Mysticeti skull has evolved from asymmetrical to symmetrical.43

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 Absent44 Vestigial45
Periotic bone External to skull, fused with tympanic bulla Fused with skull46

Classification

Size comparison of nearly all known extant cetacean species. Note the human diver at lower right for scale.

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"47 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

See also

References

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External links