walk, with both left legs moving and then both right, this ensures that the fore and hind limb will not collide while in fast motion. During this motion there is a moment where all four limbs are off the ground at the same time. Consequently, camelids large enough for human beings to ride have a typical swaying motion. Dromedary camels, bactrian camels, llamas, and alpacas are all induced ovulators. 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 drink brackish water, and some herds live in nuclear test areas. Comparative table of the seven extant species in the family Camelidae: Evolution 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 middle Eocene, in present-day North America. Among the earliest camelids was the rabbit-sized Protylopus, which still had four toes on each foot. By the late Eocene, around 35 million years ago, camelids such as Poebrotherium had lost the two lateral toes, and were about the size of a modern goat. The family diversified and prospered, but remained confined to the North American continent until only about two to three million years ago, when representatives arrived in Asia, and (as part of the Great American Interchange that followed the formation of the Isthmus of Panama) South America. A high arctic camel from this time period has been documented in the far northern reaches of Canada. 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 the earliest 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: the dromedary of northern Africa and southwest Asia; the Bactrian 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, and vicuñas. Camelids were domesticated by early Andean peoples, 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