Preorbital gland
The preorbital gland is a paired exocrine gland found in many species of hoofed animals, which is homologous to the lacrimal gland found in humans. These glands are trenchlike slits of dark blue to black, nearly bare skin extending from the medial canthus of each eye. They are lined by a combination of sebaceous and sudoriferous glands, and they produce secretions which contain pheromones and other semiochemical compounds. Ungulates frequently deposit these secretions on twigs and grass as a means of communication with other animals.
The preorbital gland serves different roles in different species. Pheromone-containing secretions from the preorbital gland may serve to establish an animal's dominance, mark its territory, or simply to produce a pleasurable sensation to the animal. Because of its critical role in scent marking, the preorbital gland is usually considered as a type of scent gland. A further function of these glands may be to produce antimicrobial compounds to fight against skin pathogens. Antimicrobial compounds found in these glands may be biosynthesized by the animal itself, or by microorganisms that live in these glands.
In cervids
have seven major external scent glands, distributed throughout their bodies. These glands are the forehead glands, the preorbital glands, the nasal glands, the interdigital glands, the preputial gland, the metatarsal glands, and the tarsal glands. Though it is not their primary function, the salivary glands also function as scent glands. Deer rely heavily on these scent glands to communicate with other members of their species, and possibly even with members of other species. A deer may rub its preorbital gland purely for pleasure.North American deer
The two major species of deer found in North America are the white-tailed deer and the mule deer. The most important sense in these animals is olfaction —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, 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 emit an odor that is significant in terms of chemical communication. Most of the time, the glands remain in the closed position, but they are capable of opening them 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.
Other deer
In juvenile red deer, the preorbital gland appears to play a role in the response to stress. The preorbital gland is closed in a relaxed calf, whereas it is opened in a stressed calf. One example of this is the signalling of hunger and satiety. Fawns open their preorbital glands as a signal that they are hungry, and close the gland after feeding, when they are no longer hungry.Other than during the rut and for the first six months after giving birth, the adult Indian muntjac is a solitary animal. Adult males in particular are well spaced and marking grass and bushes with secretions from their preorbital glands appears to be involved in the acquisition and maintenance of territory.
In bovids
The bovids comprise some 140 species of ruminants in which at least the males bear unbranched, hollow horns covered in a permanent sheath of keratin. Most species of bovids have means of spacing themselves across their habitat; territorial behavior is the most consistent type of spacing behavior.Caprids use their preorbital glands to establish social rank. For example, when competition arises between two grazing sheep, they have been observed to nuzzle each other's preorbital glands. By sending and receiving olfactory cues, this behavior appears to be a means of establishing dominance and of avoiding a fight, which would otherwise involve potentially injurious butting or clashing with the forehead.
The antilopine bovids have well-developed preorbital glands.
Among the cephalophines, members of the Philantomba and Sylvicapra genera are all solitary animals which display territorial behavior and have well developed preorbital glands. Maxwell's duiker is a solitary animal which utilizes preorbital gland secretions to mark its territory. This behavior is observed most in adult males, less frequently in females, and less still in subadults of this species. Secretions from the preorbital gland of the common duiker contain at least 33 different chemical compounds. Two thiazole compounds and an epoxy ketone are present in significantly higher concentrations in male than in female secretions, suggesting that they could serve as sex recognition cues.
The alcephine bovids have preorbital glands which secrete complex mixtures of chemical compounds. The preorbital glands of the bontebok are larger in males than in females. Their secretions contain at least forty different chemical compounds, and are deposited on grass and twigs at the borders of their territory. They then appear to transfer the secretions from the grass to their horns and forehead by waving the head from side to side across the stalk bearing the secretion. Marking of plant stalks with preorbital gland secretions is seen in both sexes. In contrast to the duikers and raphicerids, the klipspringer is a semi-gregarious species, while the hirola is fully gregarious. Nevertheless, these animals display territorial scent marking of grasses with secretions from their preorbital glands.
Differences in the social structure and marking behavior among different species may lead to a different size and position of the preorbital glands on the animal's face. For example, Günther's dik-dik is a monogamous species of antelope that lives in a permanent territory, the boundaries of which the animals mark several times a day by actively pressing the preorbital glands to grasses and low-lying plants and applying the secretions. In this territorial animal, the preorbital glands remain of considerable size throughout the year. The glands are located in large preorbital pits in the lacrimal bone, and are surrounded by specialized facial muscles that compress them to express the secretions more effectively. In contrast, the saiga antelope is a polygamous and somewhat nomadic species which does not occupy any permanent territory at any time during the year. For most of the year the preorbital glands remain small, only growing to substantial size during the rut. At that time of year, secretions ooze more or less continuously from the glands. In this nonterritorial animal, the preorbital glands are not as well-developed, lack well-developed surrounding facial muscles, and are positioned in an inconspicuous and shallow depression of the lacrimal bone.