text: The two major species of deer found in North America are the white-tailed deer (Odocoileus virginianus) and the mule deer (Odocoileus hemionus). The most important sense in these animals is olfaction (the sense of smell)—so much so that they have an accessory olfaction system.
  The vomeronasal organ, located at the base of the nasal cavity, is the sensory organ for this system. Besides locating food and water, deer rely on their two separate olfactory systems to detect the presence of predators, as well as to supply them with information about the identity, sex, dominance status and reproductive status of other deer.
  The preorbital gland of O. virginianus is about  in length, while that of O. hemionus is roughly  in length. In black-tailed deer (O. h. columbianus), a subspecies of O. hemionus, the preorbital gland measures about . In all of these animals, the preorbital glands are surrounded by muscle which is under voluntary control, at least to some extent.
  It is not entirely clear whether the preorbital gland secretions of North American deer are significant for chemical communication. Most of the time the glands remain closed, but deer are capable of opening them to emit an odor in certain circumstances. For example, a rutting male may dilate its preorbital glands in order to signal aggression to another nearby male. Female deer often open their glands while caring for their young.