A pheromone () is a secreted or excreted

chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., w ...

factor that triggers a social response in members of the same

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate ...

. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavior of the receiving individuals. There are ''

alarm An alarm device is a mechanism that gives an audible, visual or other kind of alarm signal to alert someone to a problem or condition that requires urgent attention. Alphabetical musical instruments Etymology The word ''alarm'' comes from t ...

pheromones'', ''food trail pheromones'', '' sex pheromones'', and many others that affect behavior or physiology. Pheromones are used by many organisms, from basic

unicellular A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms an ...

prokaryote A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Co ...

s to complex

multicellular A multicellular organism is an organism that consists of more than one cell, in contrast to unicellular organism. All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially un ...

eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bac ...

s. Their use among

insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pa ...

s has been particularly well documented. In addition, some

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chorda ...

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclu ...

s and

ciliates The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a different ...

communicate by using pheromones. The ecological functions and evolution of pheromones are a major topic of research in the field of chemical ecology.

Background

The

portmanteau word A portmanteau word, or portmanteau (, ) is a blend of wordsJean-Henri Fabre, Joseph A. Lintner,

Adolf Butenandt Adolf Friedrich Johann Butenandt (; 24 March 1903 – 18 January 1995) was a German biochemist. He was awarded the Nobel Prize in Chemistry in 1939 for his "work on sex hormones." He initially rejected the award in accordance with government pol ...

, and

ethologist Ethology is the scientific study of animal behaviour, usually with a focus on behaviour under natural conditions, and viewing behaviour as an evolutionarily adaptive trait. Behaviourism as a term also describes the scientific and objecti ...

Karl von Frisch Karl Ritter von Frisch, (20 November 1886 – 12 June 1982) was a German-Austrian ethologist who received the Nobel Prize in Physiology or Medicine in 1973, along with Nikolaas Tinbergen and Konrad Lorenz. His work centered on investigations ...

who called them various names, like for instance "alarm substances". These chemical messengers are transported outside of the body and affect neurocircuits, including the autonomous nervous system with

hormone A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are requir ...

cytokine Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptides and cannot cross the lipid bilayer of cells to enter the cytoplasm. Cytokines have been shown to be involved in au ...

mediated

physiological Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chem ...

changes, inflammatory signaling,

immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood spl ...

changes and/or behavioral change in the recipient. They proposed the term to describe chemical signals from

conspecific Biological specificity is the tendency of a characteristic such as a behavior or a biochemical variation to occur in a particular species. Biochemist Linus Pauling stated that "Biological specificity is the set of characteristics of living organis ...

s that elicit innate behaviors soon after the German biochemist Adolf Butenandt had characterized the first such chemical, bombykol, a chemically well-characterized pheromone released by the female

silkworm The domestic silk moth (''Bombyx mori''), is an insect from the moth family Bombycidae. It is the closest relative of '' Bombyx mandarina'', the wild silk moth. The silkworm is the larva or caterpillar of a silk moth. It is an economically i ...

to attract mates.

Categorization by function

Aggregation

Aggregation pheromones function in

mate choice Mate choice is one of the primary mechanisms under which evolution can occur. It is characterized by a "selective response by animals to particular stimuli" which can be observed as behavior.Bateson, Paul Patrick Gordon. "Mate Choice." Mate Choic ...

, overcoming host resistance by mass attack, and defense against predators. A group of individuals at one location is referred to as an aggregation, whether consisting of one sex or both sexes. Male-produced sex attractants have been called aggregation pheromones, because they usually result in the arrival of both sexes at a calling site and increase the density of conspecifics surrounding the pheromone source. Most sex pheromones are produced by the females; only a small percentage of sex attractants are produced by males. Aggregation pheromones have been found in members of the

Coleoptera Beetles are insects that form the order Coleoptera (), in the superorder Endopterygota. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 400,000 describe ...

Collembola Springtails (Collembola) form the largest of the three lineages of modern hexapods that are no longer considered insects (the other two are the Protura and Diplura). Although the three orders are sometimes grouped together in a class called E ...

Diptera Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advance ...

Hemiptera Hemiptera (; ) is an order of insects, commonly called true bugs, comprising over 80,000 species within groups such as the cicadas, aphids, planthoppers, leafhoppers, assassin bugs, bed bugs, and shield bugs. They range in size from to ...

Dictyoptera Dictyoptera (from Greek δίκτυον ''diktyon'' "net" and πτερόν ''pteron'' "wing") is an insect superorder that includes two extant orders of polyneopterous insects: the order Blattodea (termites and cockroaches together) and the o ...

, and

Orthoptera Orthoptera () is an order of insects that comprises the grasshoppers, locusts, and crickets, including closely related insects, such as the bush crickets or katydids and wētā. The order is subdivided into two suborders: Caelifera – grassh ...

. In recent decades, aggregation pheromones have proven useful in the management of many pests, such as the boll weevil ('' Anthonomus grandis''), the pea and bean weevil ('' Sitona lineatus'', and stored product weevils (e.g. '' Sitophilus zeamais'', '' Sitophilus granarius'', and '' Sitophilus oryzae''). Aggregation pheromones are among the most ecologically selective pest suppression methods. They are non-toxic and effective at very low concentrations.

Alarm

Some species release a volatile substance when attacked by a predator that can trigger flight (in

aphid Aphids are small sap-sucking insects and members of the superfamily Aphidoidea. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white woolly aphids. A ...

s) or aggression (in

ant Ants are eusocial insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from vespoid wasp ancestors in the Cretaceous period. More than 13,800 of an estimated total o ...

bee Bees are winged insects closely related to wasps and ants, known for their roles in pollination and, in the case of the best-known bee species, the western honey bee, for producing honey. Bees are a monophyletic lineage within the superfa ...

termite Termites are small insects that live in colonies and have distinct castes ( eusocial) and feed on wood or other dead plant matter. Termites comprise the infraorder Isoptera, or alternatively the epifamily Termitoidae, within the order Blat ...

s) in members of the same species. For example, '' Vespula squamosa'' use alarm pheromones to alert others to a threat. In ''

Polistes exclamans ''Polistes exclamans'', the Guinea paper wasp, is a social wasp and is part of the family Vespidae of the order Hymenoptera. It is found throughout the United States, Mexico, the Bahamas, Jamaica and parts of Canada. Due to solitary nest found ...

'', alarm pheromones are also used as an alert to incoming predators. Pheromones also exist in plants: Certain plants emit alarm pheromones when grazed upon, resulting in

tannin Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids. The term ''tannin'' (from Anglo-Norman ''tanne ...

production in neighboring plants. These tannins make the plants less appetizing to

herbivore A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthp ...

Epideictic

Epideictic pheromones are different from territory pheromones, when it comes to insects. Fabre observed and noted how "females who lay their eggs in these fruits deposit these mysterious substances in the vicinity of their clutch to signal to other females of the same species they should clutch elsewhere." It may be helpful to note that the word

epideictic The epideictic oratory, also called ceremonial oratory, or praise-and-blame rhetoric, is one of the three branches, or "species" (eidē), of rhetoric as outlined in Aristotle's ''Rhetoric'', to be used to praise or blame during ceremonies. Origin ...

, having to do with display or show (from the Greek 'deixis'), has a different but related meaning in rhetoric, the human art of persuasion by means of words.

Territorial

Laid down in the environment, territorial pheromones mark the boundaries and identity of an organism's territory. In cats and dogs, these hormones are present in the urine, which they deposit on landmarks serving to mark the perimeter of the claimed territory. In social seabirds, the preen gland is used to mark nests, nuptial gifts, and territory boundaries with behavior formerly described as ' displacement activity'.

Trail

Social insects commonly use trail pheromones. For example,

s mark their paths with pheromones consisting of volatile

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or e ...

s. Certain ants lay down an initial trail of pheromones as they return to the nest with food. This trail attracts other ants and serves as a guide. As long as the food source remains available, visiting ants will continuously renew the pheromone trail. The pheromone requires continuous renewal because it evaporates quickly. When the food supply begins to dwindle, the trail-making ceases. Pharaoh ants ('' Monomorium pharaonis'') mark trails that no longer lead to food with a repellent pheromone, which causes avoidance behaviour in ants. Repellent trail markers may help ants to undertake more efficient collective exploration. The army ant '' Eciton burchellii'' provides an example of using pheromones to mark and maintain foraging paths. When species of wasps such as '' Polybia sericea'' found new nests, they use pheromones to lead the rest of the colony to the new nesting site. Gregarious caterpillars, such as the forest tent caterpillar, lay down pheromone trails that are used to achieve group movement.

Sex

In animals, sex pheromones indicate the availability of the female for breeding. Male animals may also emit pheromones that convey information about their species and

genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...

. At the microscopic level, a number of bacterial species (e.g. ''

Bacillus subtilis ''Bacillus subtilis'', known also as the hay bacillus or grass bacillus, is a Gram-positive, catalase-positive bacterium, found in soil and the gastrointestinal tract of ruminants, humans and marine sponges. As a member of the genus ''Bacillus ...

'', ''

Streptococcus pneumoniae ''Streptococcus pneumoniae'', or pneumococcus, is a Gram-positive, spherical bacteria, alpha-hemolytic (under aerobic conditions) or beta-hemolytic (under anaerobic conditions), aerotolerant anaerobic member of the genus Streptococcus. They ar ...

'', ''

Bacillus cereus ''Bacillus cereus'' is a Gram-positive rod-shaped bacterium commonly found in soil, food, and marine sponges. The specific name, ''cereus'', meaning "waxy" in Latin, refers to the appearance of colonies grown on blood agar. Some strains are h ...

'') release specific chemicals into the surrounding media to induce the "competent" state in neighboring bacteria. Competence is a physiological state that allows bacterial cells to take up DNA from other cells and incorporate this DNA into their own genome, a sexual process called transformation. Among eukaryotic microorganisms, pheromones promote sexual interaction in numerous species. These species include the yeast ''

Saccharomyces cerevisiae ''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungus microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have b ...

'', the filamentous fungi ''

Neurospora crassa ''Neurospora crassa'' is a type of red bread mold of the phylum Ascomycota. The genus name, meaning "nerve spore" in Greek, refers to the characteristic striations on the spores. The first published account of this fungus was from an infestation ...

'' and

Mucor mucedo ''Mucor'' is a microbial genus of approximately 40 species of molds in the family Mucoraceae. Species are commonly found in soil, digestive systems, plant surfaces, some cheeses like Tomme de Savoie, rotten vegetable matter and iron oxide r ...

, the water mold '' Achlya ambisexualis'', the aquatic fungus '' Allomyces macrogynus'', the slime mold ''

Dictyostelium discoideum ''Dictyostelium discoideum'' is a species of soil-dwelling amoeba belonging to the phylum Amoebozoa, infraphylum Mycetozoa. Commonly referred to as slime mold, ''D. discoideum'' is a eukaryote that transitions from a collection of unicellular ...

'', the ciliate protozoan '' Blepharisma japonicum'' and the multicellular green algae ''

Volvox carteri ''Volvox carteri'' is a species of colonial green algae in the order Volvocales. The ''V. carteri'' life cycle includes a sexual phase and an asexual phase. ''V. carteri'' forms small spherical colonies, or coenobia, of 2000–6000 '' Chlamydom ...

''. In addition, male

copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat. Some species are planktonic (inhabiting sea waters), some are benthic (living on the ocean floor), a number of species have ...

s can follow a

three-dimensional Three-dimensional space (also: 3D space, 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called '' parameters'') are required to determine the position of an element (i.e., point). This is the inform ...

pheromone trail left by a swimming female, and male

gamete A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...

s of many animals use a pheromone to help find a female gamete for

fertilization Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Pro ...

. Many well-studied insect species, such as the ant '' Leptothorax acervorum'', the moths ''

Helicoverpa zea ''Helicoverpa zea, ''commonly known as the corn earworm, is a species (formerly in the genus ''Heliothis'') in the family Noctuidae. The larva of the moth ''Helicoverpa zea'' is a major agricultural pest. Since it is polyphagous (feeds on many ...

'' and ''

Agrotis ipsilon ''Agrotis ipsilon'', the dark sword-grass, black cutworm, greasy cutworm, floodplain cutworm or ipsilon dart, is a small noctuid moth found worldwide. The moth gets its scientific name from black markings on its forewings shaped like the letter ...

'', the bee ''

Xylocopa sonorina ''Xylocopa sonorina'', the valley carpenter bee or Hawaiian carpenter bee, is a species of carpenter bee found from western Texas to northern California, and the eastern Pacific islands.Hurd, Jr., Paul (1958).The carpenter bees of the eastern Pa ...

,'' the frog Pseudophryne bibronii, and the butterfly Edith's checkerspot release sex pheromones to attract a mate, and some

lepidoptera Lepidoptera ( ) is an order of insects that includes butterflies and moths (both are called lepidopterans). About 180,000 species of the Lepidoptera are described, in 126 families and 46 superfamilies, 10 percent of the total described sp ...

ns (moths and butterflies) can detect a potential mate from as far away as . Some insects, such as

ghost moth The ghost moth or ghost swift (''Hepialus humuli'') is a moth of the family Hepialidae. It is common throughout Europe, except for in the far south-east. Female ghost moths are larger than males, and exhibit sexual dimorphism with their diffe ...

s, use pheromones during

lek mating A lek is an aggregation of male animals gathered to engage in competitive displays and courtship rituals, known as lekking, to entice visiting females which are surveying prospective partners with which to mate. A lek can also indicate an avail ...

. Traps containing pheromones are used by farmers to detect and monitor insect populations in orchards. In addition, '' Colias eurytheme ''butterflies release pheromones, an olfactory cue important for mate selection. The effect of Hz-2V virus infection on the reproductive physiology and behavior of female ''

'' moths is that in the absence of males they exhibited calling behavior and called as often but for shorter periods on average than control females. Even after these contacts virus-infected females made many frequent contacts with males and continued to call; they were found to produce five to seven times more pheromone and attracted twice as many males as did control females in flight tunnel experiments. Pheromones are also utilized by bee and wasp species. Some pheromones can be used to suppress the sexual behavior of other individuals allowing for a reproductive monopoly – the wasp '' R. marginata ''uses this. With regard to the '' Bombus hyperboreus'' species, males, otherwise known as drones, patrol circuits of scent marks (pheromones) to find queens. In particular, pheromones for the ''Bombus hyperboreus,'' include octadecenol, 2,3-dihydro-6-transfarnesol, citronellol, and geranylcitronellol.

Sea urchin Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells ( tests) ...

s release pheromones into the surrounding water, sending a chemical message that triggers other urchins in the colony to eject their sex cells simultaneously. In plants, some homosporous ferns release a chemical called

antheridiogen Antheridiogens are a class of chemicals secreted by fern gametophytes that have "been shown to influence production of male gametangia and thus mating systems in a large number of terrestrial fern species". Antheridiogens are only observed in homos ...

, which affects sex expression. This is very similar to pheromones.

Other

This classification, based on the effects on behavior, remains artificial. Pheromones fill many additional functions. * Nasonov pheromones (worker bees) * Royal pheromones (bees) * Calming (appeasement) pheromones (mammals) * Necromones, given off by a deceased and decomposing organism; consisting of oleic and

linoleic acid Linoleic acid (LA) is an organic compound with the formula COOH(CH2)7CH=CHCH2CH=CH(CH2)4CH3. Both alkene groups are ''cis''. It is a fatty acid sometimes denoted 18:2 (n-6) or 18:2 ''cis''-9,12. A linoleate is a salt or ester of this acid. L ...

s, they allow crustaceans and hexapods to identify the presence of dead conspecifics. * Suckling:

TAA Trans Australia Airlines (TAA), renamed Australian Airlines in 1986, was one of the two major Australian domestic airlines between its inception in 1946 and its merger with Qantas in September 1992. As a result of the "COBRA" (or Common Brand ...

is present in rabbit milk and seems to play a role of pheromone inducing suckling in the newborn rabbit.

Categorization by type

Releaser

Releaser pheromones are pheromones that cause an alteration in the behavior of the recipient. For example, some organisms use powerful attractant molecules to attract mates from a distance of two miles or more. In general, this type of pheromone elicits a rapid response, but is quickly degraded. In contrast, a primer pheromone has a slower onset and a longer duration. For example, rabbit (mothers) release mammary pheromones that trigger immediate nursing behavior by their babies.

Primer

Primer pheromones trigger a change of developmental events (in which they differ from all the other pheromones, which trigger a change in behavior). They were first described in '' Schistocerca gregaria'' by Maud Norris in 1954.

Signal

Signal pheromones cause short-term changes, such as the neurotransmitter release that activates a response. For instance, GnRH molecule functions as a neurotransmitter in rats to elicit lordosis behavior.

Pheromone receptors

In the olfactory epithelium

The human

trace amine-associated receptor Trace amine-associated receptors (TAARs), sometimes referred to as trace amine receptors (TAs or TARs), are a class of G protein-coupled receptors that were discovered in 2001. TAAR1, the first of six functional human TAARs, has gained considera ...

s are a group of six

G protein-coupled receptor G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of evolutionarily-related p ...

s (i.e.,

TAAR1 Trace amine-associated receptor 1 (TAAR1) is a trace amine-associated receptor (TAAR) protein that in humans is encoded by the ''TAAR1'' gene. TAAR1 is an intracellular amine-activated and G protein-coupled receptor (GPCR) that is primarily ex ...

TAAR2 Trace amine-associated receptor 2 (TAAR2), formerly known as G protein-coupled receptor 58 (GPR58), is a protein that in humans is encoded by the ''TAAR2'' gene. TAAR2 is coexpressed with Gα proteins; however, its signal transduction mechanism ...

, TAAR5, TAAR6, TAAR8, and

TAAR9 Trace amine-associated receptor 9 is a protein that in humans is encoded by the ''TAAR9'' gene. TAAR9 is a member of a large family of rhodopsin G protein–coupled receptors (GPCRs, or GPRs). GPCRs contain 7 transmembrane domains and transduce e ...

) that – with exception for TAAR1 – are expressed in the human

olfactory epithelium The olfactory epithelium is a specialized epithelial tissue inside the nasal cavity that is involved in smell. In humans, it measures 9 cm2 and lies on the roof of the nasal cavity about 7 cm above and behind the nostrils. The olfactory ...

. In humans and other animals, TAARs in the

function as

olfactory receptor Olfactory receptors (ORs), also known as odorant receptors, are chemoreceptors expressed in the cell membranes of olfactory receptor neurons and are responsible for the detection of odorants (for example, compounds that have an odor) which give ...

s that detect volatile

amine In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent su ...

odorant An aroma compound, also known as an odorant, aroma, fragrance or flavoring, is a chemical compound that has a smell or odor. For an individual chemical or class of chemical compounds to impart a smell or fragrance, it must be sufficiently vo ...

s, including certain pheromones; these TAARs putatively function as a class of pheromone receptors involved in the olfactive detection of social cues. A review of studies involving non-human animals indicated that TAARs in the olfactory epithelium can mediate

attractive Attraction may refer to: * Interpersonal attraction, the attraction between people which leads to friendships, platonic and romantic relationships ** Physical attractiveness, attraction on the basis of beauty ** Sexual attraction * Object or even ...

aversive In psychology, aversives are unpleasant stimuli that induce changes in behavior via negative reinforcement or positive punishment. By applying an aversive immediately before or after a behavior the likelihood of the target behavior occurring in t ...

behavioral responses to a

receptor agonist An agonist is a chemical that activates a receptor to produce a biological response. Receptors are cellular proteins whose activation causes the cell to modify what it is currently doing. In contrast, an antagonist blocks the action of the ag ...

. This review also noted that the behavioral response evoked by a TAAR can vary across species (e.g., TAAR5 mediates attraction to

trimethylamine Trimethylamine (TMA) is an organic compound with the formula N(CH3)3. It is a colorless, hygroscopic, and flammable tertiary amine. It is a gas at room temperature but is usually sold as a 40% solution in water. (It is also sold in pressurize ...

in mice and aversion to trimethylamine in rats).
Figure 2: Table of ligands, expression patterns, and species-specific behavioral responses for each TAAR
/ref> In humans, hTAAR5 presumably mediates aversion to trimethylamine, which is known to act as an hTAAR5 agonist and to possess a foul, fishy odor that is aversive to humans; however, hTAAR5 is not the only olfactory receptor that is responsible for trimethylamine olfaction in humans. hTAAR5-mediated trimethylamine aversion has not been examined in published research.

In the vomeronasal organ

reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates ( lizards and snakes) and rhynchoceph ...

amphibia Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arb ...

and non-primate mammals pheromones are detected by regular

olfactory 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, ...

membranes, and also by 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. ...

(VNO), or Jacobson's organ, which lies at the base of the

nasal septum The nasal septum () separates the left and right airways of the nasal cavity, dividing the two nostrils. It is depressed by the depressor septi nasi muscle. Structure The fleshy external end of the nasal septum is called the columella or ...

between the nose and mouth and is the first stage of the accessory olfactory system. While the VNO is present in most amphibia, reptiles, and non-primate mammals, it is absent in

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightwei ...

s, adult

catarrhine The parvorder Catarrhini , catarrhine monkeys, Old World anthropoids, or Old World monkeys, consisting of the Cercopithecoidea and apes (Hominoidea). In 1812, Geoffroy grouped those two groups together and established the name Catarrhini, "Ol ...

monkeys (downward facing nostrils, as opposed to sideways), and

ape Apes (collectively Hominoidea ) are a clade of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and as well as Europe in prehistory), which together with its sister ...

s. An active role for the human VNO in the detection of pheromones is disputed; while it is clearly present in the

fetus A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal devel ...

it appears to be atrophied, shrunk or completely absent in adults. Three distinct families of vomeronasal receptors, putatively pheromone sensing, have been identified in the vomeronasal organ named V1Rs, V2Rs, and V3Rs. All are

G protein-coupled receptors G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of evolutionarily-related p ...

but are only distantly related to the receptors of the main olfactory system, highlighting their different role.

Evolution

Olfactory processing of chemical signals like pheromones exists in all animal phyla and is thus the oldest of the senses. It has been suggested that it serves survival by generating appropriate behavioral responses to the signals of threat, sex and dominance status among members of the same species. Furthermore, it has been suggested that in the evolution of

s to

s, primordial pheromone signaling between individuals may have evolved to

paracrine Paracrine signaling is a form of cell signaling, a type of cellular communication in which a cell produces a signal to induce changes in nearby cells, altering the behaviour of those cells. Signaling molecules known as paracrine factors diffuse ove ...

and endocrine signaling within individual organisms. Some authors assume that approach-avoidance reactions in animals, elicited by chemical cues, form the phylogenetic basis for the experience of emotions in humans.

Evolution of sex pheromones

Avoidance of inbreeding

Mice can distinguish close relatives from more distantly related individuals on the basis of scent signals, which enables them to avoid mating with close relatives and minimizes deleterious

inbreeding Inbreeding is the production of offspring from the mating or breeding of individuals or organisms that are closely related genetically. By analogy, the term is used in human reproduction, but more commonly refers to the genetic disorders and ...

. Jiménez et al. showed that inbred mice had significantly reduced survival when they were reintroduced into a natural habitat. In addition to mice, two species of bumblebee, in particular Bombus bifarius and Bombus frigidus, have been observed to use pheromones as a means of kin recognition to avoid inbreeding. For example, B. bifarius males display "patrolling" behavior in which they mark specific paths outside their nests with pheromones and subsequently "patrol" these paths. Unrelated reproductive females are attracted to the pheromones deposited by males on these paths, and males that encounter these females while patrolling can mate with them. Other bees of the Bombus species are found to emit pheromones as precopulatory signals, such as '' Bombus lapidarius''.

Applications

Pheromone trapping

Pheromones of certain pest insect species, such as the

Japanese beetle The Japanese beetle (''Popillia japonica'') is a species of scarab beetle. The adult measures in length and in width, has iridescent copper-colored elytra and a green thorax and head. It is not very destructive in Japan (where it is controlle ...

, acrobat ant, and the

gypsy moth ''Lymantria dispar'', also known as the gypsy moth or the spongy moth, is a species of moth in the family Erebidae. ''Lymantria dispar'' is subdivided into several subspecies, with subspecies such as ''L. d. dispar'' and ''L. d. japonica'' bei ...

, can be used to trap the respective insect for monitoring purposes, to control the population by creating confusion, to disrupt mating, and to prevent further egg laying.

Animal husbandry

Pheromones are used in the detection of

oestrus The estrous cycle (, originally ) is the set of recurring physiological changes that are induced by reproductive hormones in most mammalian therian females. Estrous cycles start after sexual maturity in females and are interrupted by anestrous ...

in sows.

Boar The wild boar (''Sus scrofa''), also known as the wild swine, common wild pig, Eurasian wild pig, or simply wild pig, is a suid native to much of Eurasia and North Africa, and has been introduced to the Americas and Oceania. The species is ...

pheromones are sprayed into the

sty A sty or pigsty is a small-scale outdoor enclosure for raising domestic pigs as livestock. It is sometimes referred to as a hog pen, hog parlor, pigpen, pig parlor, or pig-cote, although pig pen may refer to pens confining pigs that are ke ...

, and those sows that exhibit

sexual arousal Sexual arousal (also known as sexual excitement) describes the physiological and psychological responses in preparation for sexual intercourse or when exposed to sexual stimuli. A number of physiological responses occur in the body and mind ...

are known to be currently available for breeding.

Human sex pheromone controversies

While humans are highly dependent upon visual cues, when in close proximity smells also play a role in sociosexual behaviors. An inherent difficulty in studying human pheromones is the need for cleanliness and odorlessness in human participants. Though various researchers have investigated the possibility of their existence, no pheromonal substance has ever been demonstrated to directly influence human behavior in a

peer review Peer review is the evaluation of work by one or more people with similar competencies as the producers of the work ( peers). It functions as a form of self-regulation by qualified members of a profession within the relevant field. Peer revie ...

ed study.Wyatt, Tristram D. (2003). ''Pheromones and Animal Behaviour: Communication by Smell and Taste.'' Cambridge: Cambridge University Press. . p. 298 Quoting Preti & Weski (1999) "No peer reviewed data supporting the presences of ... human ... pheromones that cause rapid behavioral changes, such as attraction and/or

copulation Sexual intercourse (or coitus or copulation) is a sexual activity typically involving the insertion and thrusting of the penis into the vagina for sexual pleasure or reproduction.Sexual intercourse most commonly means penile–vaginal penetra ...

have been documented." p. 264 ...there has not yet been any hard evidence for human pheromones that might hangesexual attraction (for members of either sex) aturally/ref> Experiments have focused on three classes of possible human pheromones: axillary steroids, vaginal aliphatic acids, and stimulators of the

Axillary steroids

Axillary steroids are produced by the

testes A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testoste ...

ovaries The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. ...

apocrine Apocrine () glands are a type of exocrine gland, which are themselves a type of gland, i.e. a group of cells specialized for the release of secretions. Exocrine glands secrete by one of three means: holocrine, merocrine and apocrine. In apocr ...

glands, and

adrenal gland The adrenal glands (also known as suprarenal glands) are endocrine glands that produce a variety of hormones including adrenaline and the steroids aldosterone and cortisol. They are found above the kidneys. Each gland has an outer cortex whic ...

s. These chemicals are not biologically active until puberty when sex steroids influence their activity. The change in activity during puberty suggest that humans may communicate through odors. Several axillary steroids have been described as possible human pheromones:

androstadienol Androstadienol, or androsta-5,16-dien-3β-ol, is a 16-androstene class endogenous steroid, pheromone, and chemical intermediate to several other pheromones that is found in the sweat of both men and women. Androstadienol and androstadienone a ...

androstadienone Androstadienone, or androsta-4,16-dien-3-one, is a 16-androstene class endogenous steroid that has been described as having potent pheromone-like activities in humans. The compound is synthesized from androstadienol by 3β-hydroxysteroid dehydro ...

androstenol Androstenol, also known as 5α-androst-16-en-3α-ol (shortened to 3α,5α-androstenol or 3α-androstenol), is a 16-androstene class steroidal pheromone and neurosteroid in humans and other mammals, notably pigs. It possesses a characteristic musk ...

androstenone Androstenone (5α-androst-16-en-3-one) is a 16-androstene class steroidal pheromone. It is found in boar's saliva, celery cytoplasm, and truffle fungus. Androstenone was the first mammalian pheromone to be identified. It is found in high concentr ...

, and

androsterone Androsterone, or 3α-hydroxy-5α-androstan-17-one, is an endogenous steroid hormone, neurosteroid, and putative pheromone. It is a weak androgen with a potency that is approximately 1/7 that of testosterone. Androsterone is a metabolite of tes ...

. * Androstenol is the putative female pheromone. In a 1978 study by Kirk-Smith, people wearing surgical masks treated with androstenol or untreated were shown pictures of people, animals and buildings and asked to rate the pictures on attractiveness. Individuals with their masks treated with androstenol rated their photographs as being "warmer" and "more friendly". The best-known case study involves the synchronization of

menstrual cycle The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that make pregnancy possible. The ovarian cycle controls the production and release of eggs ...

s among women based on unconscious odor cues, the '' McClintock effect'', named after the primary investigator, Martha McClintock, of the University of Chicago. A group of women were exposed to a whiff of perspiration from other women. Depending on the time in the month the sweat was collected (before, during, or after ovulation) there was an association with the recipient woman's menstrual cycle to speed up or slow down. The 1971 study proposed two types of pheromone involved: "One, produced prior to ovulation, shortens the ovarian cycle; and the second, produced just at ovulation, lengthens the cycle". However, recent studies and reviews of the methodology have called the validity of her results into question. * Androstenone is postulated to be secreted only by males as an attractant for women, and thought to be a positive effector for their mood. It seems to have different effects on women, depending on where a female is in her menstrual cycle, with the highest sensitivity to it during ovulation. In 1983, study participants exposed to androstenone were shown to undergo changes in skin conductance. Androstenone has been found to be perceived as more pleasant to women during their time of ovulation. * Androstadienone seems to affect the limbic system and causes a positive reaction in women, improving mood. Responses to androstadienone depend on the individual and the environment they are in. Androstadienone negatively influences the perception of pain in women. Women tend to react positively after androstadienone presentation, while men react more negatively. In an experiment by Hummer and McClintock, androstadienone or a control odor was put on the upper lips of fifty males and females and they were tested for four effects of the pheromone: 1) automatic attention towards positive and negative facial expressions, 2) the strength of cognitive and emotional information as distractors in a simple reaction time task, 3) relative attention to social and nonsocial stimuli (i.e. neutral faces), and 4) mood and attentiveness in the absence of social interaction. Those treated with androstadienone drew more attention to towards emotional facial expressions and emotional words but no increased attention to neutral faces. These data suggest that androstadienone may increase attention to emotional information causing the individual to feel more focused. It is thought that androstadienone modulates on how the mind attends and processes information. While it may be expected on evolutionary grounds that humans have pheromones, these three molecules have yet to be rigorously proven to act as such. Research in this field has suffered from small sample sizes,

publication bias In published academic research, publication bias occurs when the outcome of an experiment or research study biases the decision to publish or otherwise distribute it. Publishing only results that show a significant finding disturbs the balance o ...

, false positives, and poor methodology.

Vaginal aliphatic acids

A class of aliphatic acids (volatile

fatty acids In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, f ...

as a kind of

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carbox ...

) was found in female

rhesus monkey The rhesus macaque (''Macaca mulatta''), colloquially rhesus monkey, is a species of Old World monkey. There are between six and nine recognised subspecies that are split between two groups, the Chinese-derived and the Indian-derived. Generally b ...

s that produced six types in the vaginal fluids. The combination of these acids is referred to as "copulins". One of the acids, acetic acid, was found in all of the sampled female's vaginal fluid. Even in humans, one-third of women have all six types of copulins, which increase in quantity before ovulation. Copulins are used to signal ovulation; however, as human ovulation is concealed it is thought that they may be used for reasons other than sexual communication.

Stimulators of the vomeronasal organ

The human

has

epithelia Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellu ...

that may be able to serve as a chemical sensory organ; however, the genes that encode the VNO receptors are nonfunctional

pseudogene Pseudogenes are nonfunctional segments of DNA that resemble functional genes. Most arise as superfluous copies of functional genes, either directly by DNA duplication or indirectly by reverse transcription of an mRNA transcript. Pseudogenes ar ...

s in humans. Also, while there are sensory neurons in the human VNO there seem to be no connections between the VNO and the central nervous system. The associated olfactory bulb is present in the fetus, but regresses and vanishes in the adult brain. There have been some reports that the human VNO does function, but only responds to hormones in a "sex-specific manner". There also have been pheromone receptor genes found in olfactory mucosa. Unfortunately, there have been no experiments that compare people lacking the VNO, and people that have it. It is disputed on whether the chemicals are reaching the brain through the VNO or other tissues. In 2006, it was shown that a second mouse receptor sub-class is found in the

. Called the

s (TAAR), some are activated by volatile

s found in mouse urine, including one putative mouse pheromone.

Ortholog Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a sp ...

ous receptors exist in humans providing, the authors propose, evidence for a mechanism of human pheromone detection. Although there are disputes about the mechanisms by which pheromones function, there is evidence that pheromones do affect humans. Despite this evidence, it has not been conclusively shown that humans have functional pheromones. Those experiments suggesting that certain pheromones have a positive effect on humans are countered by others indicating they have no effect whatsoever. A possible theory being studied now is that these axillary odors are being used to provide information about the immune system. Milinski and colleagues found that the artificial odors that people chose are determined in part by their

major histocompatibility complex The major histocompatibility complex (MHC) is a large locus on vertebrate DNA containing a set of closely linked polymorphic genes that code for cell surface proteins essential for the adaptive immune system. These cell surface proteins are cal ...

es (MHC) combination. Information about an individual's immune system could be used as a way of "sexual selection" so that the female could obtain good genes for her offspring. Claus Wedekind and colleagues found that both men and women prefer the axillary odors of people whose MHC is different from their own. Some body spray advertisers claim that their products contain human sexual pheromones that act as an

aphrodisiac An aphrodisiac is a substance that increases sexual desire, sexual attraction, sexual pleasure, or sexual behavior. Substances range from a variety of plants, spices, foods, and synthetic chemicals. Natural aphrodisiacs like cannabis or cocain ...

. Despite these claims, no pheromonal substance has ever been demonstrated to directly influence human behavior in a

ed study. Thus, the role of pheromones in human behavior remains speculative and controversial.

References

External links

Pherobase
the database of insect pheromones
Sexual Orientation, in the Brain
{{Authority control * Endocrinology Chemical ecology

Background

Categorization by function

Aggregation

Alarm

Epideictic

Territorial

Trail

Sex

Other

Categorization by type

Releaser

Primer

Signal

Pheromone receptors

In the olfactory epithelium

In the vomeronasal organ

Evolution

Evolution of sex pheromones

Avoidance of inbreeding

Applications

Pheromone trapping

Animal husbandry

Human sex pheromone controversies

Axillary steroids

Vaginal aliphatic acids

Stimulators of the vomeronasal organ

See also

References

Further reading

External links