Deer are ruminant mammals (ruminants) belonging to the family Cervidae. One of the hallmarks of deer is the presence of antlers (deer antlers), rather than antlers, which are bony growths that develop annually (usually in summer) especially in stags (although there are some exceptions). There are about 34 species of deer worldwide which are divided into two major groups: the Old World deer group which includes the subfamilies Muntiacinae and Cervinae; and the new world deer groups, Hydropotinae and Odocolinae.