By Phil Myers
There are two groups of living camels. One, found in northern Africa and central Asia, consists of the dromedary (one-humped camel) and bactrian camels (two-humped camel). Dromedaries and most bactrian camels exist today only in domestication. The other group, the South American camelids, includes 2 to 4 species. Llamas and alpacas, sometimes considered to be the same species, may both have been derived from the guanaco through a thousand years or more of domestication. Guanacos and vicugnas still exist in the wild, although wild populations of both are depleted and threatened.
Camelids are all large. The South American forms range in weight from 35 kg to almost 100 kg. Old World camels, however, are much larger, weighing 450 to 650 kg. Camelids vary in body shape from slender to stocky, but all have long, gracile necks; a small head; and long, slender legs. The upper lip is deeply and distinctively cleft. Their toes are splayed, and camelids are the only plantigrade or fully digitigrade ungulates (camels are sometimes grouped with several extinct families in the infraorder Tylopoda, which means "padded foot").
The skulls of camelids have an elongated 
rostrum, a well developed sagittal crest, and a complete postorobital bar. They lack horns or antlers. The cheek teeth are selenodont. Upper incisors are present; young have 3 on each side, but adults have only one, which is canine-like. The spatulate lower incisors project forward. The canines, which are present in both upper and lower jaws, are medium-sized and hooked. A wide diastema separates incisors and cheek teeth. The dental formula for (adult) Old World camels is 1/3, 1/1, 3/1-2, 3/3 = 32-34; that for New World camels is 1/3, 1/1, 2/1, 3/3 = 30.
The postcranial skeleton is notable mostly because of the condition of the legs. The 3rd and 4th metapodials fuse to form a cannon bone, as in other paraxonic ungulates, but the fusion is complete only proximally. At the end away from the body, the metapodials separate and splay wide apart. The phalanges of these two toes are also distinctive. The first two phalanges of each digit are flattened and expanded, while the last is reduced and bears a nail, not a hoof. The middle (second) phalanges are imbedded in the broad pad that makes up the sole of the foot. The ulna and fibula are much reduced, present mainly as a splint on the radius or tibia.
Camelids have a complex 3-chambered, ruminating stomach.
Wild camelids generally live in groups. These are polygynous, each containing a harem male and his females. Bachelor males also form herds in some species. Males fight for dominance by biting and "neck dominance."
All camelids are herbivorous and feed primarily but not exclusively on grasses. They are generally found in arid or semi-arid areas, and the much-studied Old World camels have a remarkable ability to conserve water.
The family Camelidae ranges back in time to the upper Eocene. It is first known from North America. Curiously, camelid feet were nearly unguligrade and probably hoofed by the Oligocene and early Miocene. The cannon bone was completely or nearly completely fused. During the Miocene and Pliocene, the ends of the cannon bones separated, and camelids returned to the digitigrade stance of their ancestors. The splayed toes of camelids give them additional surface area for supporting their weight on soft substrates like sand. Camelids probably spread to the Old World and South America during periods of low sea level during the Pliocene and Pleistocene, when land bridges connected those continents to North America.
References and literature cited:
Feldhamer, G. A., L. C. Drickamer, S. H. Vessey, and J. F. Merritt. 1999. Mammalogy. Adaptation, Diversity, and Ecology. WCB McGraw-Hill, Boston. xii+563pp.
Nowak, R.M. and J.L. Paradiso. 1983. Walker's Mammals of the World, 4th edition . John Hopkins University Press, Baltimore, MD.
Savage, R. J. G. and M. R. Long. 1986. Mammal Evolution: An Illustrated Guide. Facts on File Publications, UK. 251 pp.
Simpson, C. D. 1984. Artiodactyls. Pp. 563-587 in Anderson, S. and J. K. Jones, Jr. (eds). Orders and Families of Recent Mammals of the World. John Wiley and Sons, N.Y. xii+686 pp.
Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, N.Y. vii+576 pp.
Vaughan, T. A., J. M. Ryan, N. J. Czaplewski. 2000. Mammalogy. Fourth Edition. Saunders College Publishing, Philadelphia. vii+565pp.
Wilson, D. E., and D. M. Reeder. 1993. Mammal Species of the World, A Taxonomic and Geographic Reference. 2nd edition. Smithsonian Institution Press, Washington. xviii+1206 pp.&160;
Contributors
Phil Myers (author), Museum of Zoology, University of Michigan.
