The preorbital gland is a paired

exocrine gland Exocrine glands are glands that secrete substances on to an epithelial surface by way of a duct. Examples of exocrine glands include sweat, salivary, mammary, ceruminous, lacrimal, sebaceous, prostate and mucous. Exocrine glands are one ...

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 A sebaceous gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called sebum, which lubricates the hair and skin of mammals. In humans, sebaceous glands occur in the greatest num ...

and sudoriferous

gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormones) for release into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland). Structure De ...

s, and they produce

secretion 440px Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. The classic ...

s 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 (especially in preparation for

breeding Breeding is sexual reproduction that produces offspring, usually animals or plants. It can only occur between a male and a female animal or plant. Breeding may refer to: * Animal husbandry, through selected specimens such as dogs, horses, and r ...

), mark its

territory A territory is an area of land, sea, or space, particularly belonging or connected to a country, person, or animal. In international politics, a territory is usually either the total area from which a state may extract power resources or a ...

, 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

pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a g ...

s. Antimicrobial compounds found in these glands may be biosynthesized by the animal itself, or by

microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in old ...

s that live in these glands.

In cervids

Deer Deer or true deer are hoofed ruminant mammals forming the family Cervidae. The two main groups of deer are the Cervinae, including the muntjac, the elk (wapiti), the red deer, and the fallow deer; and the Capreolinae, including the re ...

have seven types of external scent glands distributed across their bodies. These are the

forehead gland In human anatomy, the forehead is an area of the head bounded by three features, two of the skull and one of the scalp. The top of the forehead is marked by the hairline, the edge of the area where hair on the scalp grows. The bottom of the f ...

s (on the forehead), the preorbital glands (below the eyes), the

nasal glands The nasal glands are the seromucous glands in the respiratory The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and pla ...

(inside the nostrils), the

interdigital gland Scent gland are exocrine glands found in most mammals. They produce semi-viscous secretions which contain pheromones and other semiochemical compounds. These odor-messengers indicate information such as status, territorial marking, mood, and ...

s (between the toes), the

preputial gland Preputial glands are exocrine glands that are located in the folds of skin front of the genitals of some mammals. They occur in several species, including mice, ferrets, rhinoceroses, and even-toed ungulates and produce pheromones. The glands play ...

(inside the foreskin of the deer's penis), the metatarsal glands (outside of the hind legs), and the tarsal glands (located inside of the hind legs). Although it is not their primary function, the salivary glands also function as scent glands. Deer rely heavily on the 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 (e.g., on a branch) purely for pleasure.

North American deer

The two major species of deer found in

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and th ...

are the white-tailed deer (''Odocoileus virginianus'') and the mule deer (''Odocoileus hemionus''). The most important

sense A sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of stimuli. (For example, in the human body, the brain which is part of the central nervous system re ...

in these animals is

olfaction The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. In humans, ...

(the sense of smell)—so much so that they have an accessory olfaction system. The

vomeronasal organ The vomeronasal organ (VNO), or Jacobson's organ, is the paired auxiliary olfactory (smell) sense organ located in the soft tissue of the nasal septum, in the nasal cavity just above the roof of the mouth (the hard palate) in various tetrapo ...

, located at the base of the

nasal cavity The nasal cavity is a large, air-filled space above and behind the nose in the middle of the face. The nasal septum divides the cavity into two cavities, also known as fossae. Each cavity is the continuation of one of the two nostrils. The nasal ...

, 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.

Other deer

In juvenile

red deer The red deer (''Cervus elaphus'') is one of the largest deer species. A male red deer is called a stag or hart, and a female is called a hind. The red deer inhabits most of Europe, the Caucasus Mountains region, Anatolia, Iran, and parts of ...

(''Cervus elaphus''), 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. The adult Indian muntjac (''Muntiacus muntjac'') is a solitary animal, other than during the rut (mating season) and for the first six months after giving birth. Adult males in particular are widely separated. Marking grass and bushes with secretions from their preorbital glands appears to be involved in the acquisition and maintenance of territory.

In bovids

The

bovid The Bovidae comprise the biological family of cloven-hoofed, ruminant mammals that includes cattle, bison, buffalo, antelopes, and caprines. A member of this family is called a bovid. With 143 extant species and 300 known extinct species, t ...

s (

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Bovidae) comprise some 140 species of ruminants in which at least the males bear unbranched, hollow horns covered in a permanent sheath of

keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up Scale (anatomy), scales, hair, Nail ...

. Most species of bovids have means of spacing themselves across their habitat; territorial behavior is the most consistent type of spacing behavior.

Caprids The subfamily Caprinae, also sometimes referred to as the tribe Caprini, is part of the ruminant family Bovidae, and consists of mostly medium-sized bovids. A member of this subfamily is called a caprine, or, more informally, a goat-antelope (a ...

(dwarf antelope, such as the

sheep Sheep or domestic sheep (''Ovis aries'') are domesticated, ruminant mammals typically kept as livestock. Although the term ''sheep'' can apply to other species in the genus '' Ovis'', in everyday usage it almost always refers to domesticate ...

, goats, muskox, serows, gorals, and several similar species) use their preorbital glands to establish

social rank A social class is a grouping of people into a set of hierarchical social categories, the most common being the upper, middle and lower classes. Membership in a social class can for example be dependent on education, wealth, occupation, inc ...

. For example, when competition arises between two grazing sheep (''Ovis aries''), 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 (dwarf antelope, such as the springbok, blackbuck, gazelles, dik-diks, oribi, and several similar species) 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 (''Philantomba maxwellii'') 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 compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

s. Two

thiazole Thiazole, or 1,3-thiazole, is a heterocyclic compound that contains both sulfur and nitrogen. The term 'thiazole' also refers to a large family of derivatives. Thiazole itself is a pale yellow liquid with a pyridine-like odor and the molecular fo ...

compounds and an

epoxy Epoxy is the family of basic components or cured end products of epoxy resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide functional group is also co ...

ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double b ...

are present in significantly higher concentrations in male than in female secretions, suggesting that they could serve as sex recognition cues. The alcephine bovids (

wildebeest Wildebeest ( , , ), also called gnu ( or ), are antelopes of the genus ''Connochaetes'' and native to Eastern and Southern Africa. They belong to the family Bovidae, which includes true antelopes, cattle, goats, sheep, and other even-toe ...

s, hartebeests,

hirola The hirola (''Beatragus hunteri''), also called the Hunter's hartebeest or Hunter's antelope, is a critically endangered antelope species found on the border between Kenya and Somalia. It was first described by the big game hunter and zoologi ...

bontebok The bontebok (''Damaliscus pygargus'') is an antelope found in South Africa, Lesotho and Namibia. ''D. pygargus'' has two subspecies; the nominate subspecies (''D. p. pygargus''), occurring naturally in the Fynbos and Renosterveld areas of ...

, blesbok, and several similar species) have preorbital glands which secrete complex mixtures of chemical compounds. The preorbital glands of the

(''Damaliscus pygargus pygarus'') 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 The klipspringer (; ''Oreotragus oreotragus'') is a small antelope found in eastern and southern Africa. The sole member of its genus and subfamily/tribe, the klipspringer was first described by German zoologist Eberhard August Wilhelm von Zim ...

(''Oreotragus oreotragus'') is a semi- gregarious species, while the

(''Beatragus hunteri'') 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 (''Madoqua guentheri'') is a monogamous species of

antelope The term antelope is used to refer to many species of even-toed ruminant that are indigenous to various regions in Africa and Eurasia. Antelope comprise a wastebasket taxon defined as any of numerous Old World grazing and browsing hoofed mamm ...

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 (''Saiga tatarica'') is a

polygamous Crimes Polygamy (from Late Greek (') "state of marriage to many spouses") is the practice of marrying multiple spouses. When a man is married to more than one wife at the same time, sociologists call this polygyny. When a woman is marri ...

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.

Research directions

The recent identification of several antimicrobial compounds from the secretions of animal dermal scent glands may be the beginning of a promising new area of drug development. Assuming functional analogs of these lead compounds can be synthesized and found to be effective ''in vivo'', the potential exists for producing new antimicrobial agents against pathogenic skin microorganisms.

In cervids

North American deer

Other deer

In bovids

Research directions

See also

Footnotes

Further reading