Artiodactyls are an important food source for a number of different carnivores. As artiodactyl populations decline, so too will those animals that depend on them. For example, the decline of cheetahs is often attributed habitat loss. However, cheetahs primarily prey upon small to medium sized ungulates, specifically gazelles. According to the IUCN Red List of Threatened Species, 2 species of gazelle are extinct, while 10 more are listed as vulnerable, endangered or critically endangered. In north Africa, as preferred prey species have declined, more and more cheetahs are turning to livestock for prey. Consequently, these cheetahs are then killed as pests. As a result, one of the major directives for cheetah conservation is restoration of wild prey species, most of which are small to medium-sized artiodactyls.
Artiodactyls are an important food source for many large mammalian carnivores, notably felids, canids, and ursids. Cetaceans, on the other hand, have few natural predators, save other cetaceans (killer whales, Orcinus orca), sharks, and occasionally walruses (Odobenus rosmarus). Group living ("safety in numbers") and camouflage are two defenses often employed by cetartiodactyls against predation.
Known Predators:
Anti-predator Adaptations: cryptic
Because cetaceans are so highly specialized for their aquatic lifestyle, they bear little resemblance to their artiodactyl ancestors. They have nearly hairless, fusiform bodies. They lack hind limbs except for tiny internal pelvic vestiges, and the forelimbs are modified into streamlined flippers. The tail bears a flattened fluke. In addition, cetacean skulls are highly modified so that the nares are located on the top of the head. On the other hand, most artiodactyls are specialized for cursorial locomotion, with long, hoofed limbs, and they lack the extreme aquatic specializations found in cetaceans. Most cetartiodactyls are relatively large animals, but there is an enormous range of body sizes in this group. Blue whales, Balaenoptera musculus, are the largest animals on earth, growing over 27 meters in length and weighing over 190,000 kg, whereas the smallest artiodactyl, the lesser mouse deer (Tragulus javanicus), is just 45 cm long and weighs 2 kg. Many species of cetartiodactyls exhibit sexual dimorphism, with males larger than females or vice versa. Also, male artiodactyls often bear antlers or large horns, and some male cetartiodactyls (narwhals (Monodon monoceros), tragulids, and suids) bear large tusks.
Other Physical Features: endothermic ; homoiothermic; bilateral symmetry
Sexual Dimorphism: sexes alike; female larger; male larger; ornamentation
Cetartiodactyls are relatively long-lived mammals. Most species live for at least a decade in the wild, and captivity usually prolongs life expectancy by several years. Cetaceans are especially long-lived; 116-year-old fin whales (Balaenoptera physalus) have been reported from the wild and bowhead whales (Balaena mysticetus) may live up to 200 years.
Most artiodactyls live entirely on land and reside in a range of terrestrial habitats, such as savannah, forest, mountains, desert, and farmland. One artiodactyl family, Hippopotamidae, is semi-aquatic and can be found in freshwater lakes, ponds, streams, and rivers. Cetaceans, on the other hand, are exclusively aquatic and inhabit the world's oceans, as well as some freshwater rivers and streams.
Habitat Regions: temperate ; tropical ; polar ; terrestrial ; saltwater or marine ; freshwater
Terrestrial Biomes: tundra ; taiga ; desert or dune ; savanna or grassland ; chaparral ; forest ; rainforest ; scrub forest ; mountains
Aquatic Biomes: pelagic ; lakes and ponds; rivers and streams; coastal
Other Habitat Features: agricultural ; riparian ; estuarine
Cetartiodactyls are found the world over, from north of the Arctic Circle to the waters surrounding Antarctica. Artiodactyls are native to all continents except Antarctica and Australia, and some artiodactyls are domesticated and have been introduced around the world by humans. Cetaceans inhabit all of the world's oceans and some freshwater lakes and rivers in South America, North America, and Asia.
Biogeographic Regions: nearctic (Introduced , Native ); palearctic (Introduced , Native ); oriental (Introduced , Native ); ethiopian (Introduced , Native ); neotropical (Introduced , Native ); australian (Introduced ); arctic ocean (Native ); indian ocean (Native ); atlantic ocean (Native ); pacific ocean (Native ); mediterranean sea (Native )
Other Geographic Terms: holarctic ; cosmopolitan
As a group, cetartiodactyls consume a wide array of terrestrial and aquatic food items. Most artiodactyls are herbivores, consuming grass, leaves, bark, and other plant parts. Those in the family Suidae are omnivorous, and eat small mammals, reptiles, amphibians, insects and other invertebrates, fruit, bulbs, rhizomes, fungi, carrion, and bird eggs. Cetaceans consume plankton, fish, squid, crustaceans, and aquatic birds and mammals (including other cetaceans).
Foraging Behavior: filter-feeding
Primary Diet: carnivore (Eats terrestrial vertebrates, Piscivore , Eats eggs, Insectivore , Eats non-insect arthropods, Molluscivore , Scavenger ); herbivore (Folivore , Frugivore , Lignivore); omnivore ; planktivore ; mycophage
Cetartiodactyls are primary, secondary, and higher-level consumers, filling roles of predator (most cetaceans) and prey (most artiodactyls). Terrestrial cetartiodactyls are plagued by ectoparasites such as fleas, lice, and bot flies. Cetaceans, though aquatic, are not free from external parasites either, and are host to barnacles, copepods, and whale lice. Both terrestrial and aquatic species host internal parasites as well, such as tapeworms, flukes, and nematodes. Interestingly, birds have evolved commensal relationships with both aquatic and terrestrial cetartiodactyls. Seagulls follow schools of dolphins and consume small fish stirred up by the cetaceans, and cowbirds follow herds of cattle and consume insects stirred up by the hooves of the artiodactyls. Also, some cetartiodactyl species are mutualists with animals that feed on their ectoparasites: topsmelt (Atherinops affinis) consume whale lice that live on the skin of gray whales (Eschrichtius robustus), while oxpeckers (Buphagus) remove fleas and other parasites from the skin of various African artiodactyls.
Mutualist Species:
Commensal/Parasitic Species:
Cetartiodactyls are of immense economic importance to humans. They have been hunted for thousands of years, for food, for sport, and for various body parts. They are important for ecotourism, be it a whale-watching boat off the coast of Maine or a safari on the African savannah. Several artiodactyl species have been domesticated for thousands of years and are used to produce meat, milk, leather and wool, and their dung is used as a fertilizer. Cetaceans are sometimes kept in captivity and taught to perform tricks. Many cetartiodactyls are the focus of research programs that help us to better understand many aspects of evolution, physiology, and behavior.
Positive Impacts: food ; body parts are source of valuable material; ecotourism ; research and education; produces fertilizer
Artiodactyls, especially domesticated species, sometimes carry diseases that are transmissible to humans or other domestic animals. Wild artiodactyls sometimes interfere with farming operations by eating crops.
Negative Impacts: injures humans (carries human disease); crop pest; causes or carries domestic animal disease
Cetartiodactyla is a group comprised of two orders of mammals that are superficially quite different and that, until recently, were recognized as two separate monophyletic clades. These orders are Artiodactyla, even-toed ungulates, including animals such as cows (Bovidae), camels (Camelidae), and deer (Cervidae), and Cetacea, a group of mammals that are highly specialized for an aquatic lifestyle, including baleen whales and toothed whales. Recent molecular evidence suggests that Cetacea evolved from artiodactyl ancestors, making Artiodactyla non-monophyletic unless Cetacea is included. Experts suggest the monophyletic clade representing artiodactyls and cetaceans be called Cetartiodactyla.
Currently, the IUCN classifies 54 cetartiodactyl species as data deficient, 146 as lower risk, 40 as vulnerable, 32 as endangered, 14 as critically endangered, 7 as extinct, and 2 as extinct in the wild. The biggest threats to many terrestrial species are habitat loss and fragmentation and overhunting. Several large cetacean species were hunted nearly to extinction until an international treaty banned commercial whaling in the 1980s. Populations of many large, commercially-valuable whales remain severely depleted. Today, cetaceans face threats associated with global climate change, which could have widespread impacts on their food supply in the near future.
Cetartiodactyls perceive the world through visual, tactile, auditory, and chemical means. Some cetaceans, the Odontoceti, navigate and hunt using echolocation. Terrestrial cetartiodactyls often communicate with scent; many species, such as those in the family Cervidae, have specialized glands for doing so. Communication in cetaceans is accomplished largely by sound, as sound waves travel well in water. Low frequency sounds produced by baleen whales (Mysticeti) may travel for hundreds of kilometers, allowing individuals to communicate with one another over great distances.
Communication Channels: visual ; tactile ; acoustic ; chemical
Other Communication Modes: scent marks
Perception Channels: visual ; tactile ; acoustic ; echolocation ; chemical
Humans hunt artiodactyls for their meat and skins, and as trophies. In the wild, felids and canids are the main predators of artiodactyls. With the exception of humans, felids, and canids, large artiodactyls have few predators. However, juveniles are highly vulnerable and are often targeted by smaller predators. Due to an inability to escape enclosures, livestock are vulnerable to predation and are often targeted by predators during periods of scarcity.
Many artiodactyls have some form of ornamentation, and although ornamentation is used primarily during conspecific interactions, horns, antlers, and tusks are also used during predator defense. They also use their powerful legs and sharp hooves to defend against predators. Frequently, artiodactyls use their speed to outrun predators and their sharp senses of smell, sight, and hearing detect potential threats. They often live in groups for protection and make themselves appear larger through piloerection or laterally positioning relative to predators. During a predation event, gregarious artiodactyls may stand in defensive formations that help decrease individual and group vulnerability. For example, musk oxen stand adjacent to one another in head to tail formation or in a circular formation when approached by a predator. Predators most often target old, juvenile, or sick individuals. In conjunction with feeding behavior, predation pressure has lead to important morphological adaptations resulting in cursorial, unguligrade locomotion.
Known Predators:
Anti-predator Adaptations: cryptic
In artiodactyls, the structure of the foot is especially diagnostic, specifically the number of toes and the morphology of the astragalus. Most species have either 2 or 4 toes on each foot (for exceptions see Pecari and Tayassu) as the first digit, present in most ancestral mammals, has been lost through evolution and the second and fifth digits have been significantly reduced. As a result, artiodactyls are paraxonic. The unique structure of the astragalus, which consists of a "double-pulley" arrangement of the articular surfaces, completely restricts lateral motion and allows for greater flexion and extension of the hind limb. The astragalus, in conjunction with springing ligaments in the limbs, hard hooves, relatively small feet, and elongated lightweight limbs, allows for highly developed cursorial locomotion in more derived species. In the families Camelidae, Cervidae, Giraffidae, Antilocapridae, and Bovidae, the third and fourth metapodials have become fused to create the cannon bone, which serves as the insertion point for the springing ligament in each of the four limbs. Throughout all of Artiodactyla, the range of fusion between the third and fourth metapodials varies from none to complete. Finally, residents of sandy or snowy habitats often have splayed toes, which distributes an individual's weight over a greater surface area, thereby decreasing movement costs in more fluid terrestrial substrates.
Although exceptions exist (pigs and peccaries), the vast majority of artiodactyls are obligate herbivores, consisting of browsers, grazers and mixed feeders. Although plants provide an abundant and diverse food source, mammals do not possess the enzymes necessary to break down cellulose or lignin. As a result, most artiodactyls rely on microorganisms to help break down these plant compounds. In addition to their true stomach, all artiodactyls have at least one additional chamber in which bacterial fermentation occurs. This chamber, or "false stomach", is located just before the true stomach along the gastrointestinal tract. Cervids and bovids have three false stomachs, hippos, camels, and tragulids have two, while pigs and peccaries have only one small chamber.
A majority of artiodactyls having selenodont cheek teeth, however, many species also exhibit lophodont tooth morphology. In general, browsers tend to have brachydont teeth (i.e., low crowned) while grazers have hypsodont teeth (i.e., high crowned). Within Artiodactyla, the families Suidae (pigs) and Tayassuidae (peccaries) are omnivores and have quadrate, bunodont teeth. Often, a diastema occurs between the canine and first premolar, which is especially prevalent in the lower jaw. Bovidae, Cervidae, and Giraffidae have lost their upper incisors, and several groups have lost their upper canines. However, many have retained their incisors (pigs, peccaries, hippos, and camels) and some have developed them as weapons or indicators of mate quality (some suids, cervids and musk deer). While most families have incisiform lower canines, pigs, peccaries, hippos, and camels have conically shaped canines.
Artiodactyls exhibit a great deal of variation in physical appearance. Body mass ranges from 4000 kg in hippos to 2 kg in lesser Malay mouse deer. Height ranges from 5 m in giraffes to 23 cm in lesser Malay mouse deer. Most artiodactyls have laterally positioned eyes, often with long eyelashes. They commonly have rotating ears that are round or pointed at the tips and are relatively large in relation to skull size. Most artiodactyls also have elongated and powerful legs. Many families have horns, antlers, or tusks. Horns, always consisting of bone or having a bony core, are common in many families and most often stem from the frontals which are usually larger than the parietals. Similar to horns, antlers arise from the base of the frontals and are entirely bony. Unlike horns, however, antlers are deciduous and used during the breeding season. Horns and antlers are often used in ritualized social interactions, such as male-male competition within species.
The pelage of artiodactyls typically consists of guard hair and under fur, which together help control heat exchange. Under fur tends to be short and fine and is efficient at trapping heat. Guard hairs are longer and more stout than underfur and act as a barrier against wind, rain, and snow. Pelage color varies from black to white with many shades of brown. Color patterns within the pelage vary from spots to stripes, while most young have distinctly different coats than adults. In some species, males have a ventral ridge of long hairs referred to as a ruff or dewlap and male coat color is often linked to age or social status. Species living in temperate and arctic regions shed their winter coats on a seasonal basis.
Other Physical Features: endothermic ; homoiothermic; bilateral symmetry
Sexual Dimorphism: sexes alike; female larger; male larger; sexes shaped differently; ornamentation
The lifespan of artiodactyls ranges from 8 to 40 years. Numerous studies have shown that adult male survival is lower and more variable over time than female survival. Sex-biased mortality in artiodactyls is most often attributed to sexual selection and evidence suggests a positive correlation between size-biased mortality rates and the degree of sexual dimorphism, with the larger sex exhibiting higher mortality rates (for exceptions see alpine ibex and mouflon). The correlation between mortality rates and size-dimorphism is thought to be the result of increased polygyny, resulting in increased male-male competition. It has also been hypothesized that the larger sex in sexual-size dimorphic species have higher absolute energy requirements and therefore are more susceptible to starvation. Studies also show that senescence induced mortality begins around age eight for some artiodactyl species, regardless of sex.
Artiodactyls are exceptionally diverse and globally distributed. Consequently, they inhabit a broad range of habitat types and can be found anywhere sufficient forage exists. Although artiodactyls occur from deserts to tropical forests to tundra, preferred habitat types fall into four major categories, which are linked to forage abundance and predator defense. Open grasslands provide abundant forage while allowing for early detection of approaching predators. Grasslands or meadows near steep cliffs provide forage while offering safety from potential predators in adjacent rocky ledges and steep terrain. Forests and shrublands provide abundant forage while offering cover from potential predators in dense vegetation. Finally, many species inhabit the ecotone between open areas and forests. While open areas provide abundant forage, adjacent forests provide dense cover from potential predators. Habitat-use patterns in artiodactyls are often linked with body size and taxonomy, with small to medium-sized artiodactyls found mainly in habitats with tall, dense vegetation. Most goat and sheep species (Caprinae) are found in open habitats adjacent to rocky cliffs, where they are specialized for navigating uneven terrain.
Habitat Regions: temperate ; tropical ; polar ; terrestrial
Terrestrial Biomes: tundra ; taiga ; desert or dune ; savanna or grassland ; chaparral ; forest ; rainforest ; scrub forest ; mountains
Wetlands: marsh ; swamp ; bog
Other Habitat Features: urban ; suburban ; agricultural ; riparian
Artiodactyls are distributed nearly worldwide and are native to all continents except Antarctica and Australia. Numerous introductions, consisting mainly of domestic species, have occurred in areas outside their normal range. Where introduced in areas with suitable forage, artiodactyls usually thrive.
Biogeographic Regions: nearctic (Introduced , Native ); palearctic (Introduced , Native ); oriental (Introduced , Native ); ethiopian (Introduced , Native ); neotropical (Introduced , Native ); australian (Introduced ); oceanic islands (Introduced )
Other Geographic Terms: cosmopolitan
With the exception of the suborder Suinae, artiodactyls are obligate herbivores. Typical forage includes grass, leaves, fruits, flowers, twigs, aquatic vegetation, roots, and nuts. In Suidae and Tayassuidae, diets may also include insect larvae, grubs, and eggs. Although obligate herbivores, some species of artiodactyls are opportunistic feeders (e.g., deer and giraffes), occasionally feeding on carrion. Artiodactyls with low quality diets (i.e., high fiber and low protein) are forced to compensate by ingesting large amounts of forage, chewing their cud (i.e., ruminating), and devoting a majority of their time to feeding. In addition, because mammals do not possess the enzymes needed to digest cellulose and lignin, most artiodactyls depend upon bacterial fermentation to break down these compounds.
In addition to the true stomach, or abomasum, all artiodactyls have at least one additional chamber, or false stomach, in which bacterial fermentation takes place. In the suborder Ruminantia, the digestion of poor-quality food occurs via four different pathways. First, gastric fermentation extracts lipids, proteins, and carbohydrates, which are then absorbed and distributed throughout the body via the intestines. Second, large undigested food particles form into a bolus, or ball of cud, which is regurgitated and re-chewed to help break down the cell wall of ingested plant material. Third, cellulose digestion via bacterial fermentation results in high nitrogen microbes that are occasionally flushed into the intestine and are subsequently digested by their host. These high-nitrogen microbes serve as an important protein source for many artiodactyls, especially ruminants. Finally, ruminants can store large amounts of forage in their stomachs for later digestion. All ruminants chew their cud, have three or four-chambered stomachs, and support microorganisms that breakdown cellulose.
Within the order Artiodactyla, only the suborder Suiformes is considered omnivorous. However, many species diverge from this broad classification and are considered specialized herbivores. For example, babirusas (Babyrousa babyrussa), giant forest hogs (Hylochoerus meinertzhageni), and warthogs (Phacochoerus aethiopicus) are all considered specialized herbivores. In general, suids have large heads and snouts that are used to root for food. Suidae is the most omnivorous of the three extant Suiformes families, and when given the opportunity, kill and eat small animals including rodents, snakes, and bird eggs and nestlings. Although the family Tayassuidae (i.e., javelinas and peccaries) is considered omnivorous, evidence suggests that javelinas and peccaries rely more heavily on plants than suids. Similar to suids, most tayassuids have large heads and mobile snouts that are used while rooting for food. The two species that comprise the family Hippopotamidae, Hippopotamous amphibius and Hexaprotodon liberiensis, are more specialized herbivores than either sister family. Hippopotamous amphibius individuals forage primarily on grass, while H. liberiensis also consumes leaves and fruit. Suidae and Tayassuidae have one false stomach and Hippopotamidae has two.
Species in the suborder Tylopoda are extensively specialized for dry arid habitats. As such, they can easily digest plants with high salt content (i.e., halophytes) that other artiodactyls find intolerable. Camelids are ruminating grazers and can survive in habitats with sparse vegetation. They have two false stomachs and a short, simple cecum.
Foraging Behavior: stores or caches food
Primary Diet: herbivore (Folivore , Frugivore , Granivore , Lignivore); omnivore
Artiodactyls play an integral role in the structure and function of the ecosystems in which they reside and many species have been shown to alter the density and composition of local plant communities. For example, on Isle Royale National Park, moose (Alces alces) have been shown to alter the density and composition of foraged aquatic plant communities and as a result, fecal nitrogen transferred from aquatic to terrestrial habitats via the ingestion of aquatic macrophytes increases terrestrial nitrogen availability in summer core areas. Foraging by artiodactyls has been shown to have a significant impact on plant succession and plant diversity is greater in areas subjected to foraging. As a result, foraging by artiodactyls might lead to shifts from one plant community type to another (e.g., hardwoods to conifers). In addition, moderate levels of foraging by artiodactyls may increase habitat suitability for conspecifics. For example, litter from browsed plants decomposes more quickly those not subject to browsing, thus increasing nutrient availability to the surrounding plant community. Moreover, nutrient inputs from urine and feces have been shown to contribute to longer stem growth and larger leaves in the surrounding plant community, which are preferred during foraging bouts. Finally, research has shown that the decomposition of large artiodactyl carcasses can result in elevated soil macronutrients and leaf nitrogen for a minimum of two years.
Artiodactyls are host to a diverse array of endo and ectoparasites. Many species of parasitic flatworms (Cestoda and Trematoda) and roundworms (Nematoda) spend at least part of their lifecycle in the tissues of artiodactyl hosts. Artiodactyls are also vulnerable to various forms of parasitic arthropods including ticks (Ixodoidea), lice (Phthiraptera), mites (Psoroptes and Sarcoptes), keds (Hippoboscidae), fleas (Siphonaptera), mosquitoes (Culicidae), and flies (Diptera). Artiodactyls also host various forms parasitic protozoa, including trypanosomatids, coccidians, piroplasmids, and numerous species of Giardia. In addition, various forms of bacterial and viral pathogens play an important role in artiodactyl health and population dynamics. For example, Brucella abortus, the bacteria that causes brucellosis, affects many artiodactyls and rhinderpest, also known as cattle plague, is a highly contagious viral disease caused by paramyxovirus (Morbillivirus) that is especially prevalent in ruminants. Unfortunately, evidence suggests that recent climate change is altering host-parasite dynamics across the globe, increasing transmission rates between populations of conspecifics and hybridization rates between host specific parasite forms.
Although artiodactyls can serve as host to numerous species of pathogenic bacteria and protozoa, in conjunction with anaerobic fungi, these organisms are one of the major reasons that artiodactyls are as abundant and diverse as they are today. Bacteria comprise between 60 and 90% of the microbial community present in the ruminant's gastrointestinal (GI) tract and help break down cellulose. Ciliated protozoa, which makes up 10 to 40% of the microbe community within the rumen, help bacteria break down cellulose, while also feeding on starches, proteins and bacteria. The presence of anaerobic fungi in the rumen has only been known since the early 1970's. These fungi make up between 5 to 10% of the rumen's microbial abundance and are thought to help break down the cell wall of ingested plant material. Bacteria and protozoa that pass from the upper to the lower regions of the GI tract represent a significant portion of the dietary nitrogen required by their host.
Ecosystem Impact: disperses seeds; pollinates; creates habitat; biodegradation ; soil aeration ; keystone species ; parasite
Mutualist Species:
Commensal/Parasitic Species:
Humans and their ancestors have subsisted by hunting and gathering for the majority of their evolutionary history. Artiodactyls likely served as an important food source during a significant majority of this time and continue to be important parts of the human diet. Between 72,000 and 42,000 years ago, humans began wearing clothes, which probably included the skins of many artiodactyl species. In the near east, around 10,000 years ago, goats and sheep were domesticated for subsistence purposes, followed by the domestication of cows (7,500 years ago), pigs (7,500 years ago), llamas and alpacas (6,500 years ago), and camels (3,500 years ago). The domestication of artiodactyls for subsistence purposes lead to one of the most important cultural changes in human history, the transition from a purely hunter-gatherer society to a pastoral and agricultural societies.
Economically, cattle are the most important domesticated animal world wide. In 2001, the global population of domestic artiodactyls was greater than 4.1 billion, more than 31% of which consisted of cattle. In the United States, one of the worlds top 4 beef producers, beef production is the country's fourth largest industry. In addition to meat production, artiodactyls are used for their milk, fur, skin, bone, and feces and sport hunting generates millions of dollars in North America and Europe annually. However, trophy hunting can alter the evolutionary dynamics of wild populations by imposing unnatural selective pressures for decreased ornamentation. Finally, artiodactyls play an important role in the global ecotourism movement as various species of ungulates are readily observable throughout much of their native habitat.
Positive Impacts: pet trade ; food ; body parts are source of valuable material; ecotourism ; research and education; produces fertilizer
Various forms of zoonotic pathogens use artiodactyls during critical portions of their life or viral cycle. For example, pigs can harbor several influenza virus strains simultaneously, which can hybridize and result in new and virulent strains of influenza (e.g., H1N1). In addition, artiodactyls can transmit zoonotic diseases (e.g. Mad Cow disease) to humans through meat, milk, or direct physical contact. Artiodactyls also present a potential threat to various forms of agriculture by damaging and consuming crops, serving as a potential vector of zoonotic diseases for domestic artiodactyl populations (e.g., brucellosis), and competing with livestock for resources.
Negative Impacts: injures humans (carries human disease); crop pest; causes or carries domestic animal disease
Artiodactyls are the most diverse, large, terrestrial mammals alive today. They are the fifth largest order of mammals, consisting of 10 families, 80 genera, and approximately 210 species. Although the majority of artiodactyls live in relatively open habitats, they can be found in all habitat types, including some aquatic systems, and are native to every continent, excluding Australia and Antarctica. As would be expected in such a diverse group, artiodactyls exhibit exceptional variation in body size and structure. Body mass ranges from 4000 kg in hippos to 2 kg in lesser Malay mouse deer. Height ranges from 5 m in giraffes to 23 cm in lesser Malay mouse deer.
Artiodactyls are paraxonic, that is, the plane of symmetry of each foot passes between the third and fourth digits. In all species, the number of digits is reduced by the loss of the first digit (i.e., pollex), and many species have second and fifth digits that are reduced in size. The third and fourth digits, however, remain large and bear weight in all artiodactyls. This pattern has earned them their name, Artiodactyla, which means "even-toed". In contrast, the plane of symmetry in perissodactyls (i.e., odd-toed ungulates) runs down the third toe. The most extreme toe reduction in artiodactyls, living or extinct, can be seen in antelope and deer, which have just two functional (weight-bearing) digits on each foot. In these animals, the third and fourth metapodials fuse, partially or completely, to form a single bone called a cannon bone. In the hind limb of these species, the bones of the ankle are also reduced in number, and the astragalus becomes the main weight-bearing bone. These traits are probably adaptations for running fast and efficiently.
Artiodactyls are divided into 3 suborders. Suiformes includes the suids, tayassuids and hippos, including a number of extinct families. These animals do not ruminate (chew their cud) and their stomachs may be simple and one-chambered or have up to three chambers. Their feet are usually 4-toed (but at least slightly paraxonic). They have bunodont cheek teeth, and canines are present and tusk-like. The suborder Tylopoda contains a single living family, Camelidae. Modern tylopods have a 3-chambered, ruminating stomach. Their third and fourth metapodials are fused near the body but separate distally, forming a Y-shaped cannon bone. The navicular and cuboid bones of the ankle are not fused, a primitive condition that separates tylopods from the third suborder, Ruminantia. This last suborder includes the families Tragulidae, Giraffidae, Cervidae, Moschidae, Antilocapridae, and Bovidae, as well as a number of extinct groups. In addition to having fused naviculars and cuboids, this suborder is characterized by a series of traits including missing upper incisors, often (but not always) reduced or absent upper canines, selenodont cheek teeth, a 3 or 4-chambered stomach, and third and fourth metapodials that are often partially or completely fused.
Extinction threatens nearly half of all artiodactyls and risk of extinction increases in areas with decreased economic development. Humans have hunted many species without regulation to near extinction. One of the greatest threats to artiodactyls is habitat loss. For example, the native swamp habitat of Pere David's deer was largely destroyed 3500 years ago due to the draining and cultivation. Fortunately, large herds of Pere David's deer live in numerous parks and reserves throughout their native range. In some cases, conservation efforts to increase local population growth have been so effective that population control has become necessary (e.g., Giraffa camelopardalis). In addition to habitat loss, climate change has begun to contract species ranges and forced many species move poleward. For example, moose (Alces alces), which are an important ecological component of the boreal ecosystem, are notoriously heat intolerant and are at the southern edge of their circumpolar distribution in the north central United States. Since the mid to late 1980's, demographic studies of this species have revealed sharp population declines at its southernmost distribution in response to increasing temperatures.
The IUCN Red List of Threatened Species lists 168 artiodactyl species. Seven are listed as "extinct" and two are listed as "extinct in the wild". Twenty-six species are listed as “endangered,” one is “near threatened,” and data is lacking for thirteen other species. The remaining 73 species are listed as “lower risk”. Within the United States, the U.S. Fish and Wildlife Service has listed wood bison (Bison bison athabascae), woodland caribou (Rangifer tarandus caribou), Columbian white-tailed deer (Odocoileus virginianus leucurus), key deer (Odocoileus virginianus clavium), Sonoran pronghorn (Antilocapra americana sonoriensis), Peninsular bighorn sheep (Ovis canadensis nelsoni), and Sierra Nevada bighorn sheep (Ovis canadensis sierrae) as endangered throughout at least part of their native U.S. range.
Many artiodactyl species use glandular secretions to communicate with conspecifics. Pheromones are produced my epithelial glands, which are most often located on either side of the body and some artiodactyls use pedal glands to mark trails or bedding areas. In general, artiodactyls use pheromones to communicate danger, their own physical state, to establish their presence, or to attract potential mates. For example, some members of Cervidae rake their antlers on understory vegetation to make their presence known to conspecifics. Many artiodactyls use urine or feces to mark territory, contribute to mating rituals, and may incorporate excretory actions into physical displays. For example, camels excrete feces and urine when in the presence of conspecific rivals, and some species of cervid spray urine to attract mates.
Many artiodactyls attract mates, defend territory, establish and defend hierarchical position, and send messages to conspecifics by creating a variety of sounds or vocalizations. For example, male okapis create a quiet moan to attract females, whereas hippopotami make roaring sounds in response to conspecific challengers. During mating season, American bison make guttural vocalizations (i.e., bellows) that indicate mate quality and physical condition to females. Communication among conspecifics is especially important in gregarious species.
Highly developed senses of smell, hearing, and vision help artiodactyls detect disturbances in their environment. Often, when an individual becomes aware of a disturbance they send an immediate message to conspecifics by using physical displays. Physical displays are especially important in gregarious artiodactyls, warning herd members of the presence of a threat, thereby reducing surprise attacks. For example, Grant's gazelles piloerect the hairs on their hind legs to alert fellow herd members of potential threats, and white-tailed deer lift and wave their tail from side to side to warn others of potential threats.
Communication Channels: visual ; tactile ; acoustic ; chemical
Other Communication Modes: pheromones ; scent marks
Perception Channels: visual ; tactile ; acoustic ; chemical
The majority of artiodactyls are polygynous, though a few species are seasonally monogamous (e.g., blue duiker). Artiodactyls practice two forms of polygyny, female defense polygyny, and resource defense polygyny. Female defense polygyny occurs when males mate with and defend a single female while she is in estrous. Males may also defend several females (i.e. harem) from other males, courting and mating with each individual during their period of estrous. Males may also defend specific habitat patches that attract mates because they provide abundant resources or safety from predators. This is known as resource defense polygyny and occurs in pronghorn and in many African antelope species. Lekking, a form of resource defense polygyny performed by some artiodactyls (e.g., topi), occurs when a cluster of males remain in close proximity to one another while defending individual plots of land and waiting for females to select among possible mates.
Mating System: monogamous ; polygynous
Artiodactyls usually breed only once a year, though some may breed multiple times. They tend to be polyestrous and gestation ranges from 4 to 15.5 months. Aside from Suidae, which can have as many as 12 young in a litter, artiodactyls give birth to one, sometimes two, young per year that can weigh between 0.5 and 80 kg and become sexually mature between 6 and 60 months. Timing of parturition usually coincides with seasonal plant growth. As a result, most species in temperate and arctic regions give birth during early spring, whereas tropical species give birth at the start of the rainy season. Timing of parturition is especially important for the mother, who requires an abundance of high-quality vegetation to offset the physiological costs incurred by lactation. In addition, abundant high-quality vegetation helps young grow more rapidly, which reduces risk of predation.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; viviparous ; post-partum estrous
All artiodactyls give birth to precocial young that are capable of walking within a few hours after birth. The young of some species are even capable of running within 2 to 3 hours of birth. Females are the primary caregivers and nurse until young are weaned, 2 to 12 months after birth. Artiodactyls can be placed into two different categories based on maternal care: hiders and followers. "Hider young" tend to have camouflaged coats and remain hidden while their mother leaves to forage during the day. Prior to leaving, hider mothers lead their young in a secluded area in which young will choose a place to hide. Hider mothers periodically return throughout the day to nurse and clean their young. When hider young become more capable of escaping potential predators, they begin to accompany their mother during foraging bouts, which occurs immediately after birth in follower species. Hiders tend to live in smaller groups, in areas that provide adequate shelter for young. Followers tend to be larger species that live in open habitats with little shelter for young. Both are likely forms of antipredator defenses related to the size of the young and the amount of exposure in the local environment. Offspring frequently stay with their mother for months or even years after they are weaned, and in some species of sexually segregating Bovidae and Cervidae, daughters remain with their natal herd, even after reaching sexual maturity. Female red deer, which are matriarchal, may transfer social status and part of their range to their daughters.
Parental Investment: precocial ; female parental care ; pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Provisioning: Female, Protecting: Female); post-independence association with parents; extended period of juvenile learning; inherits maternal/paternal territory; maternal position in the dominance hierarchy affects status of young
Cetartiodactyls are monogamous, polyandrous, polygynandrous, or polygynous. Polygyny, in which social groupings consist of adult females and their young and one or a few adult males, is a common cetartiodactyl strategy. It occurs in species as different as elk (Cervus elaphus) and killer whales (Orcinus orca). Sexual dimorphism in ornamentation (such as antlers) and body size indicates intense male-male competition for mates in many species.
Mating System: monogamous ; polyandrous ; polygynous ; polygynandrous (promiscuous)
In general, cetartiodactyls are not highly prolific, giving birth to just one or two young every one or two years. However, some members of the family Suidae may have 12 or more young at a time. Breeding may be either seasonal or year-round. At least one species, the minke whale (Balaenoptera acutorostrata) regularly experiences a postpartum estrus. Gestation periods are as short as four months in small artiodactyls to as long as 17 months in Baird's beaked whales (Berardius bairdii), and youngsters are weaned between 2 1/2 and 24 months of age. Age at sexual maturity varies widely as well, from 5 months to more than 10 years.
Key Reproductive Features: iteroparous ; seasonal breeding ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (Internal ); viviparous ; post-partum estrous
Most cetartiodactyls have precocial young, as it is necessary for them to be able to walk or swim from the moment of birth. Young stay with their mothers for at least four months. In some species, such as bottlenosed dolphins (Tursiops truncatus), young remain with their mothers for up to five years, long after they are weaned. Males may care for their offspring indirectly by defending family groups, but they generally do not help females raise their young.
Parental Investment: precocial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Male, Female); pre-independence (Provisioning: Female, Protecting: Male, Female); post-independence association with parents; extended period of juvenile learning; maternal position in the dominance hierarchy affects status of young
ARTIODACTYLA
The mammal order Artiodactyla (even-toed ungulates) includes ten families:
1) Camelidae (camels and relatives)
2) Suidae (pigs)
3) Tayassuidae (peccaries)
4) Hippopotamidae (hippopotamuses)
5) Tragulidae (chevrotains or mouse deer)
6) Moschidae (Musk-deer)
7) Cervidae (deer)
8) Bovidae (hollow-horned ruminants)
9) Antilocapridae (Pronghorn)
10) Giraffidae (Giraffe and Okapi)
Molecular phylogenetic studies and other evidence (some going back to the 1880s) indicate a close relationship between Artiodactyla and Cetacea (whales). The group Cetartiodactyla is a clade composed of Cetacea + Artiodactyla.
(Lewison 2011 and references therein)
The even-toed ungulates are mammals belonging to the order Artiodactyla (/ˌɑːrtioʊˈdæktɪlə/, from Ancient Greek ἄρτιος, ártios 'even', and δάκτυλος, dáktylos 'finger, toe'). They are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing posteriorly. By contrast, odd-toed ungulates bear weight on an odd number of the five toes. Another difference between the two is that many other even-toed ungulates (except for Suina) digest plant cellulose in one or more stomach chambers rather than in their intestine as the odd-toed ungulates do.
The advent of molecular biology, along with new fossil discoveries, found that Cetaceans (whales, dolphins, and porpoises) fall within this taxonomic branch, being most closely related to hippopotamuses. Some modern taxonomists thus apply the name Cetartiodactyla /sɪˌtɑːrtioʊˈdæktɪlə/ to this group, while others opt to include cetaceans within the existing name of Artiodactyla.
The roughly 270 land-based even-toed ungulate species include pigs, peccaries, hippopotamuses, antelopes, deer, giraffes, camels, llamas, alpacas, sheep, goats, and cattle. Many are herbivores, but suids are omnivorous, while cetaceans are almost exclusively carnivorous. Many of these are of great dietary, economic, and cultural importance to humans.
The oldest fossils of even-toed ungulates date back to the early Eocene (about 53 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. The fossils are classified as belonging to the family Dichobunidae; their best-known and best-preserved member is Diacodexis.[1] These were small animals, some as small as a hare, with a slim build, lanky legs, and a long tail. Their hind legs were much longer than their front legs. The early to middle Eocene saw the emergence of the ancestors of most of today's mammals.[2]
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.[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 were groups like the stocky, short-legged 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.
Suina (including pigs) have been around since the Eocene. In the late Eocene or the Oligocene, two families stayed in Eurasia and Africa; the peccaries, which became extinct in the Old World, exist today only in the Americas.
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.
The classification of artiodactyls was hotly debated because the ocean-dwelling cetaceans evolved from the land-dwelling even-toed ungulates. Some semiaquatic even-toed ungulates (hippopotamuses) are more closely related to the ocean-dwelling cetaceans than to the other even-toed ungulates.
This makes the Artiodactyla as traditionally defined a paraphyletic taxon, since it includes animals descended from a common ancestor, but does not include all of its descendants. 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.[3] An alternative approach is to include both land-dwelling even-toed ungulates and ocean-dwelling cetaceans in a revised Artiodactyla taxon.[2]
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.[5] However, they did not recognize hippopotamuses and classified the ruminants as the sister group of cetaceans. Subsequent studies established the close relationship between hippopotamuses and cetaceans; these studies were based on casein genes,[6] SINEs,[7] fibrinogen sequences,[8] cytochrome and rRNA sequences,[3][9] IRBP (and vWF) gene sequences,[10] adrenergic receptors,[11] and apolipoproteins.[12]
In 2001, the fossil limbs of a Pakicetus (amphibioid cetacean the size of a wolf) and Ichthyolestes (an early whale the size of a fox) were found in Pakistan. They were both 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, previously thought to be unique to even-toed ungulates, were also in early cetaceans.[13] The mesonychids, 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.
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 have 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 was able to confirm this, but also revealed that hippopotamuses were derived from older anthracotherians.[9][14] The newly introduced genus Epirigenys 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. Henri de Blainville recognized the similar anatomy of the limbs of pigs and hippos, and British zoologist Richard Owen coined the term "even-toed ungulates" and the scientific name "Artiodactyla" in 1848.
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 a selenodont construction (crescent-shaped cusps) and have the ability to ruminate, which requires regurgitating food and re-chewing it. Differences in stomach construction indicated that rumination evolved independently between tylopods and ruminants; therefore, tylopods were excluded from Ruminantia.
The taxonomy that was widely accepted by the end of the 20th century was:[15]
Modern cetaceans are highly adapted sea creatures which, morphologically, have little in common with land mammals; they are similar to other marine mammals, such as seals and sea cows, due to convergent evolution. However, they evolved from originally terrestrial mammals. The most likely ancestors were long thought to be mesonychids—large, carnivorous animals from the early Cenozoic (Paleocene and Eocene), which had 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, have a uniform construction.
The suspected relations can be shown as follows:[14][16]
Paraxonia CeteMolecular 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:[17][18][19][20][21]
Artiodactyla Tylopoda (camels) .png){kind=small}
Suina (pigs) _(white_background).jpg){kind=small}
Tragulidae (mouse deer) .jpg){kind=small}
Pecora (horn bearers) 
Hippopotamidae (hippopotamuses) .jpg){kind=small}
Cetacea (whales) .jpg){kind=small}
The four summarized Artiodactyla taxa are divided into ten extant families:[22]
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.
Artiodactyls are generally quadrupeds. 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 of sexual dimorphism: the males are consistently larger and heavier than the females. In deer, only the males boast antlers, and the horns of bovines are usually small or not present in females. Male Indian antelopes have a much darker coat than females.
Almost all even-toed ungulates have fur, with an 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 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 adapted differently between species:
Hippopotamuses have all four toes pointing out.
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 (while both are made of keratin, claws are curved and pointed while nails are flat and dull).[24] 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.[25]
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.
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).
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.[26]
True horns have a bone core that is covered in a permanent sheath of keratin, and are found only in the bovids. Antlers are bony structures that are shed and replaced each year; they are found in deer (members of the family 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.[27]
All these cranial appendages can serve for posturing, battling for mating privilege, and for defense. In almost all cases, they are sexually dimorphic, and 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.
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 incisors are often reduced in ruminants, and are completely absent in the upper 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 the lower jaw.
The molars of porcine have only a few bumps. In contrast, the 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.
The ruminants (Ruminantia) ruminate their food—they regurgitate and re-chew it. Ruminants' mouths often have additional salivary glands, and the oral 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: the rumen, the reticulum, the omasum, and the abomasum.[28] After the food is ingested, it is mixed with saliva in the rumen and reticulum and separates into layers of solid versus 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 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. Cellulytic microbes (bacteria, protozoa, and fungi) produce cellulase, which is needed to break down the cellulose found in plant material.[28] 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 his head to the ground—rumination can take place later, in a sheltered area.[29]
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 towards limbs that allow the animal to quickly escape predators.[30] Most species within Suina have a simple two-chambered stomach that allows an omnivorous diet. The babirusa, however, is an herbivore,[28] and has extra maxillary teeth to allow for proper mastication of plant material. Most of the fermentation occurs with the help of cellulolytic microorganisms within the caecum of the large intestine. Peccaries have a complex stomach that contains four compartments.[29] Their fore stomach has fermentation carried out by microbes and has high levels of volatile fatty acid; it has been proposed that their complex fore stomach is a means to slow digestive passage and increase digestive efficiency.[29] 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, the whales, are obligate carnivores.
Unlike other even-toed ungulates, pigs have a simple sack-shaped stomach.[28] Some artiodactyla, such as white-tailed deer, lack a gall bladder.[31]
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.[32] In some even-toed ungulates, the penis contains a structure called the urethral process.[33][34][35]
The testicles are located in the scrotum and thus outside the abdominal cavity. The ovaries of many females descend—as 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).[32]
The number of mammary glands is variable and correlates, as in all mammals, with 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 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.[36] 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.
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 hierarchy, both among males and females. Some species also live in harem 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 during 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 diurnal, crepuscular, and nocturnal artiodactyls. Some species' pattern of wakefulness varies with season or habitat.
Generally, even-toed ungulates tend to have long gestation periods, smaller litter sizes, and more highly developed newborns. As with many other mammals, species in temperate or polar regions have a fixed 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.
The 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 different natural predators depending on their size and habitat. There are several carnivores that prey on them, including large cats (e.g., lions) and bears. Other predators are crocodiles, wolves and dogs, large 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.[37]
Parasites include nematodes, botflies, fleas, lice, or flukes, but they have debilitating effects only when the infestation is severe.
Artiodactyls have been hunted by primitive humans for various reasons: for meat or fur, as well as to use their bones and teeth as weapons or tools. Their domestication began around 8000 BCE. To date, humans have domesticated goats, sheep, cattle, camels, llamas, alpacas, and pigs. Initially, livestock was used primarily for food, but they began being used for work activities around 3000 BCE.[30] Clear evidence exists of antelope being used for food 2 million years ago in the Olduvai Gorge, part of the Great Rift Valley.[30][38] 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 river Céou that was inhabited around 12,500 years ago.[39]
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.[40]
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 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.[36]
Conversely, many artiodactyls have declined significantly in numbers, and some have even gone extinct, largely due to over-hunting, and, more recently, habitat destruction. Extinct species include several gazelles, the aurochs, the Malagasy hippopotamus, the bluebuck, and Schomburgk's deer. Two species, the Scimitar-horned oryx and Père David's deer, are extinct in the wild. Fourteen species are considered critically endangered, including the addax, the kouprey, the Bactrian camel, Przewalski's gazelle, the saiga, and the pygmy hog. Twenty-four species are considered endangered.[41][42]
The even-toed ungulates are mammals belonging to the order Artiodactyla (/ˌɑːrtioʊˈdæktɪlə/, from Ancient Greek ἄρτιος, ártios 'even', and δάκτυλος, dáktylos 'finger, toe'). They are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing posteriorly. By contrast, odd-toed ungulates bear weight on an odd number of the five toes. Another difference between the two is that many other even-toed ungulates (except for Suina) digest plant cellulose in one or more stomach chambers rather than in their intestine as the odd-toed ungulates do.
The advent of molecular biology, along with new fossil discoveries, found that Cetaceans (whales, dolphins, and porpoises) fall within this taxonomic branch, being most closely related to hippopotamuses. Some modern taxonomists thus apply the name Cetartiodactyla /sɪˌtɑːrtioʊˈdæktɪlə/ to this group, while others opt to include cetaceans within the existing name of Artiodactyla.
The roughly 270 land-based even-toed ungulate species include pigs, peccaries, hippopotamuses, antelopes, deer, giraffes, camels, llamas, alpacas, sheep, goats, and cattle. Many are herbivores, but suids are omnivorous, while cetaceans are almost exclusively carnivorous. Many of these are of great dietary, economic, and cultural importance to humans.
