Artiodactyl

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Artiodactyls
)

Artiodactyls
Temporal range: 55.4–0 
Ma
 Early Eocene – Holocene
GiraffeAmerican bisonRed deerOrcaWild boarDromedary
Clockwise from center: American bison (Bison bison), dromedary (Camelus dromedarius), wild boar (Sus scrofa), orca (Orcinus orca), red deer (Cervus elaphus), and giraffe (Genus: Giraffa)
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Clade: Scrotifera
Grandorder: Ferungulata
Clade: Pan-Euungulata
Mirorder: Euungulata
Clade:
Paraxonia
Order: Artiodactyla
Owen, 1848
Subdivisions
Synonyms

Cetartiodactyla
Montgelard et al. 1997

Artiodactyls are

paraphyletic
in nature.

The roughly 270 land-based even-toed ungulate species include

peccaries, hippopotamuses, antelopes, deer, giraffes, camels, llamas, alpacas, sheep, goats and cattle. Many are herbivores, but suids
are omnivorous, whereas cetaceans are entirely carnivorous. Many of these are of great dietary, economic, and cultural importance to humans.

Evolutionary history

See also: Evolution of cetaceans

The oldest fossils of even-toed ungulates date back to the early

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 were

Entelodontidae and Anthracotheriidae. Entelodonts existed from the middle Eocene to the early Miocene in Eurasia 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 elongated muzzle. This group appeared in the middle Eocene up until the Pliocene, 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 the late Miocene and occupied Africa and Asia—they never got to the Americas.[4]

The camels (

Merycoidodontidae. 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. The North American varieties
became extinct around 10,000 years ago.

peccaries, which became extinct in the Old World, exist today only in the Americas
.

A deer-like animal wanders through a clearing.
Sivatherium was a relative of giraffes with deer-like forehead ossicones.

South America was settled by even-toed ungulates only in the Pliocene, after the land bridge at the Isthmus of Panama formed some three million years ago. With only the peccaries, lamoids (or llamas), and various species of capreoline deer, South America has comparatively fewer artiodactyl families than other continents, except Australia, which has no native species.

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 family Anoplotheriidae during the late Eocene-earliest Oligocene.

Taxonomy and phylogeny

Portrait of Richard Owen
Richard Owen coined the term "even-toed ungulate".

The classification of artiodactyls was hotly debated because ocean-dwelling cetaceans evolved from land-dwelling even-toed ungulates. Some semiaquatic even-toed ungulates (hippopotamuses) are more closely related to ocean-dwelling cetaceans than to other even-toed ungulates.

Phylogenetic classification only recognizes

monophyletic 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 infraorder Cetacea 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]

Classification

Main article: List of artiodactyls

Research history

Humpback whale swimming under water
Molecular and morphological studies confirmed that cetaceans are the closest living relatives of hippopotamuses.

In the 1990s, biological systematics used not only morphology and fossils to classify organisms, but also molecular 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 of whales and hippopotamuses is the paraphyletic group Artiodactyla.

Dan Graur and Desmond Higgins were among the first to come to this conclusion, and included a paper published in 1994.

rRNA sequences,[5][11] IRBP (and vWF) gene sequences,[12] adrenergic receptors,[13] and apolipoproteins.[14]

In 2001, the fossil limbs of a

archaeocetes ("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 the talus (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] The mesonychians, 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.

A hippo splashes in the water
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 similar skulls, 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.[11][16] The newly introduced genus Epirigenys from Eastern Africa is thus the sister group of hippos.

Historical classification of Artiodactyla

Henri de Blainville recognized the similar anatomy of the limbs of pigs and hippos,[when?] and British zoologist Richard Owen coined the term "even-toed ungulates" and the scientific name "Artiodactyla" in 1848.[17]

Internal morphology (mainly the stomach and the molars) were used for classification.

Ruminantia
.

The taxonomy that was widely accepted by the end of the 20th century was:[18][full citation needed]

Even-toed ungulates
 Suina 
 Selenodont 

 Tylopoda

 Ruminants 

 

Tragulidae

 Pecora

Historical classification of Cetacea

An illustration of a mesonychian, which looks like a wolf-like animal
The mesonychians were long considered ancestors of whales.

Modern cetaceans are highly adapted sea creatures which, morphologically, have little in common with land mammals; they are similar to other

hooves instead of claws on their feet. Their molars were adapted to a carnivorous diet, resembling the teeth in modern toothed whales, and, unlike other mammals, had a uniform construction.[citation needed
]

The suspected relations can be shown as follows:[16][19][page needed]

Paraxonia
 

 

Artiodactyla

 Cete 

Inner systematics

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 a

monophyletic
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 whales (Whippomorpha).

The presumed lineages within Artiodactyla can be represented in the following cladogram:[20][21][22][23][24]

  Artiodactyla  

  Tylopoda (camels)

  Artiofabula  

  Suina (pigs)

 Cetruminantia  
  
Ruminantia
 (ruminants)  

  Tragulidae (mouse deer)

  Pecora (horn bearers)

  
Cetancodonta
  

  Hippopotamidae (hippopotamuses)

  Cetacea (whales)

  (or Whippomorpha)  
A camel chillaxing.
Camels are now considered a sister group of Artiofabula.
A pronghorn
The pronghorn is the only extant antilocaprid.

The four summarized Artiodactyla taxa are divided into ten extant families:[25]

  • The camelids (Tylopoda) comprise only one family, Camelidae. It is a species-poor artiodactyl suborder of North American origin[26] that is well adapted to extreme habitats—the dromedary and Bactrian camels in the Old World deserts and the guanacos, llamas, vicuñas, and alpacas in South American high mountain regions.
  • The pig-like creatures (Suina) are made up of two families:
    • The pigs (
      domestic 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 the okapi.
    • The musk deer (Moschidae) is indigenous to East Asia.
    • The antilocaprids (Antilocapridae) of North America comprise only one extant species: the pronghorn.
    • The deer (
      Cervidae) are made up of about 45 species, which are characterized by a pair of antlers (generally only in males). They are spread across Europe, Asia, and the Americas. This group includes, among other species, the red deer, moose, elk (wapiti), and reindeer
      (caribou).
    • The bovids (Bovidae) are the most species-rich. Among them are cattle, sheep, caprines, and antelopes.
  • The whippomorphans include hippos and cetaceans:
    • The hippos (
      pygmy hippo
      .
    • The whales (
      Mysticeti
      )

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 infraorder Pecora (the horned ruminants) for the time being, cannot be answered.

Illustration of an Indohyus, a mouse-like mammal
Reconstruction of Indohyus

Anatomy

Blue duiker (Philantomba monticola) skeleton on display at the Museum of Osteology.

Artiodactyls are generally

Indian 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.

Limbs

mouse deer
is the smallest even-toed ungulate.

Even-toed ungulates bear their name because they have an even number of

hind 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.
    Hippopotamuses have all four toes pointing out.
  • For pigs and other biungulates the second and fifth toes are directed backwards.
    For 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, the claws are transformed into nails.

When camels have only two toes present, the claws are transformed into nails (while both are made of keratin, claws are curved and pointed while nails are flat and dull).[27] 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 foremost phalanx on the ground.[28]

cow, tapir, and horse
. Highlighted are the even-toed ungulates pig and cow.

In even-toed ungulates, the bones of the stylopodium (upper arm or thigh bone) and

zygopodiums (tibia and fibula) are usually elongated. The muscles of the limbs are predominantly localized, which ensures that artiodactyls often have very slender legs. A clavicle is never present, and the scapula
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.

Head

Many even-toed ungulates have a relatively large head. The skull is elongated and rather narrow; the frontal bone is enlarged near the back and displaces the parietal bone, which forms only part of the side of the cranium (especially in ruminants).

Horns and antlers

A gemsbok, a type of antelope
Outgrowths of the frontal bone characterize most forehead weapons carriers, such as the gemsbok and its horns.

Four families of even-toed ungulates have cranial appendages. These Pecora (with the exception of the musk deer), have one of four types of cranial appendages: true horns, antlers, ossicones, or pronghorns.[29]

True horns have a bone core that is covered in a permanent sheath of keratin, and are found only in the

Cervidae). They grow from a permanent outgrowth of the frontal bone called the pedicle and can be branched, as in the white-tailed deer (Odocoileus virginianus), or palmate, as in the moose (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 the Giraffidae. Pronghorns, while similar to horns in that they have keratinous sheaths covering permanent bone cores, are deciduous.[clarification needed][30]

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 species Rangifer tarandus, known as reindeer 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.

Teeth

A deer-pig with elongated lower canines that curve up, forming elephant-like tusks.
The canines of Suinas develop into tusks.
Dental formula
 I C P M
30–44 = 0–3 0–1 2–4 3
1–3 1 2–4 3

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 squeezing

mastication, which is characteristic of omnivores. Camels and ruminants have fewer teeth; there is often a yawning diastema
, a designated gap in the teeth where the molars are aligned for crushing plant matter.

The

lower jaw
.

The molars of porcine have only a few bumps. In contrast, camels and ruminants have bumps that are crescent-shaped cusps (selenodont).

Senses

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.

Digestive system

A warthog.
Pigs (such as this warthog) have a simple sack-shaped stomach.
A male deer
As with all ruminants, deer have such a multi-chambered stomach, which is used for better digesting plant food.

The

fungi) produce cellulase, which is needed to break down the cellulose found in plant material.[31] 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.[32]

obligate carnivores
.

Unlike other even-toed ungulates, pigs have a simple sack-shaped

gall bladder.[34]

Two Japanese serows (goat-antelopes) sit together.
The Japanese serow has glands in the eyes that are clearly visible.

Genitourinary system

The penises of even-toed ungulates have an S-shape at rest and lie in a pocket under the skin on the belly. The corpora cavernosa are only slightly developed; and an erection mainly causes this curvature to extend, which leads to an extension, but not a thickening, of the penis. Cetaceans have similar penises.[35] In some even-toed ungulates, the penis contains a structure called the urethral process.[36][37][38]

The testicles are located in the scrotum and thus outside the abdominal cavity. The ovaries of many females descend—as the testicles descend of many male mammals—and are close to the pelvic inlet at the level of the fourth lumbar vertebra. The uterus has two horns (uterus bicornis).[35]

Other

The number of

litter size. Pigs, which have the largest litter size of all even-toed ungulates, have two rows of teats 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 an udder
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 a

carotid 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.[39]

Lifestyle

Distribution and habitat

Artiodactyls are native to almost all parts of the world, with the exception of Oceania and Antarctica. Humans have introduced different artiodactyls worldwide as hunting animals.[40] Artiodactyls inhabit almost every habitat, from tropical rainforests and steppes to deserts and high mountain regions. The greatest biodiversity prevails in open habitats such as grasslands and open forests.

Social behavior

Two giraffes stand, surrounded by impalas (a type of antelope).
Artiodactyls, like impalas and giraffes, live in groups.

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 a

mating season
.

Many artiodactyls are territorial and mark their territory, for example, with glandular secretions or urine. In addition to year-round sedentary species, there are animals that migrate seasonally.

There are

nocturnal
artiodactyls. Some species' pattern of wakefulness varies with season or habitat.

Reproduction and life expectancy

Roaming wildebeests
Most artiodactyls, such as the wildebeest, are born with hair.

Generally, even-toed ungulates tend to have long

mating season, while those in tropical
areas breed year-round. They carry out polygynous 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 are

precocial
(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.

Predators and parasites

Artiodactyls have different

raptors, and for small species and young animals, large snakes. For cetaceans, possible predators include sharks, polar bears, and other cetaceans; in the latter is the orca, the top predator of the oceans.[41]

flukes, but they have debilitating effects only when the infestation is severe.[citation needed
]

Interactions with humans

Domestication

See also:
Domestication of animals
Sheep on a farm
Some artiodactyls, like sheep, have been domesticated for thousands of years.

Artiodactyls have been hunted by primitive humans for various reasons: for meat or

chew the cud, and of Cetacea, which, for the purpose of Rabbinical Law
, are considered to be scaleless fish, and thus not Kosher.

Today, artiodactyls are kept primarily for their meat, milk, and wool, fur, or hide for clothing. Domestic cattle, the water buffalo, the yak, and camels are used for work, as rides, or as pack animals.[44][page needed]

Threats

Painting of an aurochs
The aurochs has been extinct since the 17th century.

The endangerment level of each even-toed ungulate is different. Some species are

synanthropic (such as the wild 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. the Tasmanian tiger) were severely decimated by ranchers, who saw them as competition.[40]

Conversely, many artiodactyls have declined significantly in numbers, and some have even gone extinct, largely due to

endangered.[45][46]

See also

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External links

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