Evolution
In the early Miocene, bovids began diverging from the cervids (deer) and giraffids. The earliest bovids, whose presence in Africa and Eurasia in the latter part of early Miocene (20 Mya) has been ascertained, were small animals, somewhat similar to modern gazelles, and probably lived in woodland environments.[14] The Eotragus, the earliest known bovid, weighed 18 kg (40 lb) and was nearly the same in size as the Thompson's gazelle.[15] Early in their evolutionary history, the bovids split into two main clades: Boodontia and Aegodontia. This early split between Boodontia (of Eurasian origin) and Aegodontia (of African origin) has been attributed to the continental divide between these land masses. When these continents were later rejoined, this barrier was removed, and either group expanded into the other's territory.[7][16] The tribes Bovini and Tragelaphini diverged in the early Miocene.[17] Bovids are known to have reached the Americasin the Pleistocene by crossing the Bering land bridge.
Early Miocene and before
The present genera of Alcelaphinae appeared in the Pliocene. The extinct Alcelaphine genus Paramularius, that was the same in size as the hartebeest, is believed to have come into being in the Pliocene, but went extinct in the middle Pleistocene.[6] Several genera of Hippotraginae are known since the Pliocene and Pleistocene. This subfamily seems to have diverged from Alcelaphinae in the latter part of early Miocene.[17] The Bovinae is believed to have diverged from the rest of the Bovidae in the late Oligocene.[17] The Boselaphini became extinct in Africa in the early Pliocene, and their latest fossils were excavated in Langebaanweg (South Africa) and Lothagam(Kenya).
Middle Miocene
The middle Miocene marked the spread of the bovids into China and the Indian subcontinent.[15] According to Vrba, the radiation of the subfamily Alcelaphinae began in the latter part of middle Miocene.[6] The Caprinae tribes probably diverged in the early middle Miocene. Caprini emerged in the middle Miocene, and seems to have been replaced by other bovids and cervids in Eurasia.[19] The earliest fossils of the Antilopines are from the middle Miocene, though studies show the existence of the subfamily from the early Miocene.Speciation occurred in the tribe Antilopini during the middle or upper Miocene, mainly in Eurasia. Tribe Neotragini seems to have appeared in Africa by the end of Miocene, and had become widespread by the Pliocene.
Late Miocene
By the late Miocene, around 10 Mya, the bovids rapidly diversified, leading to the creation of 70 new genera. This late Miocene radiation was partly because many bovids became adapted to more open, grassland habitat.[ Aepycerotinae first appeared in the late Miocene, and no significant difference in the sizes of the primitive and modern impala has been noted. Fossils of Obivines, a tribe of Caprinae, in Africa date back to the late Miocene.[] The earliest Hippotragine fossils date back to the late Miocene, and were excavated from sites like Lothagam and Awash Valley.[17] The first African fossils of the Reduncinae date back to 6-7 Mya.[] Reduncinae and Peleinae probably diverged in the mid-Miocene.
Characteristics
The bovids show great variation in size. On the one hand, the gaur can weigh as heavy as one tonne (1,000 kg (2,200 lb)) and stands 2–3 m (6.6–9.8 ft) high at the shoulder.[23] The water buffalo can be even heavier, and weigh 1,200 kg (2,600 lb); though it is shorter than the gaur, being at most 2 m (6.6 ft).[24] On the other hand, the royal antelope is only 25 cm (9.8 in) tall and weighs at most 3 kg (6.6 lb).[25]The klipspringer is another small antelope, that stands 45–60 cm (18–24 in) at the shoulder and weighs just 10–20 kg (22–44 lb).[26]All bovids have the similar basic form - a snout with a blunt end, a pair of horns (generally present on males) immediately after the oval or pointed ears, a distinct neck and limbs, and a tail varying in length and bushiness among the species.[22] Most bovids exhibit sexual dimorphism, with males usually larger as well as heavier than females. All bovids have four toes on each foot – they walk on the central two (the hooves), while the outer two (the dew-claws) are much smaller and rarely touch the ground.
There are differences in pelage colouration as well, the colour ranging from a pale white (as in the Arabian oryx) to black (as in the black wildebeest). However, only the intermediate shades, such as brown and reddish brown (as in the reedbuck), are commonly observed.[27]In several species, females and juveniles exhibit a light-coloured coat, while that of males darkens with age. As in the wildebeest, the coat may be marked with prominent or faint stripes. In some species such as the addax, the coat colour can vary with the season.[28] Scent glands and sebaceous glands are often present.[22]
Excepting some domesticated forms, all male bovids have horns, and in many species females too possess horns. The size and shape of the horns vary greatly, but the basic structure is always a pair of simple bony protrusions without branches, often having a spiral, twisted or fluted form, each covered in a permanent sheath of keratin. The unique horn structure is the only unambiguous morphological feature of bovids that distinguish them from other pecorans.[29][30] A study revealed that there was a high correlation between horn morphology and fighting behaviour of the individual. For instance, long horns are intended for wrestling and fencing, whereas curved horns are used in ramming.[31] Another study found that males with horns directed inwards are monogamous and solitary; whereas those with horns directed outwards tend to be polygynous. These results were independent of the body size.[32]
Male horn development has been linked to sexual selection,[33][34] Horns are small spikes in the monogamous duikers and other small antelopes, whereas in the polygynous they are large and elaborately formed (for example in a spiral structure, as in the giant eland). Thus, to some extent, horns depict the degree of competition among males in a species.[27] On the other hand, the presence of horns in females is likely due to natural selection.[33][35] The horns of females are usually smaller than those of males, and are sometimes of a different shape. The horns of female bovids are thought to have evolved for defense against predators or to express territoriality, as non-territorial females, which are able to use crypsis for predator defense, often do not have horns.[35]
Anatomy
Dentition[edit]In bovids, the third and fourth metapodials are combined into the cannon bone. The ulna and fibula are reduced, and fused with the radius and tibia respectively. Long scapulae are present, whereas the clavicles are absent. Being ruminants, the stomach is composed of four chambers :the rumen (80%), the omasum, the reticulum and the abomasum. The ciliates and bacteria of the rumen ferment the complexcellulose into simpler fatty acids, which are then absorbed through the rumen wall. Bovids have a long small intestine; the length of the small intestine in cattle is 29–49 m (95–161 ft). Body temperature keeps fluctuating through the day; for instance, in goats the temperature can change slightly from nearly 37 °C (99 °F; 310 K) in the early morning to 40 °C (104 °F; 313 K) in the afternoon. Temperature is regulated through sweating in cattle, whereas goats use panting for the same. The right lung, consisting of four to five lobes, is around 1.5 times larger than the left, which has three lobes.
Most bovids bear 30 to 32 teeth.[27] While the upper incisors are absent, the upper canines are either reduced or absent. Instead of the upper incisors, bovids have a thick and tough layer of tissue called the dental pad, that provides a surface to grip grasses and foliage. They are hypsodont and selenodont, since the molars and premolars are low-crowned and crescent-shaped cusps. The lower incisors and canines project forward. The incisors are followed by a long toothless gap, known as the diastema.[36] The general dental formula for bovids is 0.0.2-3.33.1.3.3. Most members of the family are herbivorous, however, mostduikers are omnivorous. Like other ruminants, bovids have four-chambered stomachs, which allow them to digest plant material, such as grass, that cannot be used by many other animals. Ruminants (and some others like kangaroos, rabbits and termites) are able to use micro-organisms living in their guts to break down cellulose by fermentation.
Dentition
Most bovids bear 30 to 32 teeth.[27] While the upper incisors are absent, the upper canines are either reduced or absent. Instead of the upper incisors, bovids have a thick and tough layer of tissue called the dental pad, that provides a surface to grip grasses and foliage. They are hypsodont and selenodont, since the molars and premolars are low-crowned and crescent-shaped cusps. The lower incisors and canines project forward. The incisors are followed by a long toothless gap, known as the diastema.[36] The general dental formula for bovids is 0.0.2-3.33.1.3.3. Most members of the family are herbivorous, however, mostduikers are omnivorous. Like other ruminants, bovids have four-chambered stomachs, which allow them to digest plant material, such as grass, that cannot be used by many other animals. Ruminants (and some others like kangaroos, rabbits and termites) are able to use micro-organisms living in their guts to break down cellulose by fermentation.
Ecology and behaviour
Excluding the Cephalophines (duikers), Tragelaphines (spiral-horned antelopes) and the Neotragines, most African bovids are gregarious and territorial. Males are forced to disperse on attaining sexual maturity, and have to form their own territories, while females are not required to do so. Males that do not hold territories form bachelor herds. Competition takes place among males to acquire dominance, and fights tend to be more rigorous in limited rutting seasons. With the exception of migratory males, males generally hold the same territory throughout their lives.[27] In the waterbuck, some male individuals, known as "satellite males", may be allowed into the territories of other males and have to wait till the owner grows old so that they may acquire his territory.[39] Lek mating, where males gather together and competitively display to potential mates, is known to exist among topis, kobs and lechwes.[40] The Tragelaphines, cattle, sheep and goats are gregarious and non-territorial. In these species, males must gain absolute dominance over all other males, and fights are not confined to territories. Males therefore spend years in body growth.The bovids have various methods of social organization and social behaviour, which are classified into solitary and gregarious behaviour. Further, these types may each be divided into territorial and non-territorial behaviour.[27] Small bovids such as the klipspringer, oribi,steenbok are generally solitary and territorial. They hold small territories into which other members of the species, are not allowed to enter. These antelopes form monogamous pairs. Many species such as the dik-dik use pheromone secretions from the preorbital glandsand sometimes dung as well to mark their territories.[37] The offspring disperse at the time of adolescence, and males need to acquire territories prior to mating.[4] The bushbuck is the only bovid that is both solitary and non-territorial. This antelope hardly displays aggression, and tends to isolate itself or form loose herds, though in a favourable habitat several bushbuck may be found quite close to one another.
Activity
Most bovids are diurnal, although a few such as the buffalo, bushbuck, reedbuck and grysbok are exceptions. Social activity and feeding usually peaks during dawn and dusk. The bovids usually rest before dawn, during midday and after dark. Grooming is usually by licking with the tongue. Rarely do antelopes roll in mud or dust. Wildebeest and buffalo usually wallow in mud, whereas the hartebeest and topi rub their head and horns in mud and then smear it over their body. Bovids use different forms of vocal, olfactory and tangible communication. These involve varied postures of neck, head, horns, hair, legs and ears to convey sexual excitement, emotional state or alarm. Bovids usually stand motionless, with the head high and an intent stare, when they sense danger. Some like the impala, kudu and eland can even leap to heights of a few feet.[27] Bovids may roar or grunt to caution others and warn off predators, which include lion, tiger, grizzly bear, brown bear, crocodile, dhole, komodo dragon, spotted hyena and cougar.[4] In the mating season, rutting males might bellow to make their presence known to females. Muskox roar during male-male fights, and male saiga force air through their noses, producing a roar to deter rival males and attract females. Mothers also use vocal communication to locate their calves if they get separated. During fights over dominance, males tend todisplay themselves in an erect posture with a level muzzle.
Fighting techniques differ amongst the bovid families and also depend on the build. While the hartebeest fight on knees, others usually fight on all fours. Gazella species of various sizes use different methods of combat. Gazelles usually box, and in serious fights may clash and fence, consisting of hard blows from short range. Ibex and goat ram into opponents and clash horns. Wildebeest may use powerful head butting in aggressive clashes. If horns become entangled, the opponents move in a circular manner to unlock them. As a rule, only two bovids of equal build and level of defense engage in a fight, which is intended to determine the superior of the two. Individuals that are evidently inferior to others would rather flee than fight; for example, the immature males would not fight with the mature bulls. Generally bovids direct their attacks on the opponent's head rather than its body. The S-shaped horns, such as those on the impala, have various sections that help in ramming, holding and stabbing. Serious and gory fights are rare.
Diet
Most bovids alternately feed and ruminate throughout the day. While those who feed on concentrates feed and digest in short intervals, the roughage feeders take longer intervals. Only small species such as the duiker obtain their nutrition by feeding over fallen fruits for a few hours during day or night.
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