The ungulates

(Hoofed mammals)

Introduction

Ungulates or hoofed mammals are a large, diverse, and highly successful group of terrestrial mammals classified into a series of superorders and orders. Originally the term "ungulate" referred to plant-eating animals with hooves on the terminal digits (toes) of their legs. This foot structure is their unique morphological adaptation and defining characteristic. In all species, the size of at least one toe has decreased, and in many, the number of toes has been reduced through natural selection. In addition, as their common name indicates, most have evolved hooves, which are modified claws or nails at the tips of the toes. There are two basic specialized foot plans in the ungulates: mesaxonic, in which the main weight is borne on the third digit (toe) as in horses, for example; and paraxonic, in which the weight is borne equally by the third and fourth digits as seen in cattle.

Ungulates are not only the most successful and widespread group of large mammals in the world today, but they are also the single most important group of animal species directly beneficial to humans. Almost all the important domestic animals are ungulates, including horses, pigs, cattle, sheep, goats, camels, and water buffalo. From these species, humans obtain meat, milk, hides, fibers, draft animal power, and much more. These animals, along with domestic plants, fostered development of modern civilizations and continue to support humankind across the globe.

Evolution and systematics

The evolutionary story of the ungulates is complex and not always taxonomically clear. It revolves around changes towards herbivory and accompanying changes to morphology of skulls, teeth and digestive systems. The story also involved evolution of the limb structure in response to predation pressure such that the limbs became increasingly adapted for fast running (cursorial locomotion). The early evolution is particularly challenging to understand, so it is difficult to provide a clear account of their history. Many of the mammals included in the grand order Ungulata are not necessarily closely related. However, even widely separated forms show many cases of parallel evolution, often more than once and often to a remarkable degree of similarity. The evolutionary story is also problematic because ancestral ungulates are poorly represented in the fossil record resulting in an incomplete history; thus it is sometimes difficult to determine with confidence whether or not a particular fossil was an ancestral form. The taxonomy of ungulates will always change as new fossils are discovered and new evidence based on DNA and other molecular analyses help reconstruct their evolutionary history.

Traditionally, five superorders of ungulates have been recognized based on morphological evidence from fossil remains, encompassing up to 16 orders. The superorders consisted of: the Protoungulata, comprised of the primitive ungulates within the Condylarthra, the tillodonts, the litopterns, notoungulates and the Astrapotheria, as well as the Tubulidentata; the Amblypoda with three or four orders, the Dinocerata, Pyrotheria, and Desmostylia, and possibly the Pantodonta; the Paenungulata or near-ungulates, which include the Sirenia, Proboscidea, and Hyracoidea, along with extinct and distantly related Embrithopoda and Desmostylia; the Paraxonia containing the Artiodactyla; and the Mesaxonia with the Perissodactyla. Of these 16 orders, only six are extant: Tubulidentata (aardvarks), Sirenia (manatees and dugongs), Proboscidea (elephants), Hyracoidea (hyraxes), and the two modern ungulate orders, the Artiodactyla and Perissodactyla. Recent interpretations of the grand order Ungulata include the order Cetacea (whales and dolphins), while the Pantodonta, formerly included in the Amblypoda, are probably not ungulate. Of the remaining previous members of the amblypods, the uintatheres (Dinocerata) are thought to be more closely related to the paenungulates and perissodactyls, and the desmostylians are placed closer to the proboscidean root.

The following synopsis of the history of modern ungulates is best understood within the context of broad groups that included other extant orders and their common ancestry. Based almost solely on the number of digits, a large portion of modern ungulates can be placed in two morphological groups: the even-toed Artiodactyla (e.g., sheep and deer) and the oddtoed Perissodactyla (e.g., horse and rhinoceros). Although even-toed and odd-toed ungulates are about as closely related as are the rodents and primates, both ungulate orders most probably arose from within the Condylarthra. Artiodactyla were previously considered to have evolved from the mesonychids, which were themselves thought to be derived from the

raccoon-like arctocyonids. Artiodactyls are now considered to have arisen from an arctocyonid ancestor, and though the most successful and numerous of the living ungulates, this order is also probably one of the most "primitive" in terms of its origins. The arctocyonids are some of the oldest of the condylarths that first appeared in the fossil record of the late Upper Cretaceous, and eventually dispersed throughout North America and Eurasia. These primitive mammals were probably omnivores based on their teeth structure which resembled that of modern-day bears. The Perissodactyla are generally thought to have evolved from the Phenacodontidae. These were early condylarths ranging in size from modern foxes to tapirs, probably with claws on their terminal phallanges and dental adaptations towards herbivory, but otherwise showing typically primitive mammalian characteristics. The earliest phenacodontid considered leading to the perissodactyl lineage was Tetraclaenodon. This fox-sized mammal appeared in middle Paleocene and gave rise to three main groups, one of which is presumed to be a proto-perissodactyl. However, so far, fossil intermediates have not been found between Tetraclaenodon and what Leonard Radinsky considers to the earliest known primitive perissodactyl, Hyracotherium of the early Eocene.

The earliest ancestors of ungulates are poorly represented in Paleocene strata. The generally accepted picture is that ancestral ungulates evolved from a group of primitive, small-bodied mammals belonging to the order Condylarthra, the most primitive of which was probably Protungulatum. Condylarths themselves first appeared in the late Cretaceous but are one of the most characteristic groups of mammals of the Paleocene. Unlike earlier insectivorous mammals, most condylarths were probably omnivorous having bunodont cheek teeth suitable for crushing and grinding, differentiated canines, and elongated skulls. The name "condylarth" was originally given to the earliest fossil herbivores considered ancestral to ungulates by the American paleontologist E. D. Cope (1840–1897). Later it came to encompass a wide array of ancient mammals, leading some paleontologists to question the order's validity. It contains a diverse, paraphyletic assemblage

even amongst extant taxa, including the ancestors of artiodactyls, perissodactyls, cetaceans, sirenians, proboscideans, and hyraxes.

Later in the Paleocene, the Paenungulata, also called primitive or sub-ungulates, diverged from the Condylarthra. Except for the embrithopods and desmostylians, the other three orders within this superorder persist to this day, forming an interesting and diverse grouping; the aquatic Sirenia (dugong, sea cow and manatees), the Proboscidea (elephants), and the Hyracoidea (hyraxes or dassies). The relationship of cetaceans (whales) within the ungulates is somewhat controversial. Until the application of molecular techniques, morphological and fossil evidence suggested that whales had probably evolved from the mesonychids, an offshoot of the condylarthran arctocyonids considered to have given rise also to the early ungulates. In 2000, the discovery of early fossil whales in Eocene deposits from Pakistan, show that they had a double-pulley astragalus (anklebone) suggesting that cetaceans were closely related to the artiodactyls. Molecular evidence also pointed to the cetaceans being most closely related to the predecessor of the modern hippopotamus. This has led some to suggest that cetaceans should be included in the Artiodactyla and the name changed to the Cetartiodactyla. However, the earliest fossil hippos do not appear until much later than these early cetaceans, and computer analyses using morphological data from living and fossil forms also cast doubt on the hippo-whale connection. No matter whether morphological or molecular analyses are used, whales are still most closely related to artiodactyls and belong in the Ungulata.

Besides providing the ancestors of the modern ungulates, paenungulates and cetaceans, the Condylarthra also probably gave rise to several other ungulate groups including the tillodonts, litopterns, notoungulates, tubulidents and astrapotherians. The Tillodontia is an order with uncertain affinities that is found in deposits from Asia, Europe and North America. It probably evolved from the condylarths and its members had a pair of enlarged rodent-like incisors. The condylarths also gave rise to three orders that were restricted almost entirely to South America. The Litopterna, ecologically and morphologically reminiscent of modern camels and horses, had a mesaxonic limb with the weight borne by the third digit. This order included two major families: the proterotherids which evolved one- and three-toed species, illustrating another case of parallel evolution this time with the equids; and the macrauchenids, which also showed a reduction to three toes, and in some later species, the suggestions of an elongated, trunk-like upper lip that would have been used for feeding. A second order was the diverse Notoungulata, containing some of the oldest South American mammals. Notoungulates evolved a great diversity of species and form from small, rabbit-sized creatures to the giant rhinoceros-like toxodonts. This was a long-lasting order that spanned into the Pleistocene until artiodactyls, perissodactyls and proboscideans migrated into South America and out-competed the resident notoungulates. The third South American order of primitive ungulates was the Astrapotheria represented by the rhinoceros-sized Astrapotherium, which lived in the

Miocene, and based on cranial anatomy, most probably had a trunk.

The last two groups, Notoungulata and Astrapotheria, are in many ways so unlike the other early ungulates that they were not always classed with them. Although its origins are unknown, the order Tubulidentata (aardvarks) is included within the primitive ungulates based on various anatomical features. At one time, its specialized food habits, enamel-free teeth, tubular skull and long tongue, led taxonomists to class aardvarks together with the New World edentates (anteaters), but this was not supported by other key anatomical features. Much of their specialized morphology for termite feeding had been acquired by the Miocene. Besides the Tubulidentata, a second order, the Periptychoidea, differed significantly from other primitive ungulates in their dentition, brain morphology, and structure of the limbs. They sometimes were classed with pantodonts. Although some members resembled tubulidentates, the periptychoids were herbivores, lacking the characteristic skull and teeth of the termite-eating aardvark. In North America, they were the dominant condylarths in the early Paleocene ranging in size from rat- to sheep-sized forms.

The generally accepted interpretation of the phylogenetic relationships of ungulates described above was based on morphological evidence provided by fossil remains. This is now being challenged by new evidence based on genetic distances, which present a quite different view of mammalian phylogeny. Genetic data suggest that there are four superorders of mammals, Afrotheria, Xenarthra, Eurarchintoglires, and Laurasiatheria. The Sirenia, elephants, hyraxes and aardvarks are separated from the rest of the ungulates and placed into the Afrotheria, while all other modern ungulates would be placed in the superorder Laurasiatheria.

The evolutionary history of the ungulates has taken place against a backdrop of major climatic and environmental changes. These changes and their effects on plant life and habitats have driven the evolution of ungulates and other mammals. Starting in late Cretaceous and early Paleocene when primitive mammals considered ancestors of ungulates appeared, the world's temperature began increasing with only a slight cooling phase around 58 million years ago (mya) towards the end of the Paleocene. The warming trend continued, with mean annual temperatures reaching a maximum about 50 mya between the early and middle Eocene. After this, temperatures declined, at first steadily but around 35 million years ago, they dropped dramatically at the Eocene-Oligocene boundary. This was probably the result of major changes in the patterns of oceanic circulation, brought about by continental drift that had separated Greenland from Norway, and Australia from Antarctica. As vertebrate paleontologist Professor Christine Janis described, it was a "transition from the Mesozoic 'hothouse' world to the 'ice house' world" of the Neogene (Miocene and Pliocene). A much slower warming of global temperatures followed, peaking in the late Middle Miocene, but at a temperature only about half that of

the Eocene maxima. Thereafter, global temperatures once again began to cool, and from the Pliocene to present they have continued to fluctuate, but with a significantly lower amplitude and with lower maxima than in the previous epochs. Such fluctuations exerted changes to terrestrial plant communities and acted as selective pressures upon the ungulates that exploited those communities in terms of adaptations of their teeth and digestive systems to cope with the problems of eating plants.

In the northern continents, the Middle Paleocene saw the appearance of small herbivorous condylarths (phenacodontids, meniscotheriids), and also larger herbivores such as the pantodonts (including Barylamda estimated to weigh over 1,323 lb or 600 kg). Even larger were the rhinoceros-like and possibly semi-aquatic uintatheres, of which Uintatherium was estimated to weigh as much as 9,900 lb (4,500 kg). Condylarths, pantodonts, and uintatheres were present in both Asia and North America, while Tillodonts migrated into North America from Asia in the late Paleocene. In South America, the Paleocene ungulate fauna included small condylarths, the dilododontids, rhinoceros-like astrapotheres, xenungulates, litopterns, and notoungulates. By the middle Paleocene, new ungulates had appeared, some with mesodont cheek teeth, and by the late Paleocene, early South America ungulates had evolved cheek teeth that showed dental adaptations to a more fibrous diet, indicating a probable shift to more temperate, open forested habitats as the world climate became warmer. At this time, palaeanodonts, uintatheres and arctostylopids (hyrax-like notoungulates), might have dispersed from South to North America.

The warming trend continued from the Paleocene into the beginning of Eocene, and faunas indicate generally tropical habitats in the north. Conditions appeared to shift towards drier environments than in the Paleocene, creating more diverse understory habitats in which browsing ungulates could flourish. The first artiodactyls and perissodactyls appear relatively abruptly in fossil record at the beginning of the Eocene in strata from Asia, Europe, and North America. This is a major

mystery in mammalian evolutionary history. It is speculated that that their "sudden" appearance in Holarctic strata might have been because they originated elsewhere, such as in Africa, Central or South America, or India, and that the disappearance of physical barriers or the warming climate in the early Eocene allowed them to disperse. A significant faunal exchange did take place between North America and Europe starting in the early Eocene and continuing until the middle Eocene, but no evidence has yet been found for any faunal interchange with South America during the Eocene. In South America at this time, ungulates and condylarths became rare and xenungulates disappeared, while small to medium-sized litopterns and notoungulates and larger astrapotheres and pyrotheres all diversified.

Ungulates with browsing and frugivorous feeding habits also appeared, and became established by the late Eocene. At this time in North America, there was an increase in ungulates with cursorial adaptations and hypsodont cheek teeth with high crowns and short roots. Similarly in South America, a change to more fibrous vegetation is suggested by notoungulate cheek teeth because they show a greater degree of hypsodonty.

Towards the end of the Eocene, major extinctions occurred within the North American fauna, almost certainly the result of the climate change that brought about a pronounced seasonality in plant production, and hence in food supply. Many archaic ungulate groups were lost including condylarths, uintatheres, and protoungulates such as tillodonts, as well as some artiodactyls and perissodactyls. Body size increased in surviving ungulates, and the chewing surface of their cheek teeth became more complex, both probably reflecting a diet that was more fibrous, hard to digest, and less nutritious. This was the time when the artiodactyls began to flourish and diversify with the establishment of three major lineages: suines, tylopods, and ruminants. However, the perissodactyls still dominated despite having generally declined through the late Eocene in both diversity and abundance. In Europe, the impacts of climate change were not expressed until into the Oligocene, but when the climate change occurred, it too was accompanied by significant faunal extinctions. Asia, on the other hand, seemed to miss the Eocene-Oligocene extinctions and archaic forms such as condylarths, along with the more advanced pantodonts, brontotheres and tapiroids, which lingered until the mid-Oligocene.

The Oligocene was drier and cooler than the preceding Paleocene and Eocene epochs, especially in North America. The extent of tropical forests was restricted and large areas of open habitats had yet to develop, while in the northern latitudes, the broad-leaved deciduous forests in the polar region were replaced with temperate broad-leaved deciduous woodlands. Conditions were relatively stable through much of the Oligocene, although there was a gradual shift towards more seasonal vegetation types. This probably accounts for the replacement of many Eocene perissodactyl herbivores by folivorous artiodactyls (eating mainly leaves), which might have been able to feed more selectively. The most common artiodactyls were goat-sized browsers resembling modern capybaras. Hippo- and equid-like rhinocerotoids were found throughout Asia, Europe and North America, while true rhinos were small to medium-sized, either lacking horns or having horns with bony cores unlike extant species. Llama-sized camels and browsing, pony-sized equids were common in North America. Other early ungulates included anthracotheres and small, hornless pecoran ruminants in Eurasia, and oreodonts in North America. Most Oligiocene ungulates with their shorter limbs were not as well-adapted to cursorial locomotion than are modern forms. It was not until about 26 mya at the beginning of the Late Oligocene, that a second period of mammalian extinctions began. For the ungulates, this was marked mainly by the disappearance of Eocene forms including archaic rhinocerotoids, and anthracotheres in North America and paleotheres and tylopod artiodactyls in Europe.

The Miocene climate became much drier and warmer. Subtropical and tropical forest zones expanded once more, and a dry thorn-scrub developed on the western edges of the continents. In general, mammalian faunas were somewhat comparable to modern-day savanna-woodlands. Lowered sea-levels towards the end of the Early Miocene, allowed faunal exchanges between North America and Eurasia and between Africa and Eurasia. Ungulate migrations included browsing horses from North America to Eurasia, cervoid percoran ruminants and rhinos from the New to the Old World, and proboscidians from Africa to Europe. Bovids, cervids, giraffids, and giraffe-like palaeomerycids appeared in the early Miocene of Europe. These relatively large animals, which replaced the earlier, smaller ruminant forms, were notable for the diversity of their species-specific horns or antlers. Suids diversified at this time to include browser-types with tapir-like incisors. While the traguloid ruminants declined, other artiodactyls became more diverse by the middle Miocene and the bovids developed hypsodont cheek teeth. Size also increased, especially in the bovids and giraffids. In North America, camel diversity increased and the earliest equine Merychippus appeared, both groups later developing hypsodont dentition. Later in the Miocene of North America, the antilocaprids diversified as did the deer-like dromomerycids. Although there was a trend to more open grassland savannas in North America during the Miocene, sufficient productive woodlands remained for browsing peccaries and tapirs to survive. In South America, woodlands gave way to more open habitats as indicated by the trend towards increased hypsodonty (high-crowned teeth), which characterized Miocene herbivores on this continent. Litopterns adapted to these open habitats evolving single-toed forms, and although the notoungulates showed similar dental adaptations, their diversity declined during this epoch. In mid-Miocene Africa, woodland ungulates diversified including rhinos, bovids, giraffoids, suids, and traguloids.

The steady cooling trend and further drying in the late Miocene was accompanied by an expansion of savannas and a decrease in mammalian faunal diversity. Grasses had been gradually spreading since their appearance in the Eocene, but open savannas were probably not extensive until the Late Miocene, which was about the time that C₄ grasses (photo-synthesis takes place in chloroplasts) are first recorded. While C₄ grasses were taking over from C₃ grasses (photosynthesis occurs within the mesophyll) in Asia, the dominance of C₄ grasses in temperate regions occurred later in the Pliocene. True grasslands did not appear until much later, probably in the Pleistocene, but savanna grasslands were extensive in North and South America. The development of Late Miocene open savanna-like habitats in Eurasia was accompanied by hipparionid horses developing hypsodont dentition and three-toed feet, and by hypsodont bovids. Interestingly, however, the hypsodont dentition was not indicative of predominantly grazing diets. Examination of the micro-wear patterns on the teeth of these and other herbivorous ungulates suggest that feeding habits were predominantly mixed grazing-browsing. Presumably the habitat was of an open wooded savanna-type, rather than being primarily grassland. The rise of C₄ grasses and the increasing aridity of habitats are suspected to be a main selective force towards full hypsodonty. C₄ grasses contain up to three times as much silica as do C₃ species and hence teeth of animals feeding on them will wear down more quickly. In Africa around this time, more modern forms of bovids giraffes and hippos evolved, while North American hipparionid horses entered the continent via Asia and Europe.

The close of the Miocene was accompanied by major extinctions, especially of savanna-dwelling mammalian fauna, across Eurasia and North America in association with colder climates and habitat changes. Tundra and taiga ecosystems developed in the northern latitudes, and extensive dry grasslands to the south, and by the beginning of the Pliocene an ice cap may have existed in the Arctic. Global temperatures had begun to rise before the beginning of the Pliocene and continued until about 2.5 mya in the Late Pliocene, leading to the development of desert and semi-desert habitats. Modern Equus took the place of hipparionid horses in Eurasia, and along with camels, both originating in North America, entered Africa around 2.5 mya.

Around the beinning of the Pliocene, 3 to 2.5 mya, continental processes and possibly a lowering of the sea level created the Panamanian land bridge between North and South America. Animals were once more able to move between the two continents that had been separated for over 100 million years. What followed is referred to by paleontologists as the "Great American Interchange." These faunal exchanges of large mammals, which peaked in the middle Pleistocene, involved equids, tapirs, peccaries, llamas, and deer moving from North to South America along with gompotheres (related to elephants) and some large carnivores. Of the South American ungulates only a few notoungulates moved north, although two other large herbivores, glyptodonts and giant ground sloths, also entered North America. Among the ungulates that emigrated to South America, the equids eventually became extinct, as they did in North America, although these perissodactyls did survive in Africa and Eurasia.

By the end of the Pliocene global temperatures were dropping, and the following Pleistocene was a time of alternating glacial and interglacial periods repeated on a roughly 41,000-year cycle. Apart from Antarctica, most of the ice accumulation on land was in the northern hemisphere especially in North America. As this ice formed huge glaciers and ice caps, sea level fell and a land bridge formed once more between

Eurasia and North America. The interchange between Eurasia and North America was not symmetrical, with more species moving into the New World than moved in the other direction. Bovids such as bison, mountain sheep and mountain goats, mammoth along with other mammals such as wolves and cave lions, migrated across the Bering land bridge from Siberia into Alaska, and when the ice masses retreated, made their way south into the rest of the continent. Humans also entered North America around this time over the land bridge and perhaps also down the west coast of the continent. Modern horses and camels were the two ungulates that moved to the Old World and survived to the present day.

Towards the end of the Pleistocene, many species of large mammals became extinct, including many large herbivores. Eurasia lost mammoths and the woolly rhinoceros; North America lost mammoths, all but one member of the Antilocapridae, woodland musk ox, horses, and camels; and North and South America lost ground sloths, mastodons and glyptodonts. In South America, the litopterns—the last of the indigenous ungulates on this continent—finally vanished. Other species of mammals besides ungulates disappeared, and in North America, a total of between 35 and 40 large mammal species vanished between 12,000 and 9,000 years ago. Three main hypotheses have been proposed to explain these megafaunal extinctions: climate, overkill by humans, and disease (epizootic). Each has supporting evidence and some scientists suspect that the extinctions might have resulted from a combination, at least of the first two factors. Certainly climate change and the appearance of humans in North America, coincide with extinctions of large mammals.

Physical characteristics

The largest land mammal that has ever existed was an ungulate, Paraceratherium Indricotherium transouralicum. This was a Perissodactyl that belonged to the rhinoceros family and it roamed central Asia about 35 mya during the late Eocene. Although not as big as the largest dinosaurs, Paraceratherium stood over 15 ft (5 m) at the shoulder with a skull 4 ft (1.3 m) in length. It could probably browse tall vegetation to a height of 25 ft (8 m). Paraceratherium was estimated to have weighed about 20 tons (18 tonnes), or about four times heavier than the largest weight recorded for modern African elephants. This feature of relatively large body size extends to modern ungulates although some species are small. Weighing around 1.6 tons (1.5 tonnes), the hippopotamus is the heaviest of the living ungulates, while the smallest are the mouse deer or chevrotains (Tragulidae), which can weigh less than 2 lb (1 kg). The tallest living mammal, the giraffe, which stands almost 19 ft (6 m) tall, is also an ungulate. Living perissodactyls are all large animals from 440 to 7,700 lb (200–3,500 kg). The largest artiodactyls are heavier and their weights span a greater range, from 2 to 10,000 lb (less than 1 kg to about 5,000 kg).

As stated earlier, hooves surrounding the terminal toe bones or phalanges of their feet, are unique at least to modern ungulates. The hooves are composed of a hard protein material called keratin that creates a tough outer sheath protecting the terminal toe bones. Immediately under the hoof is the softer sub-unguis, and behind this is the pad. The hard outer sheath is technically the unguis and derives its name from the Latin ungula for claw or nail. The word "ungulate," the alternative name for hoofed mammals, also originates from the Latin word ungula, meaning hoof. Hooves are modified claws or nails found in most other groups of mammals.

Besides their feet, other characteristic adaptations of ungulates include their limbs, eyes, digestive systems, and teeth. They also show a wide diversity of weapons and head ornamentation such as tusks, horns, and antlers, although amongst the modern perissodactyls, only the various species of horned rhinoceroses have such specialized weapons. All of these morphological adaptations evolved to help ungulates avoid predators, gather and process food, and to interact with other members of their species for survival and reproduction.

Two of the biggest challenges facing ungulates are to avoid being killed by predators and to extract sufficient nutrients from the plants they eat. Ungulates are the main prey of large mammalian predators, and the resulting intense and constant selection pressure has led to several adaptations that help ungulates reduce the risk of being killed by a predator. The position and structure of their eyes are adapted for predator detection, their pelage and other features can make them more difficult to detect by predators, and finally, their limbs are adapted for running fast which helps them escape from predators.

Disruptive or camouflaged coats that help hide from predators are common in forest-dwelling ungulate species. Coat color can blend with the surroundings, while disruptive color patterns such as light stripes against a dark background, help break up body form in the dappled light of the tropical forest or savanna bushlands; both make detecting potential prey more difficult. Disruptive striping is seen in many tragelaphins such as the bongo (Tragelaphus eurycerus), and in the tiny forest-dwelling chevrotains (Tragulidae). Small size can also make it easier to hide and some species, such as duikers (Cephalophinae), literally dive into dense vegetation to hide from predators. This behavior has given rise to their common name which, is derived from the Dutch word for diver.

It is more difficult for large species, especially those living in open habitats, to hide. Instead, they rely primarily on vision to detect predators well in advance, and then on speed to outrun them. Ungulate vision is adapted for detecting movement over a wide field of view. The basic anatomy of their eye is similar to that of other mammals, but in the ungulate there is no central focusing spot or fovea, which allows discrimination of fine details. Also, the ungulate pupil is elliptical rather than round, and oriented horizontally, while the eyes are located on the sides, rather than on the front of the head. Eyes in this position, together with elliptical pupils, give most ungulates a field of view over 200°, and some can probably see almost as well behind as in front or to the side. So while ungulates might not easily distinguish fine details, they readily detect movements almost anywhere around them and so have a good chance of detecting approaching predators. Whether ungulates have color vision is uncertain, but based on the internal structure of their eye, they probably can distinguish some colors, but not as well as do humans, for example. It is also suggested that in some forms, the elongated skull (especially the muzzle region) not only aids with feeding, but at the same time allows an ungulate to keep its eyes above the vegetation and wary for predators. Extreme examples of elongated skulls and high placement of the eyes is seen in the hartebeests (Beatragus, Damaliscus, Alcelaphus) of Africa.

Very few species of ungulates are large enough, compared to their common predators, to be able to defend themselves physically. This is especially true if the predator normally attacks as a group. African buffalo, a large-bodied species with heavy horns, will sometimes cooperatively defend themselves against a predator such as a lion. Similarly, musk oxen will form a defensive ring against a wolf pack, shielding the young calves between their bodies. Defense, however, is the least common anti-predator strategy used by ungulates.

Instead of defense, most ungulates flee from their predators. Their limbs are adapted to allow them to run fast. The evolution of ungulate limbs is one of a gradual change towards longer, lighter limbs, with smaller, lighter feet, and specialized limb joints. Long limb bones provide an obvious advantage for speed because they provide a longer lever action, and thus longer stride length. The greatest increase in leg length came through elongation of the metapodials, the bones between the wrist/ankle and the fingers/toes. Lighter limbs also help improve speed because they require less effort to overcome the inertia of locomotion. Lightness results from musculature in the lower extremities being replaced with tendons and ligaments, and by smaller feet resulting from a decrease or loss of the lateral toes. Although the rhinoceros and hippopotamus do not show the extreme limb adaptations common in other modern ungulates, their limbs still illustrate the same basic patterns and they are capable of surprising speed.

Ancestral ungulates had the basic vertebrate plan of five digits, but during their evolution between one and four outer toes were lost, and often the outer metapodial bones were also lost or reduced in size. The number of functional toes remaining is key to whether the ungulate is classified as perissodactyl or as artiodactyl. Artiodactyls have lost the first toe and three metapodials. What remains are the two central

metapodials fused into a single unit, two functioning toes (the third and fourth), and two greatly reduced outer toes (the second and fifth) called lateral hooves or dew claws. All the toes including the dew claws are covered by hooves. A similar reduction in the number of toe bones occurred in perissodactyls, with the most extreme found in the modern horses. Zebras, asses, and horses have only the third metapodial, the cannon bone, and the associated single toe has been retained, though vestigial metapodials or splint bones may persist. In rhinos, the number of toes has been reduced to three. The result of all these reductions in the number of toes is that most modern ungulates walk with their weight on their hooves or tips of their toes. This type of gait is referred as unguligrade, and it helps the animal to move quickly over hard ground. This contrasts with plantigrade found in bears and humans which walk on the soles of their feet, and digigrade found in cats and dogs and many other species where the weight of the body is taken by the entire digits not just by the tips. The very heavy-bodied ungulates which rely less on speed, such as the rhinoceros and hippopotamus, have feet that are rather short and wide, with splayed digits that are needed to support their mass.

Increased speed and efficiency for cursorial locomotion have also been achieved through adaptations of the limb joints, and the way that they attach to the vertebral column. Many ungulate limb joints restrict movement to a strong and efficient forward-backward movement, which means that power is transferred more effectively to forward movement. For example, the ungulate astragalus is grooved, on the distal end in perissodactyls and at both ends in artiodactyls, to minimize lateral movement of the ankle joint during articulation.

The ungulate hind legs are firmly attached to the sacral section of the vertebral column via a strong ball-and-socket joint between the femur and the pelvis. In this way, the force of the limb motion is transferred directly to the body through the backbone, propelling the animal forward with each powerful stroke of the hind limbs. The front limbs in ungulates are not firmly attached to the rest of the skeleton. As in most cursorial mammals, the last upper bone of the forelimb, the scapula or shoulder blade, is not attached to the backbone, but is attached only by muscles to the upper thorax. This cushions the running animal when its forelimbs hit the ground. Additionally, flexibility is gained because ungulates, and other cursorial mammals, have lost the clavicle or collar-bone. Its absence allows the shoulder blade to move relatively freely when the forelimbs swing forward, increasing stride length and thus speed.

Besides evading and outrunning predators, ungulates also have to acquire energy and because most are herbivores, this energy must be extracted from plants. Plants are difficult to digest, and to help maximize the nutritional value of their food, ungulates not only have specialized teeth to crop and chew plants, but also unique digestive systems. In some species, even their body shape helps them feed more effectively (e.g., giraffes and gerenuks). Last, ungulates are generally social species forming small groups to large herds that lead to a range of communication systems, mating strategies, and fighting styles.

At a broad evolutionary level, herbivores can choose one of two approaches: they can feed on low-quality but abundant forage, or they can feed on high-quality but uncommon forage. These different approaches to the exploitation of plants have morphological consequences that affect feeding efficiency and thus energy acquisition. High-quality forage is typically sparse, hence many ungulates depend heavily on grasses and shrubs, which are usually abundant but generally low in nutritional quality. To help them gather sufficient quantities of food, the largest species have wide mouths and spatulate cropping incisors, allowing them to take large bunches of forage into their mouths. Many smaller species have narrower mouths that permit them to feed more selectively on the lowabundance but high-quality forage.

Ungulates, like most mammals, have heterodont dentition: incisors, canines, premolars, and molars. Except for the Suidae and Tayassuidae, which have less specialized teeth, most ungulates have highly specialized dentition that reflects their wholly herbivorous diet. To meet their energy requirements, these obligate herbivores must eat a lot of vegetation relative to their body size. Large browsing and grazing species such as the giraffe or bison use their tongue, wrapping it around a clump of vegetation to pull it into their mouth. Grazing Perissodactyls such as the Equids have retained the upper incisors and use these with the lower ones to crop grasses close to the ground. By contrast, grazing Artiodactyls lack upper incisors and instead grasp the food between their modified spatulate lower incisors and their hardened upper palate, and by jerking their mouth upward tear off a mouthful of forage. Browsers generally have narrower muzzles and incisor tooth rows than do grazing species. The former's narrow muzzle allows them to select the more nutritious parts of plants such as leaves and tips of twigs.

In most ungulates, there is a characteristic space between the lower canines and the first of the premolars, called the diastema. This might aid browsers to strip off leaves or simply be a result of an elongated jaw that evolved to allow greater feeding selectivity, and perhaps helps keep their eyes above the vegetation while feeding so they can watch for predators.

Besides the muzzle and incisors, the cheek teeth of herbivorous ungulates also show diet-related adaptations. Plants not only need to be chewed into small pieces to help digestion, but some plants such as grasses are highly abrasive. The crown of a mammalian tooth is covered in a layer of enamel, which is the hardest and most wear-resistant part of the tooth. The cheek teeth, or premolars and molars, are used for chewing, and these teeth show the greatest specialization. The molars of Suidae and Tayassuidae have bunodont teeth in which the crown consists of low conical cusps covered by a layer of enamel. These teeth are suited to masticating their diverse and generally softer, less abrasive foods. In the obligate herbivorous ungulates, however, the crown enamel of the cheek teeth is highly modified and formed into lateral or vertical folds. The tips of these enamel folds wear off quickly, creating several hard cutting edges juxtaposed to layers of softer dentine. Differential wear allows these self-maintaining ridges of enamel to create a rough surface that helps grind plants into small pieces. When the grinding surface is examined, two general enamel patterns are seen in the herbivorous ungulates: the selenodont or crescent-shaped pattern, and the lophodont or convoluted pattern. The selenodont pattern is found in artiodactyls but not in modern perissodactyls. The lophodont pattern, though typical of modern perissodactyls, is also found in premolars of some artiodactyls.

Tooth wear, especially in herbivores, is a major determinant of an animal's life span. Abrasive foods naturally wear teeth more quickly, thus potentially shortening the time that cheek teeth function to grind vegetation. Browsers (e.g., moose) consume mainly leaves, twigs, fruits, buds, and young shoots of woody plants—all foods that are much softer than tooth enamel. Species with these food habits have brachydont or low-crowned teeth, with selenodont or lophodont enamel patterns. Grazers (e.g., gnu), on the other hand, eat primarily grasses and forbs, and most grasses have phytoliths in their epidermal cells. These phytoliths are comprised of silica that abrade even the hard tooth enamel. This abrasion problem is compounded for ungulate species living in relatively dry areas, because plants growing there are invariably coated with a fine layer of silica dust that further abrades the teeth. Grazers have evolved two main adaptations to resist tooth wear. First, the diameter of the premolars and molars is enlarged, increasing the grinding surface so that the tooth takes longer to wear away. Second, in the most highly specialized grazers, the molars and sometimes premolars are hypsodont or high-crowned, having tall crowns and short roots, so again these teeth take longer to wear away, thus extending the life of the animal.

Feeding ecology

Pigs and peccaries are not obligate herbivores and eat foods other than plants. Although they mainly eat vegetation, they consume everything from roots, bulbs, and fruits to bird's eggs and insects. Of greater relevance to the evolution and radiation of ungulates are the specializations in the exploitation of plants. All other artiodactyls and perissodactyls feed almost exclusively on plants, although the types of plants eaten vary greatly. These obligate herbivores can be divided into either grazers, browsers, or mixed grazer-browsers.

The leaves and twigs of plants contain a great deal of potential energy, but they are difficult to digest because their cell walls contain cellulose and sometimes lignin. This energy is not directly accessible because no ungulate (nor any other mammal) has enzymes that can digest these two constituents. Ungulates evolved adaptations to overcome this problem by using microorganisms and fermentation to digest cellulose and some lignins. The mainly bacterial microorganisms produce the enzyme cellulase that breaks down cellulose, while the fermentation process continues the breakdown of cellulose into simpler compounds called volatile fatty acids. The main fatty acids, acetic and propionic acid, are absorbed directly into blood vessels in the gut wall, transported to the liver, and metabolized as the ungulate's primary energy source.

Artiodactyl and perissodactyl ungulates evolved distinctly different strategies for where this symbiotic microbial fermentation occurs in their digestive systems. In artiodactyls, it takes place at the front of the digestive system, referred to as fore-gut fermentation. Depending on species, there are between one and three chambers or false stomachs located before the true stomach. In the first of these, in the fore-stomach or rumen, the microbes ferment ingested plants. The most successful of the artiodactyls have evolved an added adaptation: regurgitating food (cud) to rechew it into smaller food particles, thus increasing the surface area on which digestion by microorganisms can occur.

In perissodactyls, microbial fermentation does not occur until the food has passed through the stomach, along the small intestine, and has reached the enlarged cecum located towards the end of the digestive system. Not surprisingly, this approach is referred to as hind-gut fermentation. Perissodactyls only chew their food once, and have a single stomach where digestive enzymes are released. Food is further digested in the intestines where proteins are broken down to amino acids, and sugars and carbohydrates to glucose, before being absorbed. The remaining undigested food reaches the cecum and there it is further digested by bacterial fermentation which

breaks down cellulose and other plants components into volatile fatty acids. As in the ruminant, these microbial byproducts are then absorbed and used for energy.

These two fermentation systems differ in their relative benefits and costs. There are at least two and possibly three significant benefits enjoyed by the ruminant artiodactyls. First and probably most important is the ability to regurgitate and rechew food. This increases digestive efficiency simply because it more effectively breaks the consumed plant material into very small particles, increasing the surface area on which microbes can operate and thus facilitating digestion. A second benefit comes from having the fermentation occur at the beginning of the digestive system. It means that the ruminant can obtain a valuable protein source by digesting the microorganisms themselves when they are transported out of the rumen with the digestia. Artiodactyls with greatly enlarged fore-stomachs gain a third benefit: they can harvest a large amount of food and then move to safer areas to digest it. The main cost of the fore-gut fermentor system occurs when food is of low quality. Rumen fermentation is slowed when low-quality food is ingested because crude protein in the diet limits microbe population growth and thus microbial digestion is impeded. This slower fermentation diminishes the amount of energy available to the ruminant. The ruminant is faced with being more selective in its feeding. In constrast, the hind-gut fermentors, such as some perissodactyls (e.g., modern horses), are able to increase the rate that food passes through their gut, so they extract only the most readily digestible fraction of the food and excrete the undigestible material. As a result, although they must feed almost continuously, they can be much less selective in what they eat. This allows horses to survive on poorer-quality food than artiodactyls are able to do.

Body size and feeding strategy

To survive, any animal must balance its energy budget by meeting its energy requirements through food intake and by limiting energy losses or expenditures. There are three principal constraints to meeting energy requirements through food intake: 1) the quality of available food, 2) the animal's metabolic requirements, and 3) the animal's physical capacity to eat (primarily regulated by its mouth size and stomach capacity).

The energy requirements of any mammal are approximately proportional to its body weight to the power 0.75 (W^0.75), and are closely linked to heat transfer and hence to surface area of the body. This general relationship has been shown to hold for comparisons among species, but not necessarily within species (e.g., between sexes, or between juveniles and adults). This relationship is partly explained by considering that heat transfer is a function of surface area and so larger animals have a smaller surface area-to-volume ratio than do small animals. Also, not all parts of an animal metabolize energy (heat) at the same rate. In larger animals, a higher proportion of their body is made up of structural components with relative low metabolic rates. As a consequence, small animals have a greater maintenance cost per unit body weight than do large animals.

Many aspects of the body relate to metabolic size and energy requirements, but at some point they are limited by the animal's physical size. This is true of the gastrointestinal capacity of the digestive system. One consequence is that smaller herbivores cannot develop a sufficiently large gastrointestinal tract to match their needs. Ideally, animals evolve a gastrointestinal capacity that match their metabolic requirements, but because of the link between gastrointestinal capacity and physical size, small herbivores cannot use this strategy. Small herbivores need more energy per unit of body size because of higher metabolic requirements, but their small gastrointestinal capacity limits the amount of food they can process. There are least three ways to meet this need, but because of their relatively high metabolic requirement and small gastrointestinal capacity, small herbivores (including ungulates) gain the greatest benefit from consuming higher-quality diets than do larger herbivores. This relationship between selection for high-quality foods and small body size is apparent within the Artiodactyla, especially the Bovidae. Small species of antelope, such as dik-dik, feed primarily on plants and plant parts with high nutrient quality, whereas large species such as African buffalo have broader diets and consume large quantities of low-quality forage such as grasses.

Social behavior

Ungulates are social mammals; most live in groups that can range from a pair to several thousands, although a few can be relatively solitary. Group size seems generally related to the visual density of the habitat; large groups form in open areas, and smaller ones in more closed habitats. In most species, males and female are sexually segregated and live in separate groups for most of the year, coming together during the mating season. However, in some species (e.g., equids, vicuñas) an adult male will live with a group of females and young throughout the year. The most likely reason for sexual segregation is that the sexes have different requirements for food and security habitat. Adult females need areas for raising their young that are relatively safe from predators, even if this means feeding in areas with poorer forage conditions. Males require abundant, high-quality food so they can maximize their growth and body condition for competing with other males for females. Often these different requirements can only be met in different habitats, leading to sexual segregation.

Ungulates enjoy at least two main benefits from living in groups: reduced predation and greater feeding efficiency. Reducing the chances of being killed by a predator is the main reason ungulates live in groups. Group living means that an individual can use other group members to hide behind when a predator attacks, and it also means greater vigilance because there are many pairs of eyes to keep watch for predators. Because there will always be several group members scanning for predators, other individuals can spend more time feeding than if they were alone. In species that are large compared their common predators, they might have a better chance to defend themselves collectively as a group than if they tried to defend themselves alone. However, the most important benefit ungulates gain from group living is the dilution effect. This arises simply because for each group member, the probability of being killed when a predator attacks, is inversely proportional to group size. Even in a group of two, when attacked by a single predator, the probability of an individual being killed is reduced by 50%.

Many ungulates use chemicals to communicate with each other, often depositing them on the ground, bushes or other places where conspecifics will encounter them. This means that an animal can make its presence known without actually having to be present, which can be useful for species defending large territories. Urine and feces are commonly used chemical signals, but many species also possess glands. Many of these are called epithelial glands because they are modifications of the skin or epithelium. Glands produce odoriferous chemical secretions as either volatile chemicals or waxy material, and they use these chemicals to communicate with each other for a variety of purposes. The glands themselves can be found at different locations on the body depending on species. Typical paired glands are ant- or pre-orbital glands seen as depressions, pits or slits just in front of the eye. Others are found between the hooves of some artiodactyls (pedal glands), while others can be associated with the tail (caudal glands) and hind legs.

Reproduction and mating systems

Most wild ungulates breed only once each year and the timing of birth is the main determining factor, which in turn is related to the annual cycle of plant growth. The seasonal pattern of plant growth is governed primarily by moisture and temperature. In temperate regions, this means that plants begin to grow in spring when warmer days lead to higher soil temperatures and snow melt or rainfall is also sufficient. In warmer arid areas, plant growth begins with the rainy season. Newborn ungulates need to grow rapidly so they can minimize the chances of falling prey to predators, and consequently they need to be born as early in the plant-growing season as possible. Hence, they are born either in early spring or at the beginning of the rainy season. Gestation period and birth season effectively determine when mating takes place. In some species, this is up to nine months before births and often when the adults are in their best physical condition.

Ungulates have evolved a variety of mating systems, almost all based on polygynous mating in which one male mates with several females. Basically males either defend and court a single female (tending pair), or defend a group of females (harem), mating with each as they come into heat (estrus). Only a few species form pair bonds.

The mating period in ungulates is often referred to as the rut, and is the period when males seek females coming into heat, and when females try to select a suitable mate. Courtship helps both genders achieve their goals. Professor Niko Tinbergen, one of the founders of modern ethology (animal behavior), suggested that there are up to four main functions in courtship in animals: orientation, persuasion, synchronization, and reproductive isolation. In ungulates, persuasion is probably the most important, the rest have minor roles. Although male ungulates seem to be the most active using often elaborate courtship patterns and rituals, the apparently passive females play an important role.

When a female comes into heat, chemicals in her urine act as signals that males use as cues to her reproductive condition. Males seeking females coming into estrus usually approach in a submissive or non-aggressive posture, so they can get close to investigate her. Quite often the female urinates when the male approaches and the male then tests the urine using a behavior called flehmen or lip curl. It appears that by doing this, the male is using his paired vomeronasal organs located in his upper palate to test the urine. These organs are sensitive to the chemical cues found in an estrous female's urine. Once a female coming into heat is located, the male begins to court her with species-specific courtship patterns that, if she chooses, leads to copulation. The female's role in selection of her mate, while not overt, is critical to the evolution of the mating system.

Sexual dimorphism and elaborate weapons are both usually indications of competition for mates or for attracting mates, commonly associated with polygynous mating systems. In polygynous mating, a male will usually mate with more than one female, so only a few males in a population will have the opportunity to breed. This can lead to intense competition among males, often involving ritual displays and fighting. Charles Darwin was one of the first to recognize that animal weapons function primarily for intraspecific competition (competition between members of the same species), and are used only occasionally for defense against predators (inter-specific interactions between different species). Besides their use in fighting, ungulates also frequently use their weapons for displays.

Four basic weapons systems are recognized in ungulates. The simplest are the hard, often sharp hooves, though obviously these did not evolve primarily for fighting. Some species have long canine teeth that they use for fighting. Suoids have sharply pointed upper and lower canines (tusks), camels have smaller but sharp canine teeth, and some cervoids have relatively long, dagger-like upper canines. A third weapon type are antlers typical of modern cervid deer, though not all species have them. Finally, there are horns, which are found in four living families, each with its own unique type; the rhinoceroses, the giraffes, the bovids, and the pronghorn "antelope."

The various aspects of ungulate's lives are intertwined. The habitats a species uses influence food selection and impose constraints on body and group size. In turn these affect anti-predator strategies as well as mating systems. The intricacies of these various ecological processes provide the rich morphological and behavioral differences characteristic of ungulates in modern times. Also, it is these diverse and large herbivores that sustain large predator species. The ungulate herbivores and carnivores define the mammalian assemblages that form communities throughout the world except Australia. It is from these diverse ungulates that humans have drawn sustenance and domestic animals that have supported their development.

Resources

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David M. Shackleton, PhD

Alton S. Harestad, PhD