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 of ...

found in many species of hoofed animals, which is homologous to the

lacrimal gland The lacrimal glands are paired exocrine glands, one for each eye, found in most terrestrial vertebrates and some marine mammals, that secrete the aqueous layer of the tear film. In humans, they are situated in the upper lateral region of each or ...

found in humans. These glands are trenchlike slits of dark blue to black, nearly bare skin extending from the medial

canthus The canthus (pl. canthi, palpebral commissures) is either corner of the eye where the upper and lower eyelids meet. More specifically, the inner and outer canthi are, respectively, the medial and lateral ends/angles of the palpebral fissure. T ...

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 glands, and they produce secretions which contain

pheromone A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...

s and other

semiochemical A semiochemical, from the Greek wiktionary:σημεῖον, σημεῖον (''semeion''), meaning "signal", is a chemical substance or mixture released by an organism that affects the behaviors of other individuals. Semiochemical communication c ...

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 ra ...

), 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 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 ...

. A further function of these glands may be to produce

antimicrobial An antimicrobial is an agent that kills microorganisms or stops their growth. Antimicrobial medicines can be grouped according to the microorganisms they act primarily against. For example, antibiotics are used against bacteria, and antifungals ar ...

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 germ ...

s. Antimicrobial compounds found in these glands may be

biosynthesized Biosynthesis is a multi-step, enzyme- catalyzed process where substrates are converted into more complex products in living organisms. In biosynthesis, simple compounds are modified, converted into other compounds, or joined to form macromolecules ...

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 olde ...

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 reindeer ...

have seven types of external

s distributed across their bodies. These are the forehead glands (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 pl ...

(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 se ...

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 pla ...

(inside the foreskin of the deer's penis), the

metatarsal gland The metatarsal bones, or metatarsus, are a group of five long bones in the foot, located between the tarsal bones of the hind- and mid-foot and the phalanges of the toes. Lacking individual names, the metatarsal bones are numbered from the me ...

s (outside of the hind legs), and the

tarsal glands Meibomian glands (also called tarsal glands, palpebral glands, and tarsoconjunctival glands) are sebaceous glands along the rims of the eyelid inside the tarsal plate. They produce meibum, an oily substance that prevents evaporation of the eye' ...

(located inside of the hind legs). Although it is not their primary function, the

salivary gland The salivary glands in mammals are exocrine glands that produce saliva through a system of ducts. Humans have three paired major salivary glands (parotid, submandibular, and sublingual), as well as hundreds of minor salivary glands. Salivary gla ...

s 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 the Car ...

are the

white-tailed deer The white-tailed deer (''Odocoileus virginianus''), also known as the whitetail or Virginia deer, is a medium-sized deer native to North America, Central America, and South America as far south as Peru and Bolivia. It has also been introduced t ...

(''Odocoileus virginianus'') and the

mule deer The mule deer (''Odocoileus hemionus'') is a deer indigenous to western North America; it is named for its ears, which are large like those of the mule. Two subspecies of mule deer are grouped into the black-tailed deer. Unlike the related whit ...

(''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 Stimulus (physiology), stimuli. (For example, in the human body, the brain which is part of the cen ...

in these animals is olfaction (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 tetrapods. T ...

, 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 c ...

, 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 Two forms of black-tailed deer or blacktail deer that occupy coastal woodlands in the Pacific Northwest of North America are subspecies of the mule deer (''Odocoileus hemionus''). They have sometimes been treated as a species, but virtually all r ...

(''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 wes ...

(''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 The Indian muntjac or the common muntjac (''Muntiacus muntjak''), also called the southern red muntjac and barking deer, is a deer species native to South and Southeast Asia. It is listed as Least Concern on the IUCN Red List. In popular local l ...

(''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, ...

s (

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

Bovidae) comprise some 140 species of

ruminant Ruminants (suborder Ruminantia) are ungulate, hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by Enteric fermentation, fermenting it in a specialized stomach prior to digestion, principally ...

s in which at least the males bear unbranched, hollow

horns Horns or The Horns may refer to: * Plural of Horn (instrument), a group of musical instruments all with a horn-shaped bells * The Horns (Colorado), a summit on Cheyenne Mountain * ''Horns'' (novel), a dark fantasy novel written in 2010 by Joe Hill ...

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 scales, hair, nails, feathers, ho ...

. 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 domesticated s ...

goats The goat or domestic goat (''Capra hircus'') is a domesticated species of Caprinae, goat-antelope typically kept as livestock. It was domesticated from the wild goat (''C. aegagrus'') of Southwest Asia and Eastern Europe. The goat is a membe ...

muskox The muskox (''Ovibos moschatus'', in Latin "musky sheep-ox"), also spelled musk ox and musk-ox, plural muskoxen or musk oxen (in iu, ᐅᒥᖕᒪᒃ, umingmak; in Woods Cree: ), is a hoofed mammal of the family Bovidae. Native to the Arctic, ...

serow The serows ( or ) are four species of medium-sized goat-like or antelope-like mammals of the genus ''Capricornis''. All four species of serow were until recently also classified under ''Naemorhedus'', which now only contains the gorals. Extant ...

goral The gorals are four species in the genus ''Naemorhedus''. They are small ungulates with a goat-like or antelope-like appearance. Until recently, this genus also contained the serow species (now in genus '' Capricornis''). Etymology The original ...

s, 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, incom ...

. 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 The springbok (''Antidorcas marsupialis'') is a medium-sized antelope found mainly in south and southwest Africa. The sole member of the genus ''Antidorcas'', this bovid was first described by the German zoologist Eberhard August Wilhelm ...

blackbuck The blackbuck (''Antilope cervicapra''), also known as the Indian antelope, is an antelope native to India and Nepal. It inhabits grassy plains and lightly forested areas with perennial water sources. It stands up to high at the shoulder. Ma ...

gazelle A gazelle is one of many antelope species in the genus ''Gazella'' . This article also deals with the seven species included in two further genera, '' Eudorcas'' and '' Nanger'', which were formerly considered subgenera of ''Gazella''. A third ...

dik-dik A dik-dik is the name for any of four species of small antelope in the genus ''Madoqua'' that live in the bushlands of eastern and southern Africa. Dik-diks stand about at the shoulder, are long, weigh and can live for up to 10 years. Dik- ...

oribi The oribi (; ''Ourebia ourebi'') is a small antelope found in eastern, southern and western Africa. The sole member of its genus, it was described by the German zoologist Eberhard August Wilhelm von Zimmermann in 1783. While this is the only ...

, 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 The Maxwell's duiker (''Philantomba maxwellii'') is a small antelope found in western Africa. Taxonomy The scientific name of Maxwell's duiker is ''Philantomba maxwelli''. It is classified in the genus ''Philantomba'' along with the blue 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 coll ...

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 (

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-to ...

hartebeest The hartebeest (; ''Alcelaphus buselaphus''), also known as kongoni or kaama, is an African antelope. It is the only member of the genus ''Alcelaphus''. Eight subspecies have been described, including two sometimes considered to be indepen ...

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 zoologist ...

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 Sociality is the degree to which individuals in an animal population tend to associate in social groups (gregariousness) and form cooperative societies. Sociality is a survival response to evolutionary pressures. For example, when a mother wasp ...

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 Günther's dik-dik (''Madoqua guentheri'') is a small antelope found in arid zones of East Africa. Description Günther's dik-dik is one of the smallest ungulates in Africa, weighing when fully grown. It has a yellowish-gray to reddish-brown c ...

(''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 mammals ...

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 The lacrimal bone is a small and fragile bone of the facial skeleton; it is roughly the size of the little fingernail. It is situated at the front part of the medial wall of the orbit. It has two surfaces and four borders. Several bony landmarks of ...

, and are surrounded by specialized facial muscles that compress them to express the secretions more effectively. In contrast, the

saiga antelope The saiga antelope (, ''Saiga tatarica''), or saiga, is a critically endangered antelope which during antiquity inhabited a vast area of the Eurasian steppe spanning the foothills of the Carpathian Mountains in the northwest and Caucasus in t ...

(''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 marrie ...

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 The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided ...

scent glands may be the beginning of a promising new area of

drug development Drug development is the process of bringing a new pharmaceutical drug to the market once a lead compound has been identified through the process of drug discovery. It includes preclinical research on microorganisms and animals, filing for re ...

. Assuming functional analogs of these

lead compound A lead compound (, i.e. a "leading" compound, not to be confused with various compounds of the metallic element lead) in drug discovery is a chemical compound that has pharmacology, pharmacological or biological activity likely to be therapeutical ...

s 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