- Related Topics:
- oreodont
- ruminant
- tylopod
- Archaeomeryx
- Suiformes
External glands occur in various places on artiodactyls. Preorbital glands, immediately in front of the eyes, are present in the giant forest hog (Hylochoerus meinertzhageni), in all cervids except the roe deer, and, among the bovids, in duikers, many neotragines, gazelles and their allies, and the hartebeest group. These glands are apparently required in small forest forms and have disappeared in many, but not all, open-country forms. In some, the glands are definitely connected with territorial marking; a firm object is marked by rubbing, soft vegetation by swinging the head gently from side to side. Foot, or pedal, glands are present in the African bush pig (Potamochoerus porcus), camels, tragulids, the pronghorn, some bovids, and on the back legs only of most American deer.
Inguinal (belly) glands are found in bovids, there being two in sheep, saiga, chiru, gazelles, duikers, and blackbuck, and four in members of the tribes Reduncini and Tragelaphini. Carpal (wrist) glands are present in some pigs, some gazelles and allies, and the oribi (Ourebia ourebi). Glands in other positions are rather less frequent, but postcornual ones (behind the horns) occur in the Rocky Mountain goat, the pronghorn, and the chamois (Rupicapra rupicapra), supraorbital ones in muntjacs (several species of Muntiacus). There are jaw glands in the pronghorn; neck glands in camels; dorsal glands on the back of peccaries, pronghorn, and springbok; and preputial glands (in front of the genital region) in several pigs, grysbok (Raphicerus melanotis), and the musk deer. Tail glands are found in musk deer, pronghorn, and goats; tarsal glands in pronghorn and American deer; and metatarsal glands in camels, some deer, and the impala. Pronghorn, blackbuck, gazelles, and oribi are thus particularly well equipped with glands. The use of such glands, apart from the use of preorbital glands in some species for territorial marking, is a matter for conjecture. Chital deer, when alarmed, thump the ground several times with their hind feet, which possess glands; the scent remaining on the ground may function as a danger signal. In general, mammals often mark with their glands when they are threatening other individuals of their own species.
Digestive system
The higher artiodactyls feed only on plant matter, which consists largely of cellulose and other carbohydrates and water. This necessitates adaptations of the structure and functioning of the stomach and intestines. Even pigs have enlarged stomachs—they have a pouch near the cardiac orifice (the upper opening) of the stomach—and in peccaries the stomach is more complicated. In hippopotamuses the stomach is divided into four compartments, and micro-organisms ferment food as part of the digestive process. Unlike pigs, hippopotamuses have lost the cecum (a blind pouch) further on in the gut.
In the most advanced ruminants, the much enlarged stomach consists of four parts. These include the large rumen (or paunch), the reticulum, the omasum (psalterium or manyplies)—which are all believed to be derived from the esophagus—and the abomasum (or reed), which corresponds to the stomach of other mammals. The omasum is almost absent in chevrotains. Camels have a three-chambered stomach, lacking the separation of omasum and abomasum; the rumen and reticulum are equipped with glandular pockets separated by muscular walls having sphincters (valves) and glands. The esophagus opens into the rumen, not into the area between rumen and reticulum; these and other differences suggest that camels evolved the ruminating habit independently of the true ruminants. The total stomach of the domestic ox (Bos taurus) occupies nearly three-quarters of the abdominal cavity, and, even in medium-sized cattle, the rumen alone can have a capacity of 95 to 285 litres (25 to 75 gallons), having undergone a tremendous growth in early life, with the changeover from a milk diet.
Food taken into the rumen is later regurgitated into the mouth and completely masticated, then swallowed again and passed to the reticulum, omasum, and abomasum. The regurgitation and chewing in the mouth is called rumination.
In the rumen many different species of minute protozoans (animals) and bacteria live without free oxygen. The digestion of the cellulose of plant cell walls is the main function of the fauna and flora in the rumen, since mammalian digestive juices are incapable of digesting cellulose. The contents of the plant cells are thus released for digestion. Large volumes of saliva are secreted into the rumen to help digestion. Soluble products of microbial action, mainly fatty acids, are absorbed through the rumen wall. In the omasum, some fatty acids and 60–70 percent of the water are absorbed; in the abomasum gastric juice containing hydrochloric acid is secreted, as in an ordinary mammalian stomach.
In the rumen any ingested protein is degraded into fatty acids and ammonia; the ammonia and other simple nitrogen-containing substances are used by the micro-organisms for their own cell-protein synthesis. These organisms are ultimately digested in the abomasum and small intestine, thus providing the ruminant with protein.
Many artiodactyls are adapted to living in conditions of water shortage. The best known and one of the most spectacular examples of this is the camel. Its body temperature can fluctuate according to the outside temperature, thus minimizing water loss through sweating; it excretes rather dry dung and a concentrated urine (i.e., high in urea and low in water) and is not seriously weakened by as much as a 25 percent dehydration in its body, since water is not withdrawn from the bloodstream and the continuing circulation avoids any buildup of excessive internal temperatures. The thick coat hinders the inward transference of heat from the environment (the temperature of which may often exceed the animal’s body temperature); a thirsty camel can take in water very rapidly. Oryxes and gazelles are antelopes noted for needing little water, the dorcas gazelle (Gazella dorcas) in the Sudan depending on leaves of Acacia bushes for its water. The zebu (a form of domesticated cattle) needs less water than most temperate climate breeds.
Reproductive specializations
The testes of male artiodactyls descend outside the body cavity but may regress into the abdomen in the nonbreeding season. Female pigs have many teats, but ruminants have only two to four (although domestic cattle occasionally have as many as six). Among the bovids, the alcelaphines (hartebeests, wildebeests, and relatives), gazelles, and some caprines (sheep, goats, and relatives) have two, the rest have four.
The unborn mammal within its mother breathes, feeds, and excretes through an organ called the placenta, which is connected with the tissues of the mother’s uterus (womb) wall. Hippopotamuses and pigs have an epitheliochorial placenta, a layer of fetal tissue merely pressed close against the uterus wall, but camels and ruminants possess a syndesmochorial placenta, in which the epithelium of the maternal tissues is eroded to facilitate intercommunication. This is an advance over the epitheliochorial placenta, but the artiodactyls are not particularly advanced, when compared with other mammals, in which there may be still closer association of maternal and fetal blood vessels (endothelial and hemochorial placentas). Even in many syndesmochorial placentas the uterus lining may be wholly or partly restored before the end of pregnancy. Although there is no erosion of maternal tissues in the epitheliochorial placenta, the capillaries beneath the fetal and maternal surface layers may pass just beneath the surface layers, making them thin. The actual fingerlike processes (villi), through which the placenta contacts the uterus, are evenly distributed (“diffuse” placentas) in hippopotamuses, pigs, camels, and tragulids; in higher artiodactyls they are in pockets or groups called cotyledons (“cotyledonary” placentas). It is interesting that there are few of these cotyledons in deer—for instance only five in Père David’s deer—but many in giraffes and bovids (up to 160 or 180 in giraffes and goats). The musk deer (Moschus moschiferus) is exceptional among deer in retaining a diffuse placenta.
Evolution and paleontology
The artiodactyls can be traced back to a probable descent from a group of early generalized mammals called condylarths and were certainly distinct by the Eocene Epoch, which ended about 33.9 million years ago. Fossil artiodactyls can be more or less convincingly classified in three suborders; the more primitive Suiformes, centred around pigs, the Tylopoda, centred on camels, and the Ruminantia or ruminants. The most primitive artiodactyls are the suiform group Palaeodonta, which had four functional toes on each foot, primitive, low-cusped cheek teeth, and the typical artiodactyl astragalus. The artiodactyls became more prominent in the Oligocene (between about 33.9 million and 23 million years ago) with a decline of the then dominant perissodactyls, and the later history of artiodactyls appears as successive waves of groups, each better adapted than its predecessors to the changing environment. In the suiform line, the earlier palaeodonts are succeeded by other groups such as the entelodonts, giant “pigs” of the European and North American Oligocene, characterized by very large skulls (some nearly 1 metre [3.3 feet] long), very small brains, and a large, bony flange below the eyes. The functionally two-toed ruminants succeeded four-toed suiforms in the Miocene, and within the Old World ruminants of the bovid subfamily Caprinae, the zenith of the tribe Caprini, for example, followed that of the mainly Pliocene tribe Ovibovini.
The artiodactyls had an interesting history in North America through the Paleogene and Neogene periods. Some forms, such as the entelodonts, were shared with the Old World, but others were characteristic of North America. One very prominent New World family was the merycoidodonts (or oreodonts), which lasted until the early Pliocene (about 3.6 million years ago). They had somewhat piglike proportions, short faces, a large upper canine and a caniniform first lower premolar, and selenodont molars. A close relative, Agriochoerus, had clawed feet, the function of which remains uncertain.
Camelids evolved in North America and, at or toward the end of the Neogene, spread into South America and into the Old World. By the end of the Pleistocene they all became extinct in their homeland, just as horses did. The hypertragulids were a mainly Oligocene group of chevrotain-like forms related to the Protoceratidae. The latter had horns above their noses, a position unique among artiodactyls, as well as in the usual position. The North American Miocene (23 million to 5.3 million years ago) produced some ruminants, such as Blastomeryx, that are hard to distinguish from the early palaeomerycine relatives of giraffes and deer in the Old World, which, with the North American groups, constitute the family Palaeomerycidae. Some developed horns, and the dromomerycine Cranioceras even had a third horn above the back of its skull. During the Miocene and Pliocene there finally appeared relatives of the surviving pronghorn, an example being Merycodus. Many of these North American groups have parallels with Old World groups, and the subject of North American artiodactyl evolution is of great interest. Only further finds will indicate whether Blastomeryx, the dromomerycines, Merycodus, and the pronghorns evolved from hypertragulids already in North America or sprang from some immigrant ruminant and, if the latter, whether the supposed hypertragulid Leptomeryx could be such an immigrant ruminant. It is uncertain whether the hypertragulids are nearer the tragulines or the camels, and how close the oreodonts are to the anthracotheres. Of the great New World radiation there survived after the Pleistocene only three or four camelid species and the pronghorn (deer and bovids in the Americas are immigrants), whereas in the Old World as little as 200 years ago, Eurasia and Africa had abundant deer and antelopes.
Until the Miocene there were some archaic artiodactyls in Europe, the xiphodonts, which have cautiously been taken as tylopods, and the cainotheres and anoplotheres, which are classified near anthracotheres.
A possible ruminant ancestor was Archaeomeryx from the upper Eocene of China, a small animal that already had a fused naviculo-cuboid bone in the ankle. Tragulids occurred in Africa and Eurasia back to the Miocene, and the more advanced gelocids are known from the upper Eocene and lower Oligocene. At the end of the Oligocene, the first ruminants began to appear with teeth more advanced than those of tragulids. From early in the Miocene they began to be recognizable as giraffes, deer, or antelopes, although the last were relatively uncommon before the late Miocene. Much remains to be learned about the detailed early history of these groups. Several different giraffids lived in later Miocene and early Pliocene times, but the group has since declined to only two species. Deer gradually acquired more complicated antlers, which became very large in some lineages. Different subfamilies of bovids originated in Eurasia and Africa, and it is of zoogeographic interest that representatives of African subfamilies have been found as fossils in northern India and Pakistan.
