CETACEA

Marsopas, delfines y ballenas

Cetacea

Las ballenas, los delfines y las marsopas son mamíferos que han retornado a una vida marina. Sus ancestros son mamíferos terrestres, y todos comparten las características de poseer sangre caliente y respiracón aérea por pulmones.   Existen aproximadamente 80 especies diferentes de ballenas, delfines y marsopas, con tamaños que varían entre los 90 cm hasta los 30 metros de longitud, incluyendo a los animales más grandes del mundo (las ballenas azules). Lo que los hace más sorprendentes es el hecho de que son animales similares a los perros, vacas o a los humanos y que viven en el mar la mayor parte de sus vidas. Con esto en mente, la mejor manera de entender su biología y su comportamiento es pensar en las adaptaciones que han tenido y que los han transformados en animales más parecidos a los peces que a los mamíferos.

Tanto se han adaptado a la vida en el mar, que virtualmente no pueden vivir fuera de el (salvo las orcas y los delfines que pueden estar segundos fuera del agua).  Los delfines se cuentan como los animales más perfectos en cuanto a su hidrodinamia, de hecho, la forma de los cetáceos son similares a los peces (una buena manera de diferenciarlos del otro grupo de mamíferos marinos, los pinnípedos). Sus extremidades anteriores se han minimizado a dos "aletas" o flippers y sus miembros posteriores han desaparecido por completo, dando lugar a la formación de una cola o aleta caudal cartilaginosa que usan para la locomoción.  Han perdido los pelos casi por completo, ya que usan una capa de grasa bastante gruesa para protegerse del frío.  Su nariz ya no permanece en la parte frontal del cráneo o en la trompa como los gatos, los perros u otros mamíferos terrestres, sino que ha sufrido una rotación hasta la parte superior de la cabeza formando una especie de "caño", llamado espiráculo por donde respira.  La reproducción tiene sus variaciones ya que las cópulas,  los partos y el amamantamiento deben ocurrir dentro del agua y esto conlleva a comportamientos y adaptaciones fisiológicas muy importantes. 

Los cetáceos no tienen oídos externos, y las glándulas mamárias, genitales y las aberturas anales están reducidas a hendiduras muy reducidas. No tienen pelos, salvo algunas especies que retienen pelos sensitivos en regiones cercanas a la boca.  Cuando tienen aleta dorsal, esta es usada para la estabilización.

Cetaceos del mundo (con nombre científico en inglés).

Family Balaenidae

Genus Balaena

Balaena mysticetus (bowhead whale)

Genus Eubalaena

Eubalaena australis (southern right whale)

Eubalaena glacialis (right whale)

Family Balaenopteridae

Genus Balaenoptera

Balaenoptera acutorostrata (Minke whale)

Balaenoptera borealis (Sei whale)

Balaenoptera edeni (Bryde's whale)

Balaenoptera musculus (blue whale)

Balaenoptera physalus (fin whale)

Genus Megaptera

Megaptera novaeangliae (humpback whale)

Family Eschrichtiidae

Genus Eschrichtius

Eschrichtius robustus (gray whale)

Family Neobalaenidae

Genus Caperea

Caperea marginata (pigmy right whale)

Family Delphinidae

Genus Cephalorhynchus

Cephalorhynchus commersonii (Commerson's dolphin)

Cephalorhynchus eutropia (black dolphin)

Cephalorhynchus heavisidii (Heaviside's dolphin)

Cephalorhynchus hectori (Hector's dolphin)

Genus Delphinus

Delphinus delphis (common dolphin)

Genus Feresa

Feresa attenuata (pygmy killer whale)

Genus Globicephala

Globicephala macrorhynchus (short-finned pilot whale)

Globicephala melas (long-finned pilot whale)

Genus Grampus

Grampus griseus (Risso's whale)

Genus Lagenodelphis

Lagenodelphis hosei (Fraser's dolphin)

Genus Lagenorhynchus

Lagenorhynchus acutus (Atlantic white-sided dolphin)

Lagenorhynchus albirostris (white-beaked dolphin)

Lagenorhynchus australis (Peale's dolphin)

Lagenorhynchus cruciger (hourglass dolphin)

Lagenorhynchus obliquidens (Pacific white-sided dolphin)

Lagenorhynchus obscurus (Dusky dolphin)

Genus Lissodelphis

Lissodelphis borealis (northern right whale dolphin)

Lissodelphis peronii (southern right whale dolphin)

Genus Orcaella

Orcaella brevirostris (Irrawaddy dolphin)

Genus Orcinus

Orcinus orca (killer whale)

Genus Peponocephala

Peponocephala electra (melon-headed whale)

Genus Pseudorca

Pseudorca crassidens (false killer whale)

Genus Sotalia

Sotalia fluviatilis (Tucuxi)

Genus Sousa

Sousa chinensis (Indo-Pacific hump-backed dolphin)

Sousa teuszii (Atlantic hump-backed dolphin)

Genus Stenella

Stenella attenuata (spotted dolphin)

Stenella clymene (Clymene dolphin)

Stenella coeruleoalba (striped dolphin)

Stenella frontalis (Atlantic spotted dolphin)

Stenella longirostris (Spinner dolphin)

Genus Steno

Steno bredanensis (rough-toothed dolphin)

Genus Tursiops

Tursiops truncatus (bottlenose dolphin)

Family Monodontidae

Genus Delphinapterus

Delphinapterus leucas (beluga)

Genus Monodon

Monodon monoceros (narwal)

Family Phocoenidae

Genus Australophocaena

Australophocaena dioptrica (spectacled porpoise)

Genus Neophocaena

Neophocaena phocaenoides (finless porpoise)

Genus Phocoena

Phocoena phocoena (harbour porpoise)

Phocoena sinus (Cochito)

Phocoena spinipinnis (Burmeister's porpoise)

Genus Phocoenoides

Phocoenoides dalli (Dall's porpoise)

Family Physeteridae

Genus Kogia

Kogia breviceps (pygmy sperm whale)

Kogia simus (dwarf sperm whale)

Genus Physeter

Physeter catodon (sperm whale, commonly classified as Physeter macrocephalus)

Family Platanistidae

Genus Inia

Inia geoffrensis (Amazon River dolphin)

Genus Lipotes

Lipotes vexillifer (Chinese River dolphin)

Genus Platanista

Platanista gangetica (Ganges River dolphin)

Platanista minor (Indus River dolphin)

Genus Pontoporia

Pontoporia blainvillei (Francisana)

Family Ziphiidae

Genus Berardius

Berardius arnuxii (Arnoux's beaked dolphin)

Berardius bairdii (Baird's beaked whale)

Genus Hyperoodon

Hyperoodon ampullatus (northern bottlenose whale)

Hyperoodon planifrons (southern bottlenose whale)

Genus Indopacetus

Indopacetus pacificus (Longman's beaked whale)

Mesoplodon

Mesoplodon bidens (Sowerby's beaked whale)

Mesoplodon bowdoini (Andrew's beaked whale)

Mesoplodon carlhubbsi (Hubb's beaked whale)

Mesoplodon densirostris (Blainville's beaked whale)

Mesoplodon europaeus (Gervais' beaked whale)

Mesoplodon ginkgodens (Ginko-toothed whale)

Mesoplodon grayi (Gray's beaked whale)

Mesoplodon hectori (Hector's beaked whale)

Mesoplodon layardii (straptoothed whale)

Mesoplodon mirus (True's beaked whale)

Mesoplodon peruvianus (Lesser beaked whale)

Mesoplodon stejnegeri (Stejneger's beaked whale)

Genus Tasmacetus

Tasmacetus shepherdi (Tasman beaked whale)

Genus Ziphius

Ziphius cavirostris (Curvier's beaked whale)

Mysticeti

Balaenidae | Neobalaenidae | Balaenopteridae | Eschrichtiidae

4 Familias, 6 géneros, y 11 especies

En este grupo, los dientes han sido reemplazados por barbas, que son usadas para la obtención de alimento en el mar. Además de este rasgo distintivo de las barbas, se caracterizan por sus cuerpos extremadamente grandes, que llegan a tener hasta 30 metros de longitud y pesar hasta 120 toneladas (120.000). Tienen una proporción cabeza/cuerpo muy chica, ya que el largo del cráneo llega a tener 1/3 de la longitud total del animal.

Una de las cosas que más llama la atención de estos animales, es que si bien tienen tamaños muy grandes, de hecho la ballena azul es el animal más grande de la tierra, comen a otros animales muy pequeños. Su dieta, principalmente compuesta de krill, son animales muy pequeños (menos de 1 cm). ¿ Porqué sucede esto?. Tiene que ver con la gran cantidad de krill que existen en los Océanos, en su alto contenido proteico y en su alta capacidad para reproducirse. Así, las ballenas con barbas, tuvieron que formar una estructura en la boca que les permitiera filtrar grandes cantidades de krill o pequeños peces.

Perdieron los dientes y aparecieron las barbas, parecidas varillas flexibles con pelos en uno de sus cantos, donde "enganchan" a los invertebrados.   Estas placas cuelgan de la mandíbula superior y tienen una consistencia de sustancia fibrosa córnea, que forman un inmenso filtro natural, a través de l cual la ballena tamiza los abundantes organismos planctónicos que se encuentran en las aguas superficiales, sobre todo en los océanos meridionales, donde reciben el nombre de krill.   Es posible que la evolución de las ballenas con barbas se haya iniciado a raíz del enfriamiento general de lo océanos meridionales, que tuvo lugar durante el comienzo del Oligoceno y estimuló el crecimiento de plantas microscópicas en el plancton y, asu vez, de los animales que flotan libremente y se alimentan de ellos.

Su tórax tiene una estructura muy particular; tienen solamente 1 a 2 costillas fijas al esternón. La cintura escapular y los miembros posteriores has desaparecido por completo en muchas especies y en otras permanecen como huesos vestigiales del fémur y la tibia.

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Balaenidae

Right and bowhead whales

This is a mysticete family of two genera and three species. They are found near in most waters of the temperate and polar regions.

Right and bowhead whales are large whales, reaching up to 18 m in length and over 100,000 kg weight. Their heads are huge, nearly 1/3 of their total length. The dorsal fin is either lacking. Flippers are short and rounded. The throats of balaenids are smooth, lacking the furrows or grooves of some other mysticetes.

The skull of balaenids has reduced nasals, and the frontals are barely exposed on the dorsal surface. The posterior border of the nasals and premaxillae lie anterior to the supraorbital processes of the frontals. The rostrum is high, narrow, and arched. Baleen plates are long and narrow, and they number more than 350 on each side of the upper jaw. The right and left baleen rows are separated in the front of the mouth.

Right whales feed largely on copepods, which they catch by swimming slowly, with their mouths open, through concentrations of these crustaceans. Water flows into the huge mouth and out between the baleen plates. Food is trapped on the fringes of the plates and scraped off with the tongue. They normally feed at or near the surface.

These whales live singly or in small groups of up to 3 or 4 individuals. Their stocks were severely depleted by whaling, and they remain low. The name "right whale" is said to have originated because these were the "right" whales for whalers to kill.

Neobalaenidae

Pygmy right whale

The single member of this family, the pygmy right whale (Caperea marginata), is found in the cold-temperate waters of the southern hemisphere. Usually included in the family Balaenidae in the past, they differ from members of that family in several ways. They are small, only up to about 6 m. The body is gray, lacking the striking black and white markings common in balaenids. Caperea has, however, a striikingly white mouth and tongue. It also has a small, falcate dorsal fin.

This species is poorly known. It probably does not school or migrate long distances. They are believed to feed on copepods. The species is too uncommon to be of much importance to commercial whalers.

Balaenopteridae

Rorquals

This family includes 6 species placed in 2 genera. It includes minke, Bryde's, sei, fin, humpback, and blue whales. These range in size from the relatively small minke whale, about 8-10 m in length, to the giant blue whale, at 20 - 28 m length and almost 200,000 kg weight. The shape and color of the body, and the size and shape of fins, varies considerably among species. A shared external trait is the presence of deep longitudinal grooves in the skin, running over the entire throat and chest.

The skulls of these mysticetes can be recognized by a combination of the following technical characteristics: the nasals and the nasal processes of the premaxillae extend backward beyond the supraorbital processes of the frontals; the nasals are reduced in size; the frontals are small and barely or not exposed on the dorsal surface; the supraoccipital extends forward beyond the zygomatic process of the squamosal; the rostrum is broad and flat.

The baleen plates of rorquals are short and broad. These species feed by gulping large quantities of water and straining crustaceans and fish by shooting the water out between the baleen plates.

Rorquals feed in cold currents at high latitudes during the summer, mostly on the eastern sides of the oceans. Some species range mostly offshore, others are more often found in coastal waters. Their food is primarily krill, euphausiid crustaceans, which congregate near the surface in cold water. Blue whales eat little but euphausiids; other species have a broader diet, even including some fish. During the fall, most species migrate toward equatorial latitudes. They fast for several month, living by metabolizing blubber.

Rorquals are usually seen in groups ("pods") of 2-5 individuals. Their populations have been much reduced by whaling, and most are now fully protected by international treaty.

Eschrichtiidae

Gray whale

This family contains a single living species, the gray whale. These whales are restricted today to the north Pacific (along east and west coasts); Atlantic populations are extinct.

Gray whales are large whales, ranging to over 14 m in length and over 30,000 kg in weight. They have a slender body with a low dorsal hump and no dorsal fin. The flippers are broad, and the tail has a folded ridge on its dorsal surface. The throat has two or three short grooves, unlike the multiple grooves of rorquals.

The skulls of gray whales differ from those of rorquals in that the telescoping of the bones that make up the dorsal surface is not as extreme. The nasals and nasal processes of the premaxillae extend posteriorly beyond the anterior border of the supraorbital processes of the frontals; the maxillae possess nasal processes; the nasals are large; the frontals are broadly exposed on the dorsal surface of the skull; the supraoccipital does not extend anteriorly beyond the zygomatic processes of the squamosal; and the rostrum is narrow and arched. The baleen plates are short and narrow.

Gray whales feed by swimming on their sides along the bottom, gulping mud by expanding their oral cavities rapidly (creating a strong inflow), and straining it through their fringed baleen plates. A wide variety of invertebrates is taken, but crustaceans are the main source of nutrition for this species. Like rorquals, gray whales are migratory species. They summer at high latitudes in the Pacific, migrating during autumn to the west coast of Baja California and the south coast of Korea. Pregnant females gather in shallow lagoons to give birth to calves. Gray whales travel singly or in small groups of up to a dozen or so individuals.Populations of this species were nearly driven to extinction in the early part of this century. Protected from whaling, the eastern Pacific population has made a strong comeback.

Odontoceti

Physeteridae | Monodontidae | Ziphiidae |  Delphinidae | Phocoenidae | Platanistidae

9 familias, 34 generos, y 68 especies

Este es el grupo que contiene la mayor cantidad de especies entre los mamíferos marinos. Está formado por 9 familias, 34 géneros, y 68 especies.    Su principal diferencia respecto a los Misticetos, es la presencia de dientes. Fisiológicamente tienen características únicas, tienen entre 2 y 260 dientes, dependiendo de la especie, 7 vértebras cervicales y una abertura nasal modificada a una sola estructura.

Es probable que evolucionaran  de los Arqueocetos, a fines del Eoceno hace unos 40 millones de años.

Comportamentalmente son muy complejos, la mayoría de ellos son gregarios (se agrupan en grandes cantidades de animales de la misma especie) y forman grandes manadas. Se comunican generalmente por medio de sonidos, que utilizan muy eficientemente a través de un complicado sistema de sonar de alta frecuencia. Envían sonidos para estimar la posición de determinados objetos, en forma de ecos (similar a los murciélagos) que luego analizan en su cerebro.

La presencia de dientes, revela su dieta principalmente carnívora, principalmente de peces y calamares, y en algunos casos como la orca comiendo peces, aves marinas y otros mamíferos marinos.

Las familias de Odontocetos son:

1)Physeteridae (Cachalotes), Kogia simus (Cachalote enano), Physeter catodon [Cachalote gigante), Kogiidae (Cachalote pigmeo o enano)

2) Monodontidae (Narvales y Ballenas Blancas), Delphinapterus leucas [Beluga whale] Monodon monoceros (Narval)

3) Ziphiidae (Ballenas picudas), Mesoplodon densirostris (Ballena picuda de Blainsville), Mesoplodon ginkgodens ( Ballena picuda)

4)  Delphinidae (Delfines marinos), Lissodelphis borealis (Delfín del Norte) Orcinus orca [Orca], Peponocephala electra [Melon-headed whale], Pseudorca crassidens [Falsa Orca] Tursiops truncatus [Delfín común o nariz de botella]

5) Phocoenidae (Marsopas)

6) Platanistidae (Delfines de río), Lipotes vexillifer [Delfín de Baiji]Platanista gangetica [Susu o delfín del río Ganges], Iniidae Inia geoffrensis [Boto Delfin de río sudamericano], Pontoporiidae

Nota: Existen estudios muy nuevos que sugieren que la familia Physeteridae (cachalotes) están más emparentados con los misticetos que con los odontocetos. Hasta que esto sea comprobado se los incluye en el grupo de las ballenas con dientes.

Physeteridae

Sperm whales

Three species in 2 genera (Physeter, the giant sperm whale; and Kogia, containing two species called pygmy and dwarf sperm whales) make up this family. Sperm whales are found in all oceans except the Arctic. These whales include the very large giant sperm whale, which reaches lengths of over 18 m and weights in excess of 53,000 kg, and the (relatively!) dimunitive pygmy sperm whale, which attains a mere 4 m in length and 320 kg in weight.

Physeter has an enormous head, equalling 35% of the length of the body, with a greatly-developed facial depression that contains the spermaceti organ. This structure is derived from the melon of other odontocetes, and like the melon, may serve as a sort of acoustic lens. The mouth of Physeter is considerably undershot but the lower jaw is long. The dorsal fin is is low, thick, and rounded, and the flippers are broad and rounded. Physeter also has a dorsal ridge and thick ventral keel on the tail.

The species of Kogia, in contrast, have a much smaller head; they also have spermaceti organs, but these are too small to be of commercial interest. The lower jaw is short and the jaw is undershot. The dorsal fin is low and sickle-shaped, and the flippers narrow.

The throat region of both species has numerous shallow, irregular grooves.

The facial depression of physeterids extends to the sides of the skull and roofs over the temporal fossa, hiding the zygomatic arches from dorsal view. At the rear, it terminates in a high, semicircular occipital crest. The rostrum is broad, flat, and triangular. The blowhole is asymmetrical, S-shaped, and located on the left side of the snout. The lower jaw is very long and narrow, but as noted above, does not reach the end of the rostrum. The mandibular symphysis is more than 33% the length of the rami. The number of teeth ranges from 1/8 to 0/16 in Kogia and 0/25 in Physeter.

Physeter dives to amazing depths (over 1000 m) in pursuit of its primary prey, squid. It also takes sharks, skates, and fish. Dives may last for 80 minutes or more. Females and young males form schools of 20-40 individuals. These are joined by bull males during the breeding season. Larger aggregations are occasionally seen. Young males form loose bachelor herds, but become increasingly solitary as they age. Giant sperm whales are migratory, following the summer from northern to southern hemispheres. The habits of Kogia are less well known. These whales feed primarily but not exclusively on squid, with one species foraging in deep oceanic waters and the other over the continental shelf. Kogia appear to be solitary or to live in small pods.

Giant sperm whales were extremely important to the whaling industry. Sperm oil, extracted from the spermaceti organ and from the blubber, remains liquid even at low temperatures and was used as a fine industrial lubricant. Spermaceti, which when cooled solidifies into a waxy substance, was used for making candles and ointments. Sperm whales also produce ambergris, probably from waste coalescing around indigestible substances in the intestinal tract. Ambergris is used as a fixative in the manufacture of perfume. Sperm whales are now fully protected by international law.

Curiously, while sperm whales unquestionably have teeth, recent molecular data and a reanalysis of their anatomy has suggested that they may be highly derived mysticetes.

Monodontidae

Narwhal and beluga

The two species of this family are found at high latitudes in the arctic seas and in most of the major rivers draining into them, south to the Saint Lawrence River. These are medium-sized whales, ranging from 4 - 6 m in length and up to around 1600 kg weight. Belugas have a very short, broad snout, while narwhales have a square head and appear to lack a snout altogether. The forehead in both species is high and globose. Members of this family lack a dorsal fin, although narwhals do have a distinctive ridge running along their backs. Adults are white (belugas) or white and black (narwhals).

As in the closely related delphinids, both species have a broadly expanded facial depression; this holds the melon, a fatty deposit believed to function in echolocation. The maxilla and frontal are expanded laterally, hiding the small zygomatic arch from dorsal view. The premaxillae lie flat in front of the nares, which are at the base of the short and broad rostrum. The length of the mandibular symphysis is less than 20% the length of the ramus, and in the upper jaw, the toothrows diverge posteriorly. Teeth are usually simple pegs in beluga (5/2 to 11/11 in number) but occasionally slightly 3-cusped. Narwhals have but two teeth, the one on the left is developed into a spiraled, forward-projecting tusk up to 2.7 m in length, and the other is rudimentary. The tusk of narwhals is found only in males; the teeth of females remain imbedded in their jaws.

Monodontids are generally found in schools, sometimes including more than 100 individuals. They migrate in response to the shifting ice pack. Both species feed mainly on the bottom, consuming a number of species of fish and invertebrates. The tusks presumably function in social behavior; males have been seen to fence with these structures, and occasionally broken-off pieces of tusk have been found imbedded in the heads of males of this species. Both species are highly vocal. Belugas make a sort of trilling sound and are sometimes known as "sea canaries."

Ziphiidae

Beaked whales

The Ziphiidae includes 19 species in 6 genera. It is the second largest family of cetaceans after the Delphinidae. Its members are found in all oceans.

Ziphiids are medium-sized whales, up to around 13 m in length and 11,500 kg wt. They have distinctive, long and narrow beaks. In some species, the snout is sharply set off from the rest of the head by a bulging forehead as in members of the Delphinidae; in others, however, the profile across the forehead is relatively flat. Their flippers are relatively small and oval to gently pointed in shape. Beaked whales have a small, falcate dorsal fin, which is set fairly far back on their bodies (well beyond the midpoint). The trailing edge of the fluke has no notch, unlike other cetaceans. Ziphiids have up to six short grooves on their throats. These converge anteriorly, forming a V pattern. The body color of these whales varies among species from uniform brown or gray to having contrasting white markings.

The skulls of ziphiids have an expanded facial depression like that of delphinids, but its posterior margin is very much raised. The zygomatic arch is small and hidden from dorsal view beneath the sides of the facial depression. The rostrum is very narrow, and the palate is strongly convex. The lower jaw is V-shaped and is as wide or slightly wider than the rostrum. The mandibular symphysis is relatively short, less than 1/3 the length of the ramus. The teeth vary greatly among species in number, from 19/27 in Tasmanian beaked whales to 0/1-2 in all other genera. Males of all species have 1 or 2 large functional teeth on the lower jaw; smaller, apparently non-functional teeth are sometimes seen on upper and lower jaws of several species. The teeth of females of most species remain buried in the gums, suggesting that ziphiid teeth are used mostly in social encounters.

The ziphiids are a diverse group, but the ecological and social habits of its members are not well known. They are capable of prolonged deep dives. All feed on squid; some also include fish in their diets. The social groups, insofar as is known, consist of 3-40 individuals. Some ziphiids are pursued by whalers for their oil and spermaceti.

Delphinidae

Dolphins, killer whales, pilot whales, melon-headed whales

With 32 species placed in 17 genera, this is by far the largest family of cetaceans. Delphinids are small to medium-sized cetaceans, ranging from about 1.5 m in length and 50 kg weight to almost 10 m in length and 7000 kg. Males are usually larger than females. The shape of the head of many delphinids is distinctive; the forehead appears to bulge over the beak-like rostrum due to the presence of a lens-shaped fatty deposit called a "melon." This structure may help focus the sound emitted by these animals in echolocation and feeding. Other delphinids possess a melon, but their rostrum is short and the bulging forehead merely gives the head a squared-off appearance. The bodies of most species are sleek and streamlined. Most have dorsal fins, which are usually curved (falcate), but much variation exists. Some species have striking color patterns over their bodies; others are more or less uniform. The group includes bottlenose dolphins, killer whales, pilot whales, Pacific striped dolphins, and many more.

In general, the common name "porpoise" is given to the related phocoenids, which lack a well-formed beak and have a squarish head and relatively chunky body. Most "dolphins" or delphinids generally have a distinct beak and a relatively slender, streamlined body.

In the delphinid skull, the facial depression is broadly expanded. The posterior end of the maxilla rises up above the rostrum. The zygomatic process of the squamosal is small and hidden from dorsal view by the expanded maxillae and frontals. The rostrum varies from short and broad to long and narrow. The lower jaws are fused for less than 40% of the length of their rami. The upper toothrows diverge posteriorly. The teeth are peg-like, circular in cross section, and sometimes numerous; the dental formula varies from 0/2 to 65/58.

Delphinds are found in all oceans and seas and in some river systems. They generally live in shallow water or at least stay near the surface, not making the deep and prolonged dives that characterize some other groups of cetaceans. They are fast and acrobatic swimmers, feeding on fishes and squids, which they pursue actively. Killer whales also prey on mammals such as other cetaceans and pinnipeds, as well as on birds and large fish. Like other odontocetes, delphinids echolocate and may even use high-intensity sound to stun their prey.

At least some members of this family are highly social, living in large groups (sometimes over 100,000 individuals!) and exhibiting a number of fascinating behaviors related to group living. Individuals appear to cooperate in a number of ways. One example is that these schools sometimes attack sharks, killing them by ramming them. Dolphins may work cooperatively to assist an individual that is ill or injured. Groups of dolphins often follow ships, riding the bow wave and sometimes making spectacular, acrobatic leaps. Several individuals in a school may leap in concert.Delphinids appear to be highly intelligent, adapting quickly and flexibly to novel situations. Most species, unfortunately, are poorly studied.

Phocoenidae

Porpoises

Phoecoenids include 6 species placed in 4 genera. They are found in the coastal waters of all oceans and seas of the northern hemisphere; along the coast of most of South America; and in some areas of southeastern Asia. They are also known from a few Asian rivers.

Members of this family are relatively small, from 1.5 to around 2 m in length and up to about 120 kg in weight. They have short jaws and no beak. A dorsal fin is present and triangular in some species, reduced to a ridge in others, and enormous in male Phocoena dioptrica. The flippers are fairly narrow and pointed. Some species are conspicuously marked with black, white, and gray; others are uniformly colored.

The skull is like that of the closely related delphinids, but it has distinctive swellings on the premaxillae anterior to the nares. The facial depression is broadly expanded posteriorly and hides the small zygomatic arches. The toothrows diverge posteriorly. The mandibular symphysis is relatively short, less than 20% of the length of the ramus. The teeth are numerous (from 15/15 to 30/30), and distinctively spade-shaped with 2- or 3-lobed crowns.

Some phoecoenids (Phocoena and Neophocaena) generally occupy bays, estuaries, and inlets close to shore. These porpoises are relatively slow, travelling in small groups of fewer than 6 individuals (occasionally up to 20). Others (Phocoenoides dalli) are found in offshore waters, are fast and agile swimmers, and are sometimes found in groups of up to thousands of individuals. Phoecoenids feed on a wide variety of fish and invertebrates.

Platanistidae

River dolphins

This family includes 5 species in 4 genera. In many accounts of cetaceans, its members are placed in four separate families: Iniidae (South American river dolphin), Lipotidae (Chinese river dolphin), Platanistidae (Indian river dolphins), and Pontoporidae ( La Plata river dolphin). Members of these families are found in fresh-water rivers and coastal waters in Asia and South America.

These dolphins are generally small, ranging from 1-3 m in length and from roughly 20 to 225 kg in weight, depending on species. They have a long, slender beak, above which rises a sharply differentiated, bulging forehead. The eyes are small, and in some species, appear atrophied. Unlike most other cetaceans, platanistids have a distinct neck. The flippers are broad and either rounded or sharply curved. The dorsal fin is low in all but the La Plata dolphin, in which it is moderately high. Colors tend to be muted grays, pinks, and browns, usually darker above and paler below.

Technically, these species are characterized by narrow facial depressions, with the lateral edges of the depression (formed by maxilla and frontal bones) not roofing over the temporal fossa and concealing the zygomatic arch. The zygomatic arch is strongly developed and arched. The rostrum is very long and slender; the mandibular symphysis is long, from 46 to 72% of the length of the ramus; and the teeth are numerous, ranging from 25/24 to 61/61. The teeth of most species are simple pegs, but in one species the posterior teeth are slightly tricusped, and in another they have a well defined cingulum.

The rivers used by most platanistids are muddy, and these animals probably rely on echolocation more than vision to locate their prey. They feed on fish and invertebrates, making dives that rarely last more than a few minutes. Group size varies from single individuals to 10 or 12. In general, little is known about their ecology and social behavior.

Referencias

Jefferson, Thomas A., Stephen Leatherwood, and Marc A. Webber. 1993. Marine Mammals of the World. FAO and UNEP. 320 pages.

Milinkovitch, Michel C. 1995. Molecular phylogeny of cetaceans prompts revision of morphological transformations. Trends in Ecology and Evolution 10 (8) Pgs. 305-345.

The Audubon Society. 1983. The Audubon Society Field Guide to North American Fishes, Whales and Dolphins. Alfred A. Knopf, Inc. 848 pages.

Nowak, R.M. and J.L. Paradiso. 1983. Walker's Mammals of the World, 4th edition. John Hopkins University Press, Baltimore, MD.

Savage, R. J. G. and M. R. Long. 1986. Mammal Evolution: An Illustrated Guide. Facts on File Publications, UK. 251 pp.

Rice, D. W. 1984. Cetaceans. Pp. 447-490 in Anderson, S. and J. K. Jones, Jr. (eds). Orders and Families of Recent Mammals of the World. John Wiley and Sons, N.Y. xii+686 pp.

Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, N.Y. vii+576 pp.

Wilson, D. E., and D. M. Reeder. 1993. Mammal Species of the World, A Taxonomic and Geographic Reference. 2nd edition. Smithsonian Institution Press, Washington. xviii+1206 pp. 

¿ Sabías Que?

Las ballenas y los delfines no duermen, al menos como lo hacemos los humanos. Ellos descansan en la superficie del agua por momentos mientras están nadando. En ese momento cada lado de su cerebro se "desconecta" mientras el otro lado permanece vigilante y mantiene al animal respirando.

El cuerpo de los delfines es considerado la perfección hidrodinámica. Su piel es muy suave y no tiene pelos, lo que le permite no tener prácticamente resistencia al agua mientras nada. Sus aletas son curvadas y la aleta caudal sirve para propulsión y estabilidad.  Desde los comienzos de la industria naval, el hombre ha estudiado las formas de las aletas de los delfines para el diseño de barcos y submarinos.

Los delfines se encuentran en todos los océanos y mares del mundo. Algunas especies son endémicas o viven solamente en algunas regiones, mientas que otras como el delfín común o nariz de botella son encontrados en todo el mundo. En algunos lugares se encuentran en grandes números. Aproximadamente entre 30 y 50 mil delfines viven en las costas de Japón. Algunas especies viajan juntas en grupos de a miles o más.   En general viajan en grupos familiares y no se aventuran a salir "mar adentro" en solitario, es muy difícil encontrar un delfín solitario en el océano.

Los delfines no mastican su comida aun cuando tienen tiendes muy poderosos. Usan sus dientes para sostener la comida mientras la destrozan con la ayuda de sus mandíbulas y le dan una forma cilíndrica para que luego sea fácil de digerir. Generalmente tienen que "acomodar" a los peces para ingerirlos con la cabeza adelante (es decir, lo primero que ingresa es la cabeza del pez), si esto no fuera así, tendrían una gran probabilidad de clavarse las espinas de las agallas o de las aletas de los peces espinosos.

Los Arqueocetos fueron los primeros cetáceos y aparecieron en los mares al comienzo del Eoceno, hace alrededor de 54 millones de años. Evolucionaron a partir de los mamíferos anfibios y en un principio fueron criaturas pequeñas (nunca más de 3 m de largo), cuadrúpedas, con aspecto de focas, con pocas especializaciones para la vida acuática. Pero al final del Eoceno, unos 15 millones de años después, se habían convertido en unos animales enormes, con aspecto de serpiente, muy adaptados para la vida den el mar. A este grupo pertenece Pakicetus, que es la ballena más antigua que se conoce (con aspecto de foca con extremidades en forma de aletas.

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