Camelid feet lack functional hooves, with the toe bones being embedded in a broad, cutaneous pad.[1]
Camelids are large, strictlyherbivorous animals with slender necks and long legs. They differ fromruminants in a number of ways.[2] Their dentition show traces of vestigial centralincisors in theincisive bone, and the third incisors have developed into canine-like tusks. Camelids also have truecanine teeth and tusk-likepremolars, which are separated from themolars by a gap. As in ruminants, the upper incisors are largely absent and are replaced by adental pad consisting of connective tissue covered withepithelium.[3] The musculature of the hind limbs differs from those of other ungulates in that the legs are attached to the body only at the top of the thigh, rather than attached by skin and muscle from the knee upwards. Because of this, camelids have to lie down by resting on their knees with their legs tucked underneath their bodies.[1] They have three-chamberedstomachs, rather than four-chambered ones; their upper lips are split in two, with each part separately mobile; and, uniquely among mammals, theirred blood cells are elliptical.[2] They also have a unique type ofantibodies, which lack the light chain, in addition to the normal antibodies found in other mammals. These so-calledheavy-chain antibodies are being used to developsingle-domain antibodies with potentialpharmaceutical applications.
Camelids do not have hooves; rather, they have two-toed feet with toenails and soft foot pads (Tylopoda is Greek for "padded foot"). Most of the weight of the animal rests on these tough, leathery sole pads. The South American camelids have adapted to the steep and rocky terrain by adjusting the pads on their toes to maintain grip.[4] The surface area of Camels foot pads can increase with increasing velocity in order to reduce pressure on the feet and larger members of the camelid species will usually have larger pad area, which helps to distribute weight across the foot.[5] Many fossil camelids wereunguligrade and probably hooved, in contrast to all living species.[6]
Camelids are behaviorally similar in many ways, including their walking gait, in which both legs on the same side are moved simultaneously. While running, camelids engage a unique "running pace gait" in which limbs on the same side move in the same pattern they walk, with both left legs moving and then both right, which ensures that the fore and hind limb will not collide while in fast motion. During this motion, all four limbs momentarily are off the ground at the same time.[7] Consequently, camelids large enough for human beings to ride have a typical swaying motion.
Dromedary camels, bactrian camels, llamas, and alpacas are allinduced ovulators.[8]
The three Afro-Asian camel species have developed extensive adaptations to their lives in harsh, near-waterless environments. Wild populations of the Bactrian camel are even able to drinkbrackish water, and some herds live in nuclear test areas.[9]
Comparative table of the seven extant species in the family Camelidae:
The yellow dot is the origin of the family Camelidae and the black arrows are the historic migration routes that explain the present-day distribution.
Camelids are unusual in that their modern distribution is almost the inverse of their area of origin. Camelids first appeared very early in the evolution of the even-toed ungulates, around 50 to 40 million years ago during the middleEocene,[citation needed] in present-day North America. Among the earliest camelids was the rabbit-sizedProtylopus, which still had four toes on each foot. By the lateEocene, around 35 million years ago, camelids such asPoebrotherium had lost the two lateral toes, and were about the size of a moderngoat.[6][11]
The family diversified and prospered, with the two living tribes, theCamelini andLamini, diverging in the late earlyMiocene, about 17 million years ago, but remained restricted to North America until about 6 million years ago, whenParacamelus crossed theBering land bridge intoEurasia, giving rise to the modern camels, and about 3-2 million years ago, whenHemiauchenia emigrated into South America (as part of theGreat American Interchange), giving rise to the modern llamas.[12] A population ofParacamelus continued living in North America and evolved into thehigh arctic camel, which survived until the middle Pleistocene.
The original camelids of North America remained common until the quite recent geological past, but then disappeared, possibly as a result of hunting or habitat alterations by theearliest human settlers, and possibly as a result of changing environmental conditions after the last ice age, or a combination of these factors. Three species groups survived - thedromedary of northern Africa and southwest Asia; theBactrian camel of central Asia; and the South American group, which has now diverged into a range of forms that are closely related, but usually classified as four species -llamas,alpacas,guanacos, andvicuñas. Camelids were domesticated by early Andean peoples,[13] and remain in use today.
Fossil camelids show a wider variety than their modern counterparts. One North American genus,Titanotylopus, stood 3.5 m at the shoulder, compared with about 2.0 m for the largest modern camelids. Other extinct camelids included small, gazelle-like animals, such asStenomylus. Finally, a number of very tall, giraffe-like camelids were adapted to feeding on leaves from high trees, including such genera asAepycamelus andOxydactylus.[6]
Whether the wild Bactrian camel (Camelus ferus) is a distinct species or a subspecies (C. bactrianus ferus) is still debated.[14][15] The divergence date is 0.7 million years ago, long before the start of domestication.[15]
^abClutton-Brock, Juliet (1987).A Natural History of Domesticated Mammals. Cambridge University Press. p. 208.ISBN978-0-521-34697-9.
^abFowler, M.E. (2010).Medicine and Surgery of Camelids, Ames, Iowa: Wiley-Blackwell. Chapter 1 "General Biology and Evolution" addresses the fact that camelids (including camels and llamas) are not ruminants, pseudo-ruminants, or modified ruminants.
^Palmer, D., ed. (1999).The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. pp. 274–277.ISBN978-1-84028-152-1.
^Meachen, Julie A. (2005-12-31). "A New Species of Hemiauchenia (Artiodactyla, Camelidae) from the Late Blancan of Florida".Bulletin of the Florida Museum of Natural History.45 (4).University Press of Florida:435–448.doi:10.58782/flmnh.stwk6834.
^Moore, Katherine M. (2016). "Early Domesticated Camelids in the Andes". In Capriles, Jose M.; Tripcevich, Nicholas (eds.).The Archaeology of Andean Pastoralism. University of New Mexico Press.ISBN978-0-8263-5702-1.
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