Artiodactyla

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.

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.

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