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Artiodactyls areplacental mammals belonging to theorderArtiodactyla (/ˌɑːrtioʊˈdæktɪlə/AR-tee-oh-DAK-tih-lə, from Ancient Greek ἄρτιος, ártios'even' and δάκτυλος, dáktylos'finger, toe'). Typically, they areungulates which bear weight equally on two (an even number) of their five toes (the third and fourth, often in the form of ahoof). The other three toes are either present, absent,vestigial, or pointing posteriorly. By contrast, mostperissodactyls bear weight on an odd number of the five toes. Another difference between the two orders is that many artiodactyls (except forSuina) digest plantcellulose in one or morestomach chambers rather than in theirintestine (as perissodactyls do). Molecular biology, along with new fossil discoveries, has found that cetaceans (whales,dolphins, andporpoises) fall within this taxonomic branch, being most closely related tohippopotamuses. Some modern taxonomists thus apply the nameCetartiodactyla (/sɪˌtɑːrtioʊˈdæktɪlə/) to this group, while others opt to include cetaceans within the existing name of Artiodactyla. Some researchers use "even-toed ungulates" to exclude cetaceans and only include terrestrial artiodactyls, making the termparaphyletic in nature.
The oldest fossils of even-toed ungulates date back to the earlyEocene (about 53 million years ago). Since these findings almost simultaneously appeared inEurope,Asia, andNorth America, it is very difficult to accurately determine the origin of artiodactyls. The fossils are classified as belonging to the familyDiacodexeidae;[1][2][3] their best-known and best-preserved member isDiacodexis.[2] These were small animals, some as small as ahare, with a slim build, lanky legs, and a long tail. Their hind legs were much longer than their front legs. Theearly to middle Eocene saw the emergence of the ancestors of most of today's mammals.[4]
Entelodonts were stocky animals with a large head, and were characterized by bony bumps on the lower jaw.
Two formerly widespread, but now extinct, families of even-toed ungulates wereEntelodontidae andAnthracotheriidae. Entelodonts existed from the middle Eocene to theearly Miocene inEurasia and North America. They had a stocky body with short legs and a massive head, which was characterized by two humps on the lower jaw bone. Anthracotheres had a large, porcine (pig-like) build, with short legs and an elongatedmuzzle. This group appeared in the middle Eocene up until thePliocene, and spread throughout Eurasia, Africa, and North America. Anthracotheres are thought to be the ancestors of hippos, and, likewise, probably led a similar aquatic lifestyle. Hippopotamuses appeared in thelate Miocene and occupied Africa and Asia—they never got to the Americas.[4]
The camels (Tylopoda) were, during large parts of theCenozoic, limited to North America; early forms likeCainotheriidae occupied Europe. Among the North American camels were groups like the stocky, short-leggedMerycoidodontidae. They first appeared in the late Eocene and developed a great diversity of species in North America. Only in the late Miocene or early Pliocene did they migrate from North America into Eurasia. TheNorth American varieties became extinct around 10,000 years ago.[citation needed]
Suina (includingpigs) have been around since the Eocene. In the late Eocene or theOligocene, two families stayed in Eurasia and Africa; thepeccaries, which became extinct in theOld World, exist today only in theAmericas.[citation needed]
Anoplotherium was the first fossil artiodactyl genus to be named, with a history dating back to 1804. It lived in Europe as part of the endemic familyAnoplotheriidae during the late Eocene-earliest Oligocene.
Richard Owen coined the term "even-toed ungulate".
The classification of artiodactyls was hotly debated because ocean-dwellingcetaceans evolved from land-dwelling even-toed ungulates. Somesemiaquatic even-toed ungulates (hippopotamuses) are more closely related to ocean-dwelling cetaceans than to other even-toed ungulates.[5]
Phylogenetic classification only recognizesmonophyletic taxa; that is, groups that descend from a common ancestor and include all of its descendants. To address this problem, the traditional order Artiodactyla and infraorderCetacea are sometimes subsumed into the more inclusive Cetartiodactyla taxon.[5] An alternative approach is to include both land-dwelling even-toed ungulates and ocean-dwelling cetaceans in a revised Artiodactyla taxon.[4]
Molecular and morphological studies confirmed that cetaceans are the closest living relatives of hippopotamuses.
In the 1990s, biologicalsystematics used not only morphology and fossils to classify organisms, but alsomolecular biology. Molecular biology involves sequencing an organism's DNA and RNA and comparing the sequence with that of other living beings—the more similar they are, the more closely they are related. Comparison of even-toed ungulate and cetaceans genetic material has shown that the closest living relatives ofwhales andhippopotamuses is the paraphyletic group Artiodactyla.[7][8]
Dan Graur and Desmond Higgins were among the first to come to this conclusion, and included a paper published in 1994.[9] However, they did not recognize hippopotamuses and classified theruminants as the sister group of cetaceans. Subsequent studies established the close relationship between hippopotamuses and cetaceans; these studies were based oncasein genes,[7]SINEs,[10]fibrinogen sequences,[11]cytochrome andrRNA sequences,[5][12]IRBP (andvWF) gene sequences,[13]adrenergic receptors,[14] andapolipoproteins.[8]
In 2001, the fossil limbs of aPakicetus (amphibioid cetacean the size of a wolf) andIchthyolestes (an early whale the size of a fox) were found in Pakistan. They were botharchaeocetes ("ancient whales") from about 48 million years ago (in the Eocene). These findings showed that archaeocetes were more terrestrial than previously thought, and that the special construction of thetalus (ankle bone) with a double-rolled joint surface,[clarification needed] previously thought to be unique to even-toed ungulates, were also in early cetaceans.[15] Themesonychians, another type of ungulate, did not show this special construction of the talus, and thus was concluded to not have the same ancestors as cetaceans.[citation needed]
Hippos are a geologically young group, which raises questions about their origin.
The oldest cetaceans date back to the early Eocene (53 million years ago), whereas the oldest known hippopotamus dates back only to the Miocene (15 million years ago). The hippopotamids are descended from the anthracotheres, a family of semiaquatic and terrestrial artiodactyls that appeared in the late Eocene, and are thought to have resembled small- or narrow-headed hippos. Research is therefore focused on anthracotheres (family Anthracotheriidae); one dating from the Eocene to Miocene was declared to be "hippo-like" upon discovery in the 19th century. A study from 2005 showed that the anthracotheres and hippopotamuses had very similarskulls, but differed in the adaptations of their teeth. It was nevertheless believed that cetaceans and anthracothereres descended from a common ancestor, and that hippopotamuses developed from anthracotheres. A study published in 2015 confirmed this, but also revealed that hippopotamuses were derived from older anthracotherians.[12][16] The newly introduced genusEpirigenys from Eastern Africa is thus the sister group of hippos.
Linnaeus postulated a close relationship between camels and ruminants as early as the mid-1700s.[17]Henri de Blainville recognized the similar anatomy of the limbs of pigs and hippos,[when?] and British zoologistRichard Owen coined the term "even-toed ungulates" and the scientific name "Artiodactyla" in 1848.[18]
Internal morphology (mainly the stomach and the molars) were used for classification.Suines (including pigs) and hippopotamuses have molars with well-developed roots and a simple stomach that digests food. Thus, they were grouped together as non-ruminants (porcine). All other even-toed ungulates have molars with aselenodont construction (crescent-shaped cusps) and have the ability toruminate, which requires regurgitating food and re-chewing it. Differences in stomach construction indicated that rumination evolved independently betweentylopods andruminants; therefore, tylopods were excluded fromRuminantia.[16]
The taxonomy that was widely accepted by the end of the 20th century was:[16]
Modern cetaceans are highly adapted sea creatures which, morphologically, have little in common with land mammals; they are similar to othermarine mammals, such asseals andsea cows, due toconvergent evolution. However, they evolved from originally terrestrial mammals. The most likely ancestors were long thought to be mesonychians—large, carnivorous animals from the early Cenozoic (Paleocene andEocene), which hadhooves instead ofclaws on their feet. Their molars were adapted to a carnivorous diet, resembling the teeth in moderntoothed whales, and, unlike other mammals, had a uniform construction.[19]
The suspected relations can be shown as follows:[16][20]
Molecular findings and morphological indications suggest that artiodactyls, as traditionally defined, are paraphyletic with respect to cetaceans. Cetaceans are deeply nested within the former; the two groups together form amonophyletic taxon, for which the name Cetartiodactyla is sometimes used. Modern nomenclature divides Artiodactyla (or Cetartiodactyla) in four subordinate taxa: camelids (Tylopoda), pigs and peccaries (Suina), ruminants (Ruminantia), and hippos plus cetaceans (Whippomorpha).
The four summarized Artiodactyla taxa are divided into ten extant families:[26]
The camelids (Tylopoda) comprise only one family,Camelidae. It is a species-poor artiodactyl suborder of North American origin[27] that is well adapted to extreme habitats—thedromedary andBactrian camels in the Old World deserts and theguanacos, llamas,vicuñas, andalpacas in South American high mountain regions.
The pig-like creatures (Suina) are made up of two families:
The pigs (Suidae) are limited to the Old World. These include thewild boar and the domesticated form, thedomestic pig.
The peccaries (Tayassuidae) are named after glands on their belly and are indigenous to Central and South America.
The ruminants (Ruminantia) consist of six families:
The mouse deer (Tragulidae) are the smallest and most primitive even-toed-ruminants; they inhabit forests of Africa and Asia.
The giraffe-like creatures (Giraffidae) are composed of two species: the giraffe and theokapi.
The musk deer (Moschidae) is indigenous to East Asia.
The antilocaprids (Antilocapridae) of North America comprise only one extant species: thepronghorn.
The deer (Cervidae) are made up of about 45 species, which are characterized by a pair ofantlers (generally only in males). They are spread across Europe, Asia, and the Americas. This group includes, among other species, thered deer,moose,elk (wapiti), andreindeer (caribou).
Although deer, musk deer, and pronghorns have traditionally been summarized as cervids (Cervioidea), molecular studies provide different—and inconsistent—results, so the question of phylogenetic systematics of infraorderPecora (the horned ruminants) for the time being, cannot be answered.
Artiodactyls are generallyquadrupeds. Two major body types are known: suinids and hippopotamuses are characterized by a stocky body, short legs, and a large head; camels and ruminants, though, have a more slender build and lanky legs. Size varies considerably; the smallest member, the mouse deer, often reaches a body length of only 45 centimeters (18 in) and a weight of 1.5 kilograms (3.3 lb). The largest member, the hippopotamus, can grow up to 5 meters (16 ft) in length and weigh 4.5 metric tons (5 short tons), and the giraffe can grow to be 5.5 meters (18 ft) tall and 4.7 meters (15 ft) in body length. All even-toed ungulates display some form ofsexual dimorphism: the males are consistently larger and heavier than the females. In the majority of deer species, only the males grow antlers, and the horns ofbovines are usually small or not present in females. MaleIndian antelopes have a much darker coat than females.
Almost all even-toed ungulates have fur, with the exception being the nearly hairless hippopotamus. Fur varies in length and coloration depending on the habitat. Species in cooler regions can shed their coat. Camouflaged coats come in colors of yellow, gray, brown, or black tones.
Even-toed ungulates bear their name because they have an even number oftoes (two or four)—in some peccaries, thehind legs have a reduction in the number of toes to three. The central axis of the leg is between the third and fourth toe. The first toe is missing in modern artiodactyls, and can only be found in now-extinct genera. The second and fifth toes are adapted differently between species:
Hippopotamuses have all four toes pointing out.
In pigs and other biungulates, the second and fifth toes are directed backwards.
When camels have only two toes present, the claws are transformed into nails.
When camels have only two toes present, theclaws are transformed intonails (while both are made ofkeratin, claws are curved and pointed while nails are flat and dull).[28] These claws consist of three parts: the plate (top and sides), the sole (bottom), and the bale (rear). In general, the claws of the forelegs are wider and blunter than those of the hind legs, and they are farther apart. Aside from camels, all even-toed ungulates put just the tip of the foremostphalanx on the ground.[29]
Diagrams of hand skeletons of various mammals, left to right:orangutan,dog,pig,cow,tapir, andhorse. Highlighted are the even-toed ungulates pig and cow.
In even-toed ungulates, the bones of the stylopodium (upper arm or thigh bone) andzygopodiums (tibia and fibula) are usually elongated. The muscles of the limbs are predominantly localized, which ensures that artiodactyls often have very slender legs. Aclavicle is never present, and thescapula is very agile and swings back and forth for added mobility when running. The special construction of the legs causes the legs to be unable to rotate, which allows for greater stability when running at high speeds. In addition, many smaller artiodactyls have a very flexible body, contributing to their speed by increasing their stride length.
Many even-toed ungulates have a relatively large head. The skull is elongated and rather narrow; thefrontal bone is enlarged near the back and displaces theparietal bone, which forms only part of the side of the cranium (especially in ruminants).
Outgrowths of the frontal bone characterize most forehead weapons carriers, such as thegemsbok and its horns.
Four families of even-toed ungulates have cranial appendages. ThesePecora (with the exception of themusk deer), have one of four types of cranial appendages: true horns,antlers,ossicones, orpronghorns.[30]
True horns have a bone core that is covered in a permanent sheath of keratin, and are found only in thebovids.Antlers are bony structures that are shed and replaced each year; they are found in deer (members of the familyCervidae). They grow from a permanent outgrowth of the frontal bone called the pedicle and can be branched, as in thewhite-tailed deer (Odocoileus virginianus), or palmate,[31] as in themoose (Alces alces).Ossicones are permanent bone structures that fuse to the frontal or parietal bones during an animal's life and are found only in theGiraffidae.Pronghorns, while similar to horns in that they have keratinous sheaths covering permanent bone cores, are deciduous.[clarification needed][32]
All these cranial appendages can serve for posturing, battling for mating privilege, and for defense. In almost all cases, they are sexually dimorphic, and are often found only on the males. One exception is the speciesRangifer tarandus, known asreindeer in Europe or caribou in North America, where both sexes can grow antlers yearly, though the females' antlers are typically smaller and not always present.
There are two trends in terms of teeth within Artiodactyla. The Suina and hippopotamuses have a relatively large number of teeth (with some pigs having 44); their dentition is more adapted to a squeezingmastication, which is characteristic ofomnivores. Camels and ruminants have fewer teeth; there is often a yawningdiastema, a designated gap in the teeth where the molars are aligned for crushing plant matter.
Theincisors are often reduced in ruminants, and are completely absent in theupper jaw. The canines are enlarged and tusk-like in the Suina, and are used for digging in the ground and for defense. In ruminants, the males' upper canines are enlarged and used as a weapon in certain species (mouse deer, musk deer,water deer); species with frontal weapons are usually missing the upper canines. The lower canines of ruminants resemble the incisors, so that these animals have eight uniform teeth in the frontal part of thelower jaw.
The molars of porcine have only a few bumps. In contrast, camels and ruminants have bumps that are crescent-shaped cusps (selenodont).
Artiodactyls have a well-developed sense of smell and sense of hearing. Unlike many other mammals, they have a poor sense of sight—moving objects are much easier to see than stationary ones. Similar to many other prey animals, their eyes are on the sides of the head, giving them an almost panoramic view.
Pigs (such as thiswarthog) have a simple sack-shaped stomach.As with allruminants,deer have such a multi-chambered stomach, which is used for better digesting plant food.
Theruminants (Ruminantia) ruminate their food—they regurgitate and re-chew it. Ruminants' mouths often have additionalsalivary glands, and theoral mucosa is often heavily calloused to avoid injury from hard plant parts and to allow easier transport of roughly chewed food. Their stomachs are divided into three to four sections: therumen, thereticulum, theomasum, and theabomasum.[33] After the food is ingested, it is mixed with saliva in therumen andreticulum and separates into layers of solid versus liquid material. The solids lump together to form abolus (also known as thecud); this is regurgitated by reticular contractions while theglottis is closed. When the bolus enters the mouth, the fluid is squeezed out with the tongue and re-swallowed. The bolus is chewed slowly to completely mix it with saliva and to break it down. Ingested food passes to the "fermentation chamber" (rumen and reticulum), where it is kept in continual motion by rhythmic contractions. Cellulyticmicrobes (bacteria,protozoa, andfungi) producecellulase, which is needed to break down the cellulose found in plant material.[33] This form of digestion has two advantages: plants that are indigestible to other species can be digested and used, and the duration of the actual food consumption shortened; the animal spends only a short time out in the open with its head to the ground—rumination can take place later, in a sheltered area.[34]
Tylopoda (camels, llamas, and alpacas) andchevrotains have three-chambered stomachs, while the rest ofRuminantia have four-chambered stomachs. The handicap of a heavy digestive system has increased selective pressure towards limbs that allow the animal to quickly escape predators.[35] Most species withinSuina have a simple two-chambered stomach that allows for anomnivorous diet. Thebabirusa, however, is a herbivore,[33] and has extramaxillary teeth to allow for proper mastication of plant material. Most of the fermentation occurs with the help of cellulolytic microorganisms within thecaecum of thelarge intestine.Peccaries have a complex stomach that contains four compartments.[34] Their fore stomach has fermentation carried out by microbes and has high levels ofvolatile fatty acid; it has been proposed that their complex fore-stomach is a means to slow digestive passage and increase digestive efficiency.[34] Hippopotamuses have three-chambered stomachs and do not ruminate. They consume around 68 kilograms (150 lb) of grass and other plant matter each night. They may cover distances up to 32 kilometers (20 mi) to obtain food, which they digest with the help of microbes that produce cellulase. Their closest living relatives, thewhales, areobligate carnivores.
The penises of even-toed ungulates have an S-shape at rest and lie in a pocket under the skin on the belly.[37] Thecorpora cavernosa are only slightly developed; and anerection mainly causes this curvature to extend, which leads to an extension, but not a thickening, of thepenis. Cetaceans have similar penises.[38] In some even-toed ungulates, the penis contains a structure called theurethral process[39][40][41] orpenile vermiform appendix.[42]
TheJapanese serow has glands in the eyes that are clearly visible.
The number ofmammary glands is variable and correlates, as in all mammals, withlitter size. Pigs, which have the largest litter size of all even-toed ungulates, have two rows ofteats lined from the armpit to the groin area. In most cases, however, even-toed ungulates have only one or two pairs of teats. In some species these form anudder in the groin region.
Secretory glands in the skin are present in virtually all species and can be located in different places, such as in the eyes, behind the horns, the neck, or back, on the feet, or in the anal region.
Artiodactyls have acarotid rete heat exchange that enables them, unlike perissodactyls which lack one, to regulate their brain temperature independently of their bodies. It has been argued that its presence explains the greater success of artiodactyls compared to perissodactyls in being able to adapt to diverse environments from the Arctic Circle to deserts and tropical savannahs.[43]
Artiodactyls are native to almost all parts of the world, with the exception ofOceania andAntarctica. Humans have introduced different artiodactyls worldwide as hunting animals.[44] Artiodactyls inhabit almost every habitat, fromtropical rainforests andsteppes todeserts and highmountain regions. The greatestbiodiversity prevails in open habitats such asgrasslands and open forests.
The social behavior of even-toed ungulates varies from species to species. Generally, there is a tendency to merge into larger groups, but some live alone or in pairs. Species living in groups often have ahierarchy, both among males and females. Some species also live inharem groups, with one male, several females, and their common offspring. In other species, the females and juveniles stay together, while males are solitary or live in bachelor groups and seek out females only duringmating season.
Many artiodactyls areterritorial and mark their territory, for example, withglandular secretions or urine. In addition to year-round sedentary species, there are animals thatmigrate seasonally.
There arediurnal,crepuscular, andnocturnal artiodactyls. Some species' pattern of wakefulness varies with season or habitat.
Most artiodactyls, such as thewildebeest, are born with hair.
Generally, even-toed ungulates tend to have longgestation periods, smaller litter sizes, and more highly developed newborns. As with many other mammals, species intemperate orpolar regions have a fixedmating season, while those intropical areas breed year-round. They carry outpolygynous mating behavior, meaning a male mates with several females and suppresses all competition.
The length of the gestation period varies from four to five months for porcine, deer, and musk deer; six to ten months for hippos, deer, and bovines; ten to thirteen months with camels; and fourteen to fifteen months with giraffes. Most deliver one or two babies, but some pigs can deliver up to ten.
The newborns areprecocial (born relatively mature) and come with open eyes and are hairy (with the exception of the hairless hippos). Juvenile deer and pigs have striped or spotted coats; the pattern disappears as they grow older. The juveniles of some species spend their first weeks with their mother in a safe location, where others may be running and following the herd within a few hours or days.
Life expectancy is typically twenty to thirty years; as in many mammals, smaller species often have a shorter lifespan than larger species. The artiodactyls with the longest lifespans are the hippos, cows, and camels, which can live 40 to 50 years.
Artiodactyls have differentnatural predators depending on their size and habitat. There are severalcarnivores that prey on them, includinglarge cats (e.g.,lions) andbears. Other predators arecrocodiles,wolves anddogs, largeraptors, and for small species and young animals, largesnakes. For cetaceans, possible predators include sharks, polar bears, and other cetaceans; in the latter is the orca, the top predator of the oceans.[45]
Some artiodactyls, likesheep, have been domesticated for thousands of years.
Artiodactyls have been hunted by primitive humans for various reasons: for meat orfur, as well as to use their bones and teeth as weapons or tools. Theirdomestication began around 8000 BCE. To date, humans have domesticatedgoats,sheep,cattle, camels, llamas, alpacas, andpigs. Initially, livestock was used primarily for food, but they began being used for work activities around 3000 BCE.[35] Clear evidence exists of antelope being used for food 2 million years ago in theOlduvai Gorge, part of theGreat Rift Valley.[35][46]Cro-Magnons relied heavily on reindeer for food, skins, tools, and weapons; with dropping temperatures and increased reindeer numbers at the end of the Pleistocene, they became the prey of choice. Reindeer remains accounted for 94% of bones and teeth found in a cave above the riverCéou that was inhabited around 12,500 years ago.[47] In general, most even-toed ungulates can be consumed as akosher meat, with the principal exception ofSuina (pigs, etc.) andhippopotamids, which are even-toed but do notchew the cud, and ofCetacea, which, for the purpose ofrabbinic law, are considered to be scaleless fish, and thus not kosher.
Today, artiodactyls are kept primarily for their meat, milk, andwool, fur, orhide for clothing. Domestic cattle, thewater buffalo, theyak, and camels are used for work, as rides, or aspack animals.[48][page needed]
Theaurochs has been extinct since the 17th century.
The endangerment level of each even-toed ungulate is different. Some species aresynanthropic (such as thewild boar) and have spread into areas that they are not indigenous to, either having been brought in as farm animals or having run away as people's pets. Some artiodactyls also benefit from the fact that their predators (e.g., theTasmanian tiger) were severely decimated by ranchers, who saw them as competition.[44]
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