Deer are also ungulates with an even number of toes, two on each foot touching the ground and two vestigial toes that do not contact the ground. They are ruminants, which means that they have stomachs with multiple chambers (four), which allows them to chew, digest, regurgitate and chew their food some more. Compared with other ruminants, deer have antlers on their heads. Antlers consist of a bone core covered with skin and fur. Unlike the ossicones of giraffes, deer antlers are shed and regrown each year. Usually only the males have antlers, but in some species like the caribou, the females also have antlers.
The two main groups of deer include: Old World deer (30 species) and New World deer (18 species). These names refer to the regions in which they originated, although species of each are found in the opposite regions today, everywhere but Australia. Examples of Old World deer include elk, red deer, sambar and rusa deer. The two groups differ only in the way that ligaments connecting the bones in their feet are arranged.