Even-toed ungulate

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Even-toed ungulates
Temporal range: 55–0 Ma
Early Eocene-Holocene
Giraffe American Bison Red deer Killer whale Wild Boar DromedaryThe Artiodactyla.jpg
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Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Mammalia
Subclass:
Theria
Infraclass:
Eutheria
(unranked):
Exafroplacentalia
Magnorder:
Boreoeutheria
Superorder:
Laurasiatheria
(unranked):
Scrotifera
Order:
Artiodactyla

Owen, 1848
Subclades

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The even-toed ungulates (order Artiodactyla) are ungulates (hoofed animals) whose weight is borne approximately equally by the third and fourth toes, rather than mostly or entirely by the third as in odd-toed ungulate (perissodactyls), such as horses.

The name Artiodactyla comes from (Greek: ἄρτιος (ártios), "even", and δάκτυλος (dáktylos), "finger/toe"), so the name "even-toed" is a translation of the description. This group includes pigs (family Suidae), peccaries (family Tayassuidae), hippopotamuses (family Hippopotamidae), camels (genus Camelus in family Camelidae), llamas (genus Lama (genus) in family Camelidae), chevrotains or mouse deer (family Tragulidae), deer (family Cervidae), giraffes (family Giraffidae), pronghorn (genus Antilocapra in family Antilocapridae), antelopes (sections of family Bovidae including subfamily Antilopinae), goat-antelopes (which include sheep, goats and others; subfamily Caprinae in family Bovidae), and cattle (genus Bos in family Bovidae). Some exclude whales (infraorder Cetacea), although DNA sequence and anatomical data indicate they share a common ancestor, making the group paraphyletic. This school of thought names the joint group Cetartiodactyla (from Cetacea and Artiodactyla). Conversely, Grove and Grubb, 2011, citing Heglen and others, include Cetacea as a sister group of Hippopotamidae, since they are "deeply nested" within the phylogeny of the Artiodactyla, rather than a sister group which could be comprehended by a parent "Cetartiodactyla". They further contend, on the same authority, that just as Carnivora subsumed Pinnipedia without a name change, no name change is required for Artiodactyla.

Of the roughly 220 artiodactyl species, many are of great dietary, economic, and cultural importance to humans.

A further distinguishing feature of the group is the shape of the astragalus (talus), a bone in the ankle joint, which has a double-pulley structure. This gives the foot greater flexibility.

Taxonomy and phylogeny

Richard Owen coined the term "even-toed ungulate"

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See also: Evolution of cetaceans

The classification of artiodactyls is hotly debated. This is because there is a morphologically defined order, yet cetaceans have evolved from them, and some groups (hippopotamuses) are more closely related to them than to other even-toed ungulates. This makes the artiodactyls a phylogenetic (defined through the tribal development) classification - which is significant in recent times -in essence, a paraphyletic taxon, that is, a group which, although descended from a common ancestor, does not include all descendants. The phylogenetic classification is recognized as only a monophyletic taxa, that is, groups that are descended from a common ancestor and include all descendants including their ancestors; in essence, the order Artiodactyla and infraorder Cetacea are summarized in the taxon Cetartiodactyla. [1]

Classification

Research History

Molecular and morphological studies confirmed that cetaceans are the closest living relatives of hippopotamuses

In the 1990s, the biological systematics not only went in accordance with aspects of the morphology and the fossil findings, but also by means of molecular biology. It seeks, by sequencing the DNA and RNA, to obtain genetic information and compare it with the data of other living beings to elicit the degree of relationship based on the degree of similarity notes. This method was, and still is, used in the classification of many living things and has changed the classification of many taxa significantly. These methods were performed between even-toed ungulates and cetaceans with the surprising result that the closest living relatives of the whales and hippopotamuses is the paraphyletic group Artiodactyla.

Among the first who came to that conclusion, Dan Graur and Desmond Higgins, included a study published in 1994.[6] However, they did not recognize hippopotamuses and classified the ruminants as the sister group of cetaceans. Subsequent studies then came to the conclusion that the hippopotamuses are the closest living relative of cetaceans; these studies were based upon Caseingenen,[7] SINEs,[8] fibrinogen sequences,[9] cytochrome and rRNA sequences,[10][11][12] IRBP - and vWF gene sequences,[13] adrenergic receptors,[14] and apolipoproteins.[15] In one of these studies, the scientific names of Cetacea and Artiodactyla was established in 1997 and for the first time, the name "Cetartiodactyla", proposed by Claudine Montgelard, Francois M. Catzeflis and Emmanuel Douzery, was composed.

In Pakistan, 2001, parts of the limb skeleton of a creature as big as a wolf, Pakicetus, and another as large as a fox, Ichthyolestes, were discovered; they were two archaeocetes from the Eocene from about 48 million years ago. These findings not only showed that archaeocetes were more terrestrial than previously thought to, but also showed the special construction of the ankle bone (talus) with a double-rolled joint surface. This feature has long been considered an exclusive feature of even-toed ungulates and because it has now been discovered even in early cetaceans, the close relationship of the two groups could be assigned morphologically. [16] In more modern cetaceans, there was a comprehensive reduction of the hind limbs that made the construction of the hind legs of these animals inconclusive about possible lineages between the two. The mesonychid did not show this special construction of the talus, thus a shared lineage with cetaceans was excluded.

Although the specific construction of the talus supported a close relationship between artiodactyls and cetaceans, the question of whether the even-toed ungulates are paraphyletic was not answered. Therefore, morphological studies were conducted to support the findings of molecular biology between that of hippopotamuses and cetaceans. The arrangement of cusps of the molars, the construction of the metatarsal bones, and the skull support a sister group relationship between these two taxa, but the striking commonality, the loss of fur and the sebaceous glands, is controversial.

Hippos are a geologically young group, which raises questions about their origin

The oldest member of cetaceans dates back to the early Eocene (53 million years ago), whereas the oldest known hippopotamus dates back only to the Miocene (15 million years ago). Because of the 40 million year gap between cetaceans and hippopotamuses in the fossil record, doubts have arisen. Given the relatively good fossil fund rate of even-toed ungulates, it seems unlikely that there are no remains of the ancestors of hippos. Some studies declared the late emergence of hippos is that they are relatives of peccaries and split recently, but this seems unlikely because of the molecular findings. The focus of research is therefore focused on the Anthracotheriidae, one dating from the Eocene to Miocene which has already been described in their discovery in the 19th century as being "hippo-like". A study from 2005 showed that they have a very similar skull structure, but the hippos have a different tooth design. As a possible scenario was nevertheless believed that cetaceans and the Anthracotheriidae descended from a common ancestor, and the hippopotamusess were developed from the Anthracotheriidae. A study published in a 2015 study was able to confirm this, but also revealed that the hippopotamuses, not as it was suspected, were phylogenetically much more developed, but can be derived from more original members of Anthracotherien. [17][18] The newly introduced genus Epirigenys from eastern Africa is thus the sister group of hippos.

Morphological classification of Artiodactyla

Linnaeus postulated a close relationship between camels and ruminants. Henri de Blainville recognized the similar construction of the limbs of these animals between those of pigs and hippos, and the British zoologist Richard Owen coined the term "even-toed ungulates" and the scientific name Artiodactyla in 1848.

Since then, the composition of this group was clear and was hardly ever questioned. The construction of the internal systematics (stomach and the molars) served for classification. Suinas and hippopotamuses have molars that have well-developed roots which formed early, and a simple stomach which digests food directly without rehashing. Thus, they were grouped together as non-ruminants, or Porcine. All other even-toed ungulates have molars with a selenodont construction (crescent-shaped cusps) and have the ability to ruminate. Differences in stomach construction indicated that rumination evolved independently between tylopods and ruminants; therefore tylopods were excluded from Ruminantia.

From a purely morphological point of view, the suspected classifications, which were widely accepted by the end of the 20th century, were:[19]

Even-toed ungulates 
 Suina 

  Suoidea


 Hippopotamidae


 Selenodont 

 Tylopoda

 Ruminants 

 Tragulidae


 Pecora




Morphological classification of Cetacea

The mesonychid were long considered ancestors of whales

Modern cetaceans are highly accustomed sea creatures that phenotypically have little in common with other mammals - they're similar to other marine mammals such as seals and sea cows which occurred due to convergence. It stands to reason, however, that they must have evolved from terrestrial mammals. The most likely candidates for the ancestors of cetaceans were long thought to be mesonychids. These were large, carnivorous animals from the early Cenozoic (Paleocene and Eocene), who had hooves instead of claws on their feet. Their molars were adapted to a carnivorous diet, resembling the teeth in modern toothed whales, which are aligned for a fish based diet, and, unlike other mammals, have a uniform construction.

The suspected relations can be as follows:[20][21]

Paraxonia 

 Artiodactyla

 Cete 

 Mesonychia †


 Cetacea



Inner systematics

The molecular findings and morphological indications suggests that even-toed ungulates are paraphyletic to cetaceans and form a taxon Cetartiodactyla; the monophyly of Cetartiodactyla is well protected by a total of molecular and anatomical references. Shared modern nomenclatures divides Cetartiodactyla in five subordinate taxa that are also monophyletic: the camels (Tylopoda), the porcine (Suina), the ruminants (ruminant), the hippo (Ancodonta), and the whales (Cetacea).

The ruminants are likely to be more closely related to whales and hippos than with the other even-toed ungulates - this has so far only been explored by molecular biology but not investigated morphologically and is therefore controversial. The presumed lineages within Cetartiodactyla can be represented in the following cladogram:[22]

Cetartiodactyla 

Tylopoda (camels)

 Artiofabula 

  Suina pigs and allies

 Cetruminantia 
 Ruminantia (ruminants) 

 Tragulidae (mouse deer)


 Pecora (forehead weapon carriers)


 Cetancodonta/Whippomorpha 

 Hippopotamidae (hippopotamus)


 Cetacea (whales)





Camels are now considered a sister group of Artiofabula.
The pronghorn is the only extant member of Gabelhorn carriers

The pronghorn is the only extant representative of Gabelhorn carrier. The four summarized Cetartiodactyla taxa are divided into ten extant families:[23]

  • The camels (Tylopoda) comprise only one family, Camelidae. It is a species-poor group of animals that are well adapted to extreme habitats - the camels of deserts and the llamas, vicuna, and alpaca of high mountain regions.

The pigs (Suina) are made up of two families:

    • The Suina Suidae are limited to the old world. These include the wild boar and the domesticated form, the 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 (Giraffidae) are composed of two outwardly different species: the giraffe and the okapi.
    • The musk deer (Moschidae) is a kind of stag indigenous to East Asia.
    • The Gabelhorn carriers (Antilocapridae) comprise only one species: the pronghorn.
    • The deer (Cervidae) are made up of about 45 species, which are characterized by a pair of antlers, worn only the males generally. They're spread across Europe, among other species, the deer, red deer, the elk and reindeer.
  • The bovines (Bovidae) are the most species-rich. Among them are the cattle, the caprine, the gazelle-like and more than antelopes designated groups.
  • The Hippos (Hippopotamidae) comprise two types, the hippo and the pygmy hippo.
  • The whales (Cetacea) comprise 72 species and two parvorders: toothed whales (Odontoceti) and baleen whales (Mysticeti)

The largest systematic problem within the subordinate taxa regards the ruminants. It is generally accepted that the mouse deer is the sister group of the remaining five families, but the systematics is confusing within the forehead weapons carrier. Although deer, musk deer and Gabelhorn carriers, have traditionally been summarized as cervids (Cervioidea), molecular studies provide different - and inconsistent - results, so that the question of phylogenetic systematics of forehead weapons carrier, for the time being, cannot be answered.

Reconstruction of Indohyus

In December 2007, Hans Thewissen, professor at the Department of Anatomy of Northeastern Ohio Universities Colleges of Medicine and Pharmacy, hypothesized an alternative family tree. According to his studies, the next of kin of early whales were an extinct group called Raoellidae, and both taxa put together form the sister group of the remaining artiodactyl members, including the hippo.

His findings come from the study of a new skeleton found in the Kashmir in Pakistan. It was a member of the genus Indohyus, which is a member of the Raoellidae family. Mainly due to a bony ring on temporal bone (bulla), called the involucrum, which was previously associated only with cetaceans, as well as other features of the premolars and the bone structure, the close relationship was established.[24]

Outer systematics

Cetartiodactyla is classified within the higher mammals to the superiority of the Laurasiatheria, a mammalian group named after their putative origins in the extinct continent Laurasia. Which major groups within the Laurasiatheria and Cetartiodactyla are more closely related is still unresolved, however, the previously proposed set of ungulates is not a natural group. Various molecular studies can recognize different possible lineages: a taxon together with the perissodactyls and ferae (pangolins and carnivores), or a common taxon, Fereuungulata, but what remains unclear is whether the odd-toed ungulates are more closely related to Cetartiodactyla or ferae. Another recently drawn up hypothesis assigns Perissodactyla, ferae and bats to a taxon, pegasoferae, together and sees Cetartiodactyla as its sister group.[25]

Anatomy

Artiodactyls have similar characteristics between each other, particularly the construction of the limbs. One common feature (synapomorphies) is the special construction of the anklebone with two castors (trochlea tali proximalis and distalis). [16] However, the discovery of this special anklebone in archaeocetes has had many, in the 2001 discussion about systematics, rework their hunting methods.

The mouse deer is the smallest even-toed ungulate

Artiodactyls are generally medium to large quadrupeds. There are two major body types, which differ significantly in physical aspects. Suinas and hippopotamuses are characterized by a stocky body, short legs, and a large head. Camels and ruminants on the other hand have a more slender build and lanky legs. Size varies considerably, as the smallest member, the mouse deer, often reaches only 45 centimetres (18 in) in body length and a weight of 1.5 kilograms (3.3 lb), whereas the largest members, the hippopotamus, can grow up to 5 metres (16 ft) in length and 4.5 metric tons (5.0 short tons) in head weight, and the giraffe can grow to be 5.5 metres (18 ft) high and 4.7 metres (15 ft) in body length. All species display some form of sexual dimorphism: the males are consistently larger and heavier than the females; gender differences also exist in the forehead weapons, as in deer where usually only the males boast antlers, and the horns of bovines are usually small or not present in the females, and the coat of Blackbucks differ between the genders in that the males coat is much darker than that of the females.

Almost all members of this order have fur. One exception is the nearly hairless hippopotamus. Fur coverage 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

Even-toed ungulates bear their name because they have an even number of toes (two or four) - in some peccaries the 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 designed differently between species: in the hippos, they are directed forward and fully functional; for the other even-toed ungulates, they are arranged in reverse or completely reduced. For pigs and deer, the toes are still in contact with soft, muddy ground and increase the contact surface area. In most cases, however, they no longer touch the ground. In some groups, like the camels and giraffes, regression has progressed so far that the second and fifth toe are not even present.

  • Hippopotamuses have all four toes pointing outwards

  • For pigs and many 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. These claws are made of keratin, just like human nails, and consist of three parts: the plate (top and sides), the sole (bottom), and the bale (rear), which can, however, grow in varying degrees. In general, the claws of the forelegs are wider and blunter than the hind legs and the gape is farther apart. Aside from camels, all even-toed ungulates put just the tip of the foremost phalanx on the ground. In camels, the horn formations on the feet reduces the toes that rest on an elastic pad made of connective tissue, which forms a wide sole area. [26]

Hand skeletons of various mammals, left to right: orangutan, dog, pig, cow, tapir, horse

In even-toed ungulates, there is a tendency to coalescence metapodials (metacarpal and metatarsals). For Suinas and hippopotamuses, they are still isolated and coupled only by a taut connective tissue. 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 ensures a rigid position of the lower limbs. Rotational movement of the legs are nearly impossible, but the immobility causes a higher 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. The selection pressure to obtain high speeds with the flight of the specialized construction of the limbs increases the energy saved during slow movement, such as during food intake.

Head and teeth

Many even-toed ungulates have a relatively large head. The facial skeleton is elongated and rather narrow, and the nasal bones run forward in one or two points. The frontal bone is enlarged backwards and displaces the parietal bone, which forms only part of the side of the cranium, especially in ruminants.

Horns and antlers

Outgrowths of the frontal bone characterize most forehead weapons carriers, such as the Gemsbok

Four families of even-toed ungulates have end arms. The group of forehead weapons carriers, Pecora, also includes unarmed members such as the musk deer and water deer. The end weapons are mostly outgrowths of the frontal bone, which are built differently. The giraffes have bone pins, which are sheathed in hairy skin. Deer grow antlers with a peg-shaped bone structure which consists of bone and is shed each year after the breeding season. The horns of the bovines are usually maintained for their entire life. The skin that covers the bone pins separates horn cells which eventually forms a hard sheath. The oldest horn layers are continuously moved toward the horn tip. With the exception of domestic breeds, all bovines have two horns. The Gabelhorn have horns that are formed similarly to the bovines, except the horn sheaths are shed annually. The end weapons can serve for posturing, battling for mating privilege, and for defense. In almost all cases, they are sexually dimorphic, in that females lack them.

Teeth

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

The dentition of artiodactyls varies between species; it can, however, discern two trends. The Suina and hippopotamuses have a relatively large number of teeth (with some pigs containing 44). The dentition is more adapted to a squeezing mastication, which is the tendency for omnivores such as pigs. With camels and ruminants, the number of teeth is reduced; there is often a yawning diastema, a designated gap in the teeth where the molars are aligned on a mill for crushing plant matter.

The incisors are often reduced in ruminants, as they are completely absent in the upper jaw, and they instead press the lower incisors against a dental plate. The canines are pronounced differently: in the Suina, they are enlarged and tusk-like, and are used for digging in the ground and for defense; in ruminants, the upper canine in males of species without end arms (mouse deer, musk deer, water deer...) is enlarged and used as a weapon in the battle for mating privileges; species with frontal weapons, usually, are missing the upper canine. The lower canines of ruminants resemble the incisors, so that these animals have eight uniform teeth in the front part of the lower jaw.

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

Senses

Their environment primarily serves a sense of smell, which is very well developed in artiodactyls, as with most mammals. Their sense of hearing is also well-developed. Unlike many other mammals, they have a poor sense of sight which is least pronounced in ruminants and camels. Above all, motion perception is developed, and stationary objects are not well perceived. Similar to many other animals, which must be constantly mindful of predators that are eye laterally attached to the head, they have an almost complete panoramic view which helps with the earliest possible detection of threats.

Digestive system

Pigs have a simple sack-shaped stomach, unlike the other artiodactyls
Deer have such a multi-chambered stomach as with all ruminants, which is used for better utilization of the plant food

Pigs have a simple sack-shaped stomach, unlike other even-toed ungulates.[27]

Deer have such a multi-chambered stomach, as with all ruminants, which is used for better utilization of the plant food. As an adaptation to the indigestible plant food, artiodactyls have some peculiarities of the digestive tract developed which are especially strong in ruminants. The mouth often has additional salivary glands and the oral mucosa is often heavily callused to avoid injury from hard plant parts and to allow easier transport of roughly chewed food.

The stomach of ruminants is divided into three to four sections: the rumen, the reticulum, the omasum and the abomasum.[27] Rumination occurs in the ruminants (Ruminantia and Tylopoda), whereby food is regurgitated and rechewed then broken down by microbes in the stomach. After ingestion of plant material, it is mixed with saliva in the rumen and reticulum and separates into layers of solid and liquid material. The solids lump together to form a bolus (also known as the cud), this is regurgitated by reticular contractions while the glottis is closed. When the bolus enters the mouth, the fluid is squeezed out with the tongue and reswallowed. The bolus is chewed slowly to completely mix it with saliva and to break down the particle size. Ingested food passes to the 'fermentation chamber' (rumen and reticulum) where it is kept in continual motion by rhythmic contractions of this organ. Cellulytic microbes (bacteria, protozoa, and fungi) produce cellulase, which is needed to break down the cellulose found in plant material. Without this mutual symbiosis, ruminants would find plant material indigestible.[27]This form of digestion has two advantages: the indigestible plants are best digested and utilized, and the duration of the actual food consumption is - especially with the unfavorable perception of the environmental posture with the head close to the ground - shortened, which is, in view of the threat of predators, an advantage; rumination can take place in sheltered places.[28]

Tylopoda (camels, llamas, and alpacas) and chevrotains have three-chambered stomachs, while the rest of Ruminantia have four-chambered stomachs. The handicap of a heavy digestive system has increased selective pressure for limb bone adaptations to escape predators.[29] Most species within Suina have a simple two-chambered stomach that allows an omnivorous diet, the babirusa, however, is a herbivore.[27] They have extra maxillary teeth to allow for the proper mastication of plant material. Most of the fermentation occurs with the help of cellulolytic microorganisms within the caecum. Peccaries, however, have a complex stomach that contains four compartments.[28] Microbial fermentation with the formation of high volatile fatty acid levels has been observed in the fore stomach; it has been proposed that their complex fore stomach is a means to slow digestive passage and increase digestive efficiency.[28] Hippopotamuses have three-chambered stomachs and do not ruminate. They consume around 68 kg of grass and other plant matter each night. They may cover large distances (up to 20 miles) to obtain their food, which they digest with the help of microbes that produce cellulase. Their closest living relatives, the whales, are obligate carnivores.

Genitourinary system

The Japanese serow has glands in the eyes that are clearly visible

The construction of the urinary and sexual organs of even-toed ungulates also shows some peculiarities. The penis is curved S-shape at rest and rests in a pocket under the skin on the belly. The corpora cavernosa is developed only slightly and the erection causes mainly an extension of this curvature and thus an extension, but a thickening, of the penis. This construction of the penis is found in a similar manner with cetaceans. The testicles are located in the scrotum and thus outside the abdominal cavity. The ovaries of many females make a descent (prolapse ovary) - comparable to the testicle descent 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).[30]

The number of mammary glands is variable and correlates, as in all mammals, with the litter size. In pigs, there are two rows of teats are lined from the armpit to the groin area. This is due to their large litter size, the biggest out of all even-toed ungulates. In most cases, however, due to a reduction of the number of young, even-toed ungulates have only one or two pairs of teats. These form, in some species, an udder in the groin region.

Secretory glands in the skin are present in virtually all types and can be located in different places, as in the eyes, behind the horns, the neck or back, on the feet, or in the anal region.

Distribution and habitat

Artiodactyls are native to almost all parts of the world, with the exception of Oceania - Australia, New Zealand and the many remote pacific islands - with pigs being feral, and Antarctica. Today, the latter inhabit Africa and Asia. In the Americas, they are relatively species-poor, and are found only in South America, where only peccaries, llamas, vicunas, and capreolinae occur. Humans have introduced different artiodactyls worldwide as hunting animals among others, for example, pigs, as these animals are found almost everywhere today, where there are, or were, people. [31]

Artiodactyls inhabit almost every habitat, from tropical rain forests and steppes to the desert areas and high mountain regions and to the oceans (Cetacea). The greatest biodiversity prevails in open habitats such as grasslands and open forests. These animals are marked bottom dwellers, and only a few species lead a semi-aquatic, e.g., hippos. Some species have colonized the high mountains and can climb excellently.

Lifestyle

Social behavior

Artiodactyls, like impalas and giraffes, live in groups

Artiodactyls often live in groups. The social behavior of even-toed ungulates varies from species to species. Generally, however, there is a tendency to merge into larger groups, but there are also solitary or in pairs. Species living in groups, often have a hierarchy, both among males and females. But some species also live in harem groups, which means that a single male gathers several females and their common offspring around and no other rivals are tolerated. In other species the females and juveniles make up for the greater part of the year on their own groups, while males are solitary or live in bachelor groups and seek the females groups only during the mating season. Many occur during mating season for fierce fighting around the mating privilege between the males, which are discharged to the end weapons.

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 take seasonal migrations in search of better grazing places.

There are diurnal, crepuscular, and nocturnal members as well as species associated with the day-night scheme which may vary depending on the season or habitat.

Reproduction and life expectancy

Many artiodactyls, like wildebeest, are born with hair

Generally, even-toed ungulates have a tendency to long gestation period, smaller litter size and high level of development of the newborn. As with many other mammals, species in temperate or polar regions have a fixed mating season, while breeding can take place year-round in more tropical areas. They have a polygynous mating behavior, meaning a male so often pairs with several females, and competition will not be tolerated. The copulation is usually done by the mammals typical "ride up". In camels exclusively it is performed while lying down.

The length of the gestation period varies 4–5 months for porcine, deer, and musk deer, 6 to 10 months with hippos, deer and bovines, 10 to 13 months with camels, and 14 to 15 months with giraffes. Most deliver one or two babies, but some pigs can deliver up to ten.

The newborns are precocial and come with open eyes and, with the exception of generally hairless hippos, hairy to the world. In deer and pigs, the striped or spotted coat in juveniles disappear as they grow. The juveniles of some species spend their first weeks with their mother in a safe storage location, where others may soon run after birth and follow the herd within a few hours or days.

The life expectancy is typically 20 to 30 years, as in many mammals smaller species often have a shorter lifespan than larger species. The oldest animals like hippos, cows and camels, can reach 40 to 50 years.

Predators and parasites

Artiodactyls have different natural predators depending on their size and habitat. There are several carnivores who would prey on such mammals such as cats, dogs or bears. Other predators are, for example, crocodiles, large raptors and, for small species and young animals, giant snakes.

Parasitic infection can come around from nematodes, botflies, fleas, louse or flukes, but they have debilitating effects only when the infestation is severe.

Interactions with humans

Domestication

Some artiodactyls, like sheep, have been domesticated for thousands of years

Artiodactyls have been hunted by primitive humans for various reasons: for meat, to process their fur into clothing, and to use their forehead weapons, bones and teeth as weapons or tools. The domestication of animals began around the 8,000 B.C.E. To date, humans have domesticated goats, sheep, cattle and pigs. Initially, livestock was used primarily as food suppliers, but then later, around 3,000, the animals were used for work activities.[29] Clear evidence exists of antelope being used for food 2 million years ago in the Olduvai Gorge, part of the Great Rift Valley.[29][32] 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. By around 12,500 years ago, reindeer remains accounted for 94% of bones and teeth found in a cave above the Céou River.[33]

Today, artiodactyls are kept for various reasons. These are primarily for their meat, their milk, and the processing of their wool, fur, or hide (leather) into clothing. They are also used for work, to ride, or are used as pack animals, such as the domestic cattle, the water buffalo, the yak, or camels. Cattle are the basis of a multibillion-dollar industry worldwide. The international trade in beef in 2000 was over US$30 billion and represented only 23% of world beef production.[34]

Two basic types of domestication were used. On one hand, the animals in different breeds were bred, which are spread throughout the world and partly differ significantly from the wild species, as with domestic cattle, domestic pigs, domestic goats and domestic sheep. Other pets have largely remained in their region of origin and, when compared with the wild species, changed little, like the reindeer, some cows (like the water buffalo, the banteng, the gaur, and yak), and camels (like the dromedary, the llama, and alpaca).[29]

Some are not only used for food assistance, but also for sport. These practices are conducted under partly severe criticism and have driven some species, such as the Alpine ibex and the Arabian Oryx, to the brink of extinction.

Threats

Aurochs has been wiped out since the 17th century

The hazard level of each even-toed ungulate is different. Some species are synanthropic (such as the wild boar) and can spread their range or have been brought by humans as farm animals or runaway pets in regions where they were not indigenous to. Some artiodactyls also benefit from the fact that their predators were severely decimated as competitors of the ranchers, such as Thylacine.[31]

Conversely, many artiodactyls have declined significantly in their population, and some were even eradicated. The reasons for this is over-hunting, and, more recently though, increasing habitat destruction. Currently extinct are several gazelle species (such as the Arabian gazelle), the Malagasy hippopotamus, the Blue Buck, and the Schomburgk's Deer. Two species, the Arabian Oryx and the Saudi Gazelle, are considered by the IUCN as being extinct in the wild. 14 species are considered Critically Endangered, including the Addax, the Kouprey, the red deer, Przewalski's gazelle, the Saiga, and the pygmy hog. Twenty four species are considered endangered.[35][36]

Evolution

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See also: Evolution of cetaceans

The oldest fossils of even-toed ungulates date back to the early Eocene (about 55 million years ago). Since these findings almost simultaneously appeared in Europe, Asia and North America, it is very difficult to accurately determine the origin of artiodactyls. These ancient creatures are classified in the family Dichobunidae - their best-known and best-preserved member is Diacodexis. [37] These were small animals, with some as small as a hare, with a slim build, lanky legs, and a long tail. The hind legs were much longer than the front legs. The early to middle Eocene saw the emergence of the ancestors of most of today's mammals. The phylogeny of even-toed ungulates proceeded relatively uniformly and unspectacularly, giant or bizarre animals (such as perissodactyls), are hardly known in this group.[2]

The earliest cetaceans evolved in the early Eocene (49 million years ago) and have believed to have developed in the Indian subcontinent. From then on, they separated from the other even-toed ungulates, due to the aquatic life, into their own lineage.[38]

Entelodonts were stocky animals with a large head, and were characterized by bony bumps on the lower jaw

Two formerly widespread, but now extinct, groups of even-toed ungulates were Entelodont and Anthracotheriidae. The Entelodont 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. Anthracotheriidae had a large, porcine 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. Anthracotheriidae are thought to be the ancestors of hippos, and, likewise, they probably led a similar aquatic lifestyle. The hippos even appeared in the Late Miocene and occupied Africa and Asia - they never got to the Americas.[2]

The camels (Tylopoda) were, during large parts of the Cenozoic, limited to North America; early forms like Cainotheriidae occupied Europe. Among the North American camels included groups like the stocky, short-legged Merycoidodontidae which was equipped with frontal weapons. 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 America into Eurasia. North American variety went extinct around 10,000 years ago.

Members of the Suina have been around since the Eocene. In the late Eocene or Oligocene, the two still existing families remained restricted to Eurasia and Africa, and the peccaries, which became extinct in the Old World, exist today only in America.

Sivatherium was a group of giraffes with deer-like forehead weapons

Sivatherium were a group consisting of the giraffes with deer-like forehead weapons, and were the predecessors of giraffes and bovines. The ruminants were designed with many early forms. These early forms, which are classified as Tragulina, lived up to the Miocene in Africa, Eurasia and North America, and then died out, except for the mouse deer. In the Oligocene or Miocene, the other members, with the majority remained confined to the Old World, developed. The giraffes (Giraffidae), emerging alongside the existing species, developed the stag-like end weapons. The deer (Cervidae) developed numerous species, but remained limited to Eurasia. Bovines and deer arrived relatively late in North America, however, the group with Gabelhorn carriers spread with numerous species, of which today only one species, the pronghorn, survived.[2]

South America was settled by the even-toed ungulates only in Pliocene some three million years ago when the land bridge at the Isthmus of Panama formed. With peccaries, llamas and deceit deer, the South American artiodactyls, compared to the other continents, has remained poor in species.

References

  1. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  2. ↑ 2.0 2.1 2.2 2.3 Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  4. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  5. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  6. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  7. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  8. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  9. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  10. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  11. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  12. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  13. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  14. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  15. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  16. ↑ 16.0 16.1 Lua error in package.lua at line 80: module 'strict' not found.
  17. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  18. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  19. ↑ etwa noch bei Nowak (1999) oder Hendrichs (2004)
  20. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  21. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  22. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  23. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  24. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  25. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  26. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  27. ↑ 27.0 27.1 27.2 27.3 Lua error in package.lua at line 80: module 'strict' not found.
  28. ↑ 28.0 28.1 28.2 Lua error in package.lua at line 80: module 'strict' not found.
  29. ↑ 29.0 29.1 29.2 29.3 Lua error in package.lua at line 80: module 'strict' not found.
  30. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  31. ↑ 31.0 31.1 Lua error in package.lua at line 80: module 'strict' not found.
  32. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  33. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  34. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  35. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  36. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  37. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  38. ↑ Lua error in package.lua at line 80: module 'strict' not found.

External links

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