The uropygial gland, informally known as the preen gland or the oil gland, is a bilobed
sebaceous gland
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 number ...
possessed by the majority of birds used to distribute the gland's oil through the plumage by means of
preening. It is located dorsally at the base of the tail (between the fourth
caudal vertebra and the
pygostyle) and is greatly variable in both shape and size. In some species, the opening of the gland has a small tuft of feathers to provide a
wick for the preen oil (see below). It is a
holocrine gland enclosed in a connective tissue capsule made up of glandular
acini
An acinus (; plural, acini; adjective, acinar or acinous) refers to any cluster of cells that resembles a many-lobed "berry," such as a raspberry ('' acinus'' is Latin for "berry"). The berry-shaped termination of an exocrine gland, where the ...
that deposit their oil secretion into a common collector tube ending in a variable number of pores (openings), most typically two. Each lobe has a central cavity that collects the secretion from tubules arranged radially around the cavity. The gland secretion is conveyed to the surface via ducts that, in most species, open at the top of a papilla (nipple-like structure).
Etymology
From :
Medieval Latin
Medieval Latin was the form of Literary Latin used in Roman Catholic Western Europe during the Middle Ages. In this region it served as the primary written language, though local languages were also written to varying degrees. Latin functioned ...
, from
Ancient Greek
Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic peri ...
(''ouropugion''), from (''oura'') 'tail' and (''puge'') 'rump'.
Distribution amongst species
The gland is invariably present during
embryonic development
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...
, whereas it can be
vestigial
Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...
in adults of certain orders,
families, genera and species. Some or all species in at least nine families of birds lack a uropygial gland, mostly the ones unable to fly or the ones that produce
powder down for feather maintenance.
These include
kiwis (''Apterygidae''),
emu
The emu () (''Dromaius novaehollandiae'') is the second-tallest living bird after its ratite relative the ostrich. It is endemic to Australia where it is the largest native bird and the only extant member of the genus ''Dromaius''. The emu' ...
(''Dromaiidae''),
ostriches (''Struthionidae''),
rheas
The rheas ( ), also known as ñandus ( ) or South American ostriches, are large ratites (flightless birds without a keel on their sternum bone) in the order Rheiformes, native to South America, distantly related to the ostrich and emu. Most taxo ...
(''Rheidae''),
cassowaries
Cassowaries ( tpi, muruk, id, kasuari) are flightless birds of the genus ''Casuarius'' in the order Casuariiformes. They are classified as ratites (flightless birds without a keel (bird anatomy), keel on their sternum bones) and are native t ...
(''Casuariidae''),
mesites (''Mesitornithidae''),
bustards (''Otididae''),
pigeons and doves (''Columbidae''),
amazon parrot
Amazon parrots are parrots in the genus ''Amazona''. They are medium-sized, short-tailed parrots native to the Americas, with their range extending from South America to Mexico and the Caribbean. ''Amazona'' is one of the 92 genera of parrots t ...
s (''Psittacidae''),
frogmouths
The frogmouths are a group of nocturnal birds related to owlet-nightjars, swifts, and hummingbirds. Species in the group are distributed in the Indomalayan and Australasian realms.
Biology
They are named for their large flattened hooked bill and ...
(''Podargidae''), and
woodpeckers
Woodpeckers are part of the bird family Picidae, which also includes the piculets, wrynecks, and sapsuckers. Members of this family are found worldwide, except for Australia, New Guinea, New Zealand, Madagascar, and the extreme polar regions. M ...
(''Picidae'').
These birds typically find other means to stay clean and dry, such as taking a
dust bath
Dust bathing (also called sand bathing) is an animal behavior characterized by rolling or moving around in dust, dry earth or sand, with the likely purpose of removing parasites from fur, feathers or skin. Dust bathing is a maintenance behavior ...
. Researchers have been unable to correlate the presence or absence of the uropygial gland with factors such as distribution, climate, ecology, or flightlessness.
Gland secretion
The uropygial gland secretes an oil (preen oil) through the dorsal surface of the skin via a
grease nipple
Grease fitting on a bearing
A grease fitting, grease nipple, Zerk fitting, grease zerk, or Alemite fitting is a metal fitting used in mechanical systems to feed lubricants, usually lubricating grease, into a bearing under moderate to high p ...
-like nub or papilla.
The oil contains a complex and variable mixture of substances formed greatly of
aliphatic
In organic chemistry, hydrocarbons ( compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. ''aleiphar'', fat, oil). Aliphatic compounds can be saturated, like hexane, or ...
monoester
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides are fa ...
waxes, formed of
fatty acids and monohydroxy wax-alcohols. However, some types of
diester
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides are ...
wax
Waxes are a diverse class of organic compounds that are lipophilic, malleable solids near ambient temperatures. They include higher alkanes and lipids, typically with melting points above about 40 °C (104 °F), melting to give low ...
es called uropygiols and containing hydroxyfatty acids and/or alkane-diols exist in the secretions of the uropygial gland of some groups of birds.
Preen gland secretion of some, but not all, birds have shown to be
antimicrobial. Some birds harbor bacteria in their preen gland, which to date, have (exclusively) been isolated from preen glands (e.g. ''
Enterococcus phoeniculicola'' and ''
Corynebacterium uropygiale
''Corynebacterium uropygiale'' is a bacterium described in 2016 following thorough investigations using a polyphasic approach including MALDI-TOF mass spectrometry, phylogeny of 16S rRNA and ''rpoB'' genes and DNA fingerprinting. To date, it ha ...
''). Some of those bacteria add to the antimicrobial properties of preen wax.
A bird will typically transfer preen oil to its body during preening by rubbing its
beak
The beak, bill, or rostrum is an external anatomical structure found mostly in birds, but also in turtles, non-avian dinosaurs and a few mammals. A beak is used for eating, preening, manipulating objects, killing prey, fighting, probing for food ...
and head against the gland opening and then rubbing the accumulated oil on the feathers of the body and wings, and on the skin of the feet and legs. Tailward areas are usually preened utilizing the beak, although some species, such as
budgerigars
The budgerigar ( ; ''Melopsittacus undulatus''), also known as the common parakeet or shell parakeet, is a small, long-tailed, seed-eating parrot usually nicknamed the budgie ( ), or in American English, the parakeet. Budgies are the only spe ...
, use the feet to apply the oil to feathers around the vent.
History
Emperor Frederick II
Frederick II (German: ''Friedrich''; Italian: ''Federico''; Latin: ''Federicus''; 26 December 1194 – 13 December 1250) was King of Sicily from 1198, King of Germany from 1212, King of Italy and Holy Roman Emperor from 1220 and King of Jerusa ...
, in his thirteenth-century treatise on falconry, was seemingly the first to discuss the function of the uropygial gland of birds. He believed that its product not only oiled the plumage but also provided a poison which was introduced by the claws of
hawks
Hawks are birds of prey of the family Accipitridae. They are widely distributed and are found on all continents except Antarctica.
* The subfamily Accipitrinae includes goshawks, sparrowhawks, sharp-shinned hawks and others. This subfamily a ...
and
owls
Owls are birds from the Order (biology), order Strigiformes (), which includes over 200 species of mostly Solitary animal, solitary and Nocturnal animal, nocturnal birds of prey typified by an upright stance, a large, broad head, binocular vi ...
thus bringing quicker death to their prey. However, studies in 1678 on the question of the toxic nature of the uropygial gland secretion found no evidence to support Frederick's contention.
Function
Several researchers have reported differences in the relative gland weights attributing them to factors like seasonal changes, habitat, body weight, inter-individual variations, and sex. Significant differences are found in the relative gland size between males and females in most species, however, no coherent explanation has as yet been found for these results.
Many ornithologists believe the function of the uropygial gland differs among various species of birds.
Feather and body integrity
Preen oil is believed to help maintain the integrity of
feathers.
In waterbirds, preen oil maintains the flexibility of feathers and keeps feather barbules from breaking. The interlocking
barbule
Feathers are epidermal growths that form a distinctive outer covering, or plumage, on both avian (bird) and some non-avian dinosaurs and other archosaurs. They are the most complex integumentary structures found in vertebrates and a premier ...
s, when in good condition, form a barrier that helps repel water (see below). In some species, preen oil is also believed to maintain the integrity of the horny beak and the scaly skin of the legs and feet. It has also been speculated that in some species, preen oil contains a precursor of
vitamin D
Vitamin D is a group of fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, and many other biological effects. In humans, the most important compounds in this group are vitamin D3 (c ...
; this precursor is converted to vitamin D by the action of sunlight and then absorbed through the skin.
Courtship and pheromone production
Some researchers have postulated that the change in preen oil
viscosity
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the inte ...
may be related to the formation of the more brilliant plumage required for
courtship
Courtship is the period wherein some couples get to know each other prior to a possible marriage. Courtship traditionally may begin after a betrothal and may conclude with the celebration of marriage. A courtship may be an informal and private m ...
, although later research did not find support for this idea. The results of other studies suggest that the gland in females may be involved in the production and secretion of
lipids
Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include ...
with female
pheromone activity.
Waterproofing effect
The uropygial gland is strongly developed in many waterbirds, such as
ducks
Duck is the common name for numerous species of waterfowl in the family Anatidae. Ducks are generally smaller and shorter-necked than swans and geese, which are members of the same family. Divided among several subfamilies, they are a form t ...
,
petrels,
pelican
Pelicans (genus ''Pelecanus'') are a genus of large water birds that make up the family Pelecanidae. They are characterized by a long beak and a large throat pouch used for catching prey and draining water from the scooped-up contents before s ...
s and in the
osprey
The osprey (''Pandion haliaetus''), , also called sea hawk, river hawk, and fish hawk, is a diurnal, fish-eating bird of prey with a cosmopolitan range. It is a large raptor reaching more than in length and across the wings. It is brown o ...
and the
oilbird. A study examining the gland's mass relative to body weight in 126 bird species showed the absence of a clear-cut correlation between the gland's mass and the degree of birds' contact with water.
Anecdotal reports indicating that the waterproofing effect of the
hydrophobic
In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, th ...
uropygiols might be increased by
electrostatic
Electrostatics is a branch of physics that studies electric charges at rest (static electricity).
Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word for amber ...
charge to the oiled feather through the mechanical action of preening are not supported by scientific studies.
Antiparasitic effect
The taxonomic richness of avian louse burdens covaries positively with uropygial gland size (relative to body size) across avian taxa, suggesting
coevolution
In biology, coevolution occurs when two or more species reciprocally affect each other's evolution through the process of natural selection. The term sometimes is used for two traits in the same species affecting each other's evolution, as well ...
between gland size and parasite
biodiversity
Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic (''genetic variability''), species (''species diversity''), and ecosystem (''ecosystem diversity'') l ...
. The
hoopoe (''Upupa epops'') uropygial gland harbours
symbiotic
Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
whose excretions reduce the activity of feather-degrading bacteria and thus help to preserve the plumage.
''In vitro'' studies suggest that the preen oils of
rock doves (''Columba livia'') are effective against
lice
Louse ( : lice) is the common name for any member of the clade Phthiraptera, which contains nearly 5,000 species of wingless parasitic insects. Phthiraptera has variously been recognized as an order, infraorder, or a parvorder, as a result o ...
, however ''in vivo'' studies which removed the preen gland from captive birds had no significant effect on louse loads over the course of four months (but did reduce plumage quality), suggesting that preen oil may not be an important defense against lice in this species.
Similarly, there was no evidence to support a role of the gland secretion in attracting biting midges and black flies in
blue tits (''Cyanistes caeruleus'') and
feral pigeon
Feral pigeons (''Columba livia domestica'' or ''Columba livia forma urbana''), also called city doves, city pigeons, or street pigeons,Nagy, Kelsi, and Johnson, Phillip David. ''Trash animals: how we live with natures filthy, feral, invasive, an ...
s (''Columba livia'').
Cosmetic effect
Secretions of the uropygial gland of
greater flamingos (''Phoenicopterus roseus'') contain
carotenoid
Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic compound, organic pigments that are produced by plants and algae, as well as several bacteria, and Fungus, fungi. Carotenoids give the characteristic color to pumpki ...
s, organic pigments which give flamingos their pink colour. During the breeding season, greater flamingos increase the frequency of their spreading uropygial secretions over their feathers and thereby enhance their colour. This cosmetic use of uropygial secretions has been described as applying "make-up".
The chemical composition
The biochemical composition of preen oil has been reported to significantly vary among birds in several ways: for example, the qualitative and quantitative content of the volatile compounds, chain length of diols, lipid concentration, ester saturation, type of fatty acids and their percentage composition. This variation is due to a range of factors, including species, age, sex and season.
In chickens, a limited number of studies have investigated the specific effect of diet on the fatty acid composition of preen oil in meat chickens. The preen oil of meat chickens is dominated by saturated fatty acids, which make up to 97% of the total fatty acids, with 13 different medium to long chain saturated fatty acids (C8:0 to C22:0) detected. The preen oil contained several odd-chain fatty acids, which suggests they may be derived from lipolysis by the uropygial gland and/or its microbiome. Diet and gender had small but significant effects on levels of specific saturated fatty acid in the preen oil. The fatty acid composition of the preen oil did not reflect the more diverse fatty acid compositions of the diet or whole blood. Therefore, this clearly indicate that measuring the fatty acid profile of preen oil is not a suitable alternative approach for predicting the fatty acid composition of the blood of meat chickens.
See also
*
Sebaceous gland
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 number ...
References
External links
Diagram of uropygial gland structure
{{DEFAULTSORT:Uropygial Gland
Bird anatomy