Artiodactyls, or even-toed ungulates, are mammals characterized by having an even number of toes, typically two or four on each foot. These animals belong to the order Artiodactyla and stand in contrast to the odd-toed ungulates of the order Perissodactyla, which have either one or three toes. While odd-toed ungulates rely on intestinal digestion, artiodactyls use one or more specialized stomach chambers to break down their food more efficiently. odd-toed ungulates
This group represents the fifth-largest mammalian order (around 420 extant species), comprising pigs, peccaries, deer, antelopes, giraffes, camels, and ruminants like sheep and cattle. Though some researchers often exclude cetaceans from this order, evidence from fossil records and molecular studies suggest that they are most closely related to hippopotamuses and, hence, are artiodactyls.
Being a diverse mammalian group, their body size varies largely across the different members. The smallest artiodactyl, the mouse deer, measures only 18 in (45 cm) and weighs 3.3 lb (1.5 kg). In contrast, the largest member (by weight), the hippopotamus, weighs 4.5 mt and reaches up to 16 ft (5 m) in length. The giraffe is the tallest (and thus largest by body length) and grows up to 18 ft (5.5m) in height.
Their bodies are typically covered by a thick pelage or fur (except for hippopotamuses, which are nearly hairless). The pelage comprises an external layer of guard hair and an internal underfur, both insulating in nature. The guard hair, as the name suggests, shields the animals against external forces like wind, rain, and snow. Some members, like cattle, have a ventral ridge of long hairs called ruff or dewlap that hangs below the throat.
The pelage color ranges from black and brown to white, with many shades of yellow and gray, which helps them to camouflage. Those living in temperate and polar regions shed this coat seasonally.
In general, many artiodactyls have large heads with a narrow, elongated skull. The frontal bone is particularly enlarged at the back and displaces the parietal bone. In some groups, like hippopotamuses, ruminants, and most camels, the postorbital bar of the skull is complete, while in others, such as pigs and peccaries, it is incomplete.
Although the brain is moderately developed, the olfactory and the auditory regions are quite advanced, aiding in acute sensory perception. The foldings of the cerebral hemispheres are often more complex in larger artiodactyls than in small ones.
In members of the four families Bovidae, Cervidae, Giraffidae, and Antilocapridae, the skull is modified into cranial appendages, like true horns, antlers, ossicones, and pronghorns. These structures are typically found in males (hence sexually dimorphic) and aid in sexual selection and defense.
| Family | Cranial Appendage |
|---|---|
| Bovidae | True horns with a bony core covered in a permanent sheath of keratin. |
| Cervidae | Long and branched bony antlers that are shed and replaced every year. These structures are temporarily covered with a soft, vascular skin called velvet. |
| Giraffidae | Permanent bony ossicones, which are fused with frontal or parietal bones. They are permanently covered with skin and fur. |
| Antilocapridae | Forked or branching pronghorns with bony cores like true horns. They are covered in keratinous sheaths. |
Artiodactyls that are omnivores, such as suines and hippopotamuses, have a large number of teeth, with some, like pigs, having as many as 44. On the other hand, herbivores, like camels and ruminants, possess fewer teeth with a wide gap or diastema between them. In general, the dental formula of artiodactyls is 0-3. 0-1. 2-4. 3/1-3. 1. 2-4. 3 = 30 – 44.
In ruminants, the incisors are often reduced (absent in the upper jaw), while the canines are enlarged and used as weapons in certain groups, like musk deer and water deer. Similarly, in suines, the canines are large and tusk-like, helping them dig the ground and defend themselves.
As their name suggests, these animals possess either two or four (even-numbered) toes on each foot. However, one group, the peccaries, have three toes on their hindlimbs.
All extant artiodactyls have lost the first toe through evolution, while the second and fifth toes have been considerably reduced. Thus, they are paraxonic, meaning the leg axis passes between the third and fourth toes. It is the third and the fourth toes that bear the weight of these ungulates.
The talus or astragalus bone of these animals is unique, with a double-pulley-like arrangement of the articulating surfaces. Such an arrangement restricts lateral movement or rotation, thereby allowing greater flexion and stability when running at high speeds.
In families like Camelidae, Cervidae, Giraffidae, Antilocapridae, and Bovidae, the two principal metacarpal and metatarsal bones in artiodactyls are fused to form the cannon bone. This bone serves as an insertion point for springing ligaments of their limbs.
The bones of stylopodium (upper arm and thigh bones) and zygopodium (tibia and fibula) are typically long, and a shoulder blade or scapula helps in articulating the upper arm bone. The scapula swings back and forth during runs, adding to the agility of artiodactyls. The clavicle is absent in these animals.
The anatomical specificities of their digestive system vary across the different groups.
In male artiodactyls, the testes are located in a sac-like scrotum that hangs outside the abdominal cavity. The intromittent organ, the penis, is S-shaped when at rest and lies in a pocket under the belly.
In females, the ovaries also descend from the abdominal cavity and lie close to the pelvic inlet near the fourth lumbar vertebra. The uterus comprises two uterine horns and is hence called uterus bicornis.
Depending on the litter size, the number of mammary glands varies across different artiodactyl groups. Since pigs have the largest litter size, they have multiple teats lined in two rows extending from their armpits to the groin.
Ruminants typically have two to four teats, but some domestic cattle may occasionally have extra teats, bringing the total to six. All bovids have four teats, except some, like gazelles, hartebeests, wildebeests, sheep, and goats. In some members, like cows and goats, the teats form a single mass called udder under the belly.
The terms ‘even-toed ungulates’ and ‘Artodactyla’ were coined by the British zoologist Sir Richard Owen in 1848.
Initially, the morphology of the stomach and the molars was used to classify these animals. Based on such a criterion, pigs and hippopotamuses were initially grouped together under the suborder Suina since both have molars with well-developed roots and a simple stomach. Other artiodactyls that ruminate and possess molars with crescent-shaped cusps were grouped under the suborder Ruminantia. However, tylopods are distinctly different from ruminants in stomach construction and were found to have evolved independently from them. Thus, the suborder Tylopoda was excluded from Ruminantia.
Recent molecular studies suggest that cetaceans (infraorder Cetacea) are deeply nested within the order Artiodactyla. Thus, the two groups form a monophyletic taxon called the Cetartiodactyla, often used synonymously with Artiodactyla itself.
In modern classification, around 420 species of artiodactyls are grouped under four suborders: Tylopoda (camels and llamas), Suina (pigs and peccaries), Whippomorpha (hippopotamuses and cetaceans), and Ruminantia (ruminants).
These mammals are found worldwide, except in Oceania and Antarctica. Though they live in all habitats, ranging from tropical rainforests and oceans to deserts and mountains, they are most abundant in grasslands and open forests.
Different groups of artiodactyls are distributed across different regions of the world.
Except for pigs, all artiodactyls are obligate herbivores and feed on plants. Ruminants consume cellulose-rich plant materials, including grass, hay, and leguminous plants. They also feed on roughage rich in fiber, such as grains. Hippopotamuses eat around 150 lb (68 kg) of grass every night.
Pigs are the only artiodactyls that are omnivores and feed on both plants and animals, like worms and fish.
Depending on the species, artiodactyls are diurnal (giraffes and gazelles), crepuscular (white-tailed deer), or nocturnal (water chevrotains). While many species are sedentary and stay in the same place throughout the year, others, like wildebeests, migrate annually.
Being a diverse group, different artiodactyls follow different movement patterns, like walking, ambling, or galloping. Naturally, different members have different locomotion speeds. While camels move at around 9 to 10 mph (14 to 16 kmph), the Thomson’s gazelle has a speed of about 50 mph (80 kmph).
In normal walking, these animals move their legs in the following sequence: left front, right rear, right front, and left rear. When trotting, each foot lifts off before the one ahead touches the ground, while in galloping, the front legs leave the ground one after the other, followed by the hind legs.
Camels amble by moving both legs on each side together, with the right front and hind limbs, as well as the left front and hind limbs, moving in unison. Giraffes and okapis follow this gait, too, though giraffes, having a short body and great height, move their long necks back and forth with each stride.
These ungulates generally have social tendencies and merge into large social herds and family groups, though some, like okapi, prefer living alone, gathering only while mating. Large groups maintain a social hierarchy within both males and females.
While some artiodactyls, like bovids, live in harems, comprising one male, several females, and their offspring, in others, the females stay together with the juveniles. In the latter scenario, the males either live solitarily or in bachelor groups, seeking the females only during the breeding season.
Many species are territorial and mark their areas with urine or special glandular secretions. The simplest territorial organization is observed in the common wild pig (Sus scrofa), which has a small home range where it rests, feeds, and wallows. In contrast, male Uganda kobs (Kobus kob) maintain 30 to 40 territories on their breeding grounds, with each territory having a diameter of 50 to 100 ft.
They usually communicate through some form of signaling, like glandular secretions or vocalizations.
Most species produce chemical pheromones from their epithelial or pedal glands to mark their territories, assert their presence, or attract mates. Some artiodactyls also use their urine for the same purposes. For instance, deer spray urine on potential mates to draw their attention, while camels often expel feces and urine to their rivals.
They also make use of sounds to communicate with their conspecifics. For instance, hippopotamuses roar when challenged by contenders, while male okapis attract their female mates by moaning quietly. Similarly, male American bison bellow to the females to prove their sexual fitness and quality.
They have a lifespan that varies widely, from 8 to 50 years. However, most species typically live between 20 and 30 years. While hippos, cows, and camels live between 40 to 50 years, some cetaceans, like bowhead whales, have reportedly survived as many as 200 years.
Artiodactyls living in tropical regions breed year-round, while those in the temperate and polar regions have a fixed breeding season when they mate. They are generally polygynous, where a single male mates with multiple females; however, some, like the blue duiker, are seasonally monogamous (a single male mates with a single female).
Many advanced artiodactyls engage in courtship behavior. The male often sniffs or licks the female’s genitals and urine and later raises his head with curled lips. Such behavior is called the Flehmen response, and it helps the male recognize which females are in heat. In kudus and bushbucks, the males also follow the females and nuzzle their necks before mounting. Similarly, in Thomson’s gazelles, the male runs close behind the female and taps her hindlimb with his forelimb. Such leg contact helps test the female’s readiness or habituate her to touch prior to mating.
These mammals tend to have long gestation periods, ranging from 4 to 5 months in chevrotain, porcine, and musk deer to 14 months in the Bactrian camel. Hippos and bovines have a gestation period of 6 to 10 months, while giraffes are typically pregnant for 14 to 15 months.
Females of many gregarious species prefer living solitarily before giving birth. For instance, female chitals or axis deer live near patches of dense bush or high grass to which they retreat when threatened. Similarly, female collared peccaries also withdraw to burrows before giving birth.
They generally give birth to precocial, relatively mature young in small litter sizes. The young weigh about one-tenth of their mothers and are hairy with open eyes. Generally, most modern artiodactyls bear one young at each birth, but some species, like the Chinese water deer, give birth to twins or triplets. Some pigs, like wild boars, can give birth to up to ten (or even twelve) piglets.
While some young are mature enough to run about within a few hours or days of their birth, others stay with their mothers for the first few weeks. Wildebeest calves represent an extreme level of precocity by rising within five minutes of birth. In about 24 hours, they can move as fast as adults.
It is primarily the mothers that take care of the young till they are weaned in about 2 to 12 months after birth. Depending on maternal care, the young are either hiders or followers. Hiders have camouflaged coats and tend to remain hidden in well-sheltered areas while their mothers leave for foraging. Followers, on the other hand, live in open habitats with little shelter and thus have to follow their mothers as they forage.
Depending on the body size, artiodactyls are preyed upon by a wide variety of natural predators. These include lions, tigers, bears, crocodiles, large snakes, wolves, and dogs. Large raptorial birds, like vultures, also feed on them. Cetaceans, in particular, are hunted down by sharks, polar bears, and other cetaceans.
A wide range of invertebrates, including nematodes, botflies, fleas, lice, and flukes, parasitize these animals and live off their body tissues and fluids.
In camels with two toes, the digits end in keratinized claws, which in turn are transformed into pointed nails for better grip in the sand. The claws of their forelimbs are wider and more blunt than those of the hindlimbs.
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