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A fanning honeybee exposes Nasonov's gland (white – at tip of abdomen) releasing pheromone to entice swarm into an empty hive

A pheromone (from Ancient Greek φέρω (phérō) 'to bear' and hormone) is a chemical that is secreted or excreted by an organism, which triggers a social response in members of the same species.[1] There are alarm pheromones, food trail pheromones, sex pheromones, and many others that affect behavior or physiology. Pheromones are used by many organisms, from basic unicellular prokaryotes to complex multicellular eukaryotes.[2] Their use among insects has been particularly well documented. In addition, some vertebrates, plants and ciliates communicate by using pheromones. The ecological functions and evolution of pheromones are a major topic of research in the field of chemical ecology.[3]

Background

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The portmanteau word "pheromone" was coined by Peter Karlson and Martin Lüscher in 1959, based on the Greek φέρω phérō ('I carry') and ὁρμων hórmōn ('stimulating').[4] Pheromones are also sometimes classified as ecto-hormones ("ecto-" meaning "outside"[5]). They were researched earlier by various scientists, including Jean-Henri Fabre, Joseph A. Lintner, Adolf Butenandt, and ethologist Karl von Frisch who called them various names, such as "alarm substances". These chemical messengers are transported outside of the body and affect neurocircuits, including the autonomous nervous system with hormone or cytokine mediated physiological changes, inflammatory signaling, immune system changes and/or behavioral change in the recipient.[6] They proposed the term to describe chemical signals from conspecifics 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 to attract mates.[7]

Categorization by function

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Aggregation

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Aggregation of bug nymphs
Aggregation of the water springtail Podura aquatica

Aggregation pheromones function in mate choice, 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.[8] Aggregation pheromones have been found in members of the Coleoptera, Collembola,[9] Diptera, Hemiptera, Dictyoptera, and Orthoptera. 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.[10]

Alarm

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Main article: Alarm signal
A milkweed aphid on narrow-leaf milkweed is attacked by a hoverfly larvae. It thrashs and release pheromones and sticky wax from its cornicles. Nearby aphids flee. Video played at 4X speed.

Some species release a volatile substance when attacked by a predator that can trigger flight (in aphids) or aggression (in ants, bees, termites, and wasps)[11][12][13][14][15] in members of the same species. For example, Vespula squamosa use alarm pheromones to alert others to a threat.[16] In Polistes exclamans, alarm pheromones are also used as an alert to incoming predators.[17] Pheromones also exist in plants: Certain plants emit alarm pheromones when grazed upon, resulting in tannin production in neighboring plants.[18] These tannins make the plants less appetizing to herbivores.[18]

An alarm pheromone has been documented in a mammalian species. Alarmed pronghorn, Antilocapra americana flair their white rump hair and exposes two highly odoriferous glands that releases a compound described having the odor "reminiscent of buttered popcorn". This sends a message to other pronghorns by both sight and smell about a present danger. This scent has been observed by humans 20 to 30 meters downwind from alarmed animals. The major odour compound identified from this gland is 2-pyrrolidinone.[19]

Epideictic

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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, 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

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Dogs communicate using pheromones and olfactory signals in urine.[20]

Laid down in the environment, territorial pheromones mark the boundaries and identity of an organism's territory. Cats and dogs deposit these pheromones by urinating on landmarks that 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'.[21]

Trail

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Main article: Trail pheromone

Social insects commonly use trail pheromones. For example, ants mark their paths with pheromones consisting of volatile hydrocarbons. 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.[22] 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.[23] Repellent trail markers may help ants to undertake more efficient collective exploration.[24] 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.[25]

Sex

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Main article: Sex pheromone
Male Danaus chrysippus showing the pheromone pouch and brush-like organ in Kerala, India

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.

At the microscopic level, a number of bacterial species (e.g. Bacillus subtilis, Streptococcus pneumoniae, Bacillus cereus) release specific chemicals into the surrounding media to induce the "competent" state in neighboring bacteria.[26] 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.[27] These species include the yeast Saccharomyces cerevisiae, the filamentous fungi Neurospora crassa and Mucor mucedo, the water mold Achlya ambisexualis, the aquatic fungus Allomyces macrogynus, the slime mold Dictyostelium discoideum, the ciliate protozoan Blepharisma japonicum and the multicellular green algae Volvox carteri. In addition, male copepods can follow a three-dimensional pheromone trail left by a swimming female, and male gametes of many animals use a pheromone to help find a female gamete for fertilization.[28]

Many well-studied insect species, such as the ant Leptothorax acervorum, the moths Helicoverpa zea and Agrotis ipsilon, the bee Xylocopa sonorina, the frog Pseudophryne bibronii, and the butterfly Edith's checkerspot release sex pheromones to attract a mate, and some lepidopterans (moths and butterflies) can detect a potential mate from as far away as 10 km (6.2 mi).[29][30] Some insects, such as ghost moths, use pheromones during lek mating.[31] 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.[32] In mealworm beetles, Tenebrio molitor, the female preference of pheromones is dependent on the nutritional condition of the males.

The effect of Hz-2V virus infection on the reproductive physiology and behavior of female Helicoverpa zea 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.[33]

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.[34] With regard to the Bombus hyperboreus species, males, otherwise known as drones, patrol circuits of scent marks (pheromones) to find queens.[35] In particular, pheromones for the Bombus hyperboreus, include octadecenol, 2,3-dihydro-6-transfarnesol, citronellol, and geranylcitronellol.[36]

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

Other

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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 acids, they allow crustaceans and hexapods to identify the presence of dead conspecifics.[37]
  • Suckling: TAA is present in rabbit milk and seems to play a role of pheromone inducing suckling in the newborn rabbit.[38]

Categorization by type

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Releaser

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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.[21]

Primer

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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.[39]

Signal

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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.[6]

Pheromone receptors

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In the olfactory epithelium

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Further information: Trace amine-associated receptor

The human trace amine-associated receptors are a group of six G protein-coupled receptors (i.e., TAAR1, TAAR2, TAAR5, TAAR6, TAAR8, and TAAR9) that – with exception for TAAR1 – are expressed in the human olfactory epithelium.[40] In humans and other animals, TAARs in the olfactory epithelium function as olfactory receptors that detect volatile amine odorants, including certain pheromones;[40][41] these TAARs putatively function as a class of pheromone receptors involved in the olfactive detection of social cues.[40][41]

A review of studies involving non-human animals indicated that TAARs in the olfactory epithelium can mediate attractive or aversive behavioral responses to a receptor agonist.[41] This review also noted that the behavioral response evoked by a TAAR can vary across species (e.g., TAAR5 mediates attraction to trimethylamine in mice and aversion to trimethylamine in rats).[41] 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;[41][42] however, hTAAR5 is not the only olfactory receptor that is responsible for trimethylamine olfaction in humans.[41][42] As of December 2015, hTAAR5-mediated trimethylamine aversion has not been examined in published research.[42]

In the vomeronasal organ

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Further information: Vomeronasal receptor

In reptiles, amphibia and non-primate mammals pheromones are detected by regular olfactory membranes, and also by the vomeronasal organ (VNO), or Jacobson's organ, which lies at the base of the nasal septum between the nose and mouth and is the first stage of the accessory olfactory system.[43] While the VNO is present in most amphibia, reptiles, and non-primate mammals,[44] it is absent in birds, adult catarrhine monkeys (downward facing nostrils, as opposed to sideways), and apes.[45] An active role for the human VNO in the detection of pheromones is disputed; while it is clearly present in the fetus 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 but are only distantly related to the receptors of the main olfactory system, highlighting their different role.[43]

Evolution

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Olfactory processing of chemical signals like pheromones exists in all animal phyla and is thus the oldest of the senses.[citation needed] 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.[46]

Furthermore, it has been suggested that in the evolution of unicellular prokaryotes to multicellular eukaryotes, primordial pheromone signaling between individuals may have evolved to paracrine and endocrine signaling within individual organisms.[47]

Some authors assume that approach-avoidance reactions in animals, elicited by chemical cues, form the phylogenetic basis for the experience of emotions in humans.[48]

Evolution of sex pheromones

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Avoidance of inbreeding

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See also: Inbreeding avoidance

Mice can distinguish close relatives from more distantly related individuals on the basis of scent signals,[49] which enables them to avoid mating with close relatives and minimizes deleterious inbreeding.[50]

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.[51] For example, B. bifarius males display "patrolling" behavior in which they mark specific paths outside their nests with pheromones and subsequently "patrol" these paths.[51] 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.[51] Other bees of the Bombus species are found to emit pheromones as precopulatory signals, such as Bombus lapidarius.[52]

Applications

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Pheromone trapping

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Main article: Pheromone trap

Pheromones of certain pest insect species, such as the Japanese beetle, acrobat ant, and the spongy moth, 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

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Pheromones are used in the detection of oestrus in sows. Boar pheromones are sprayed into the sty, and those sows that exhibit sexual arousal are known to be currently available for breeding.

Human sex pheromone controversies

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Further information: Human sex pheromones
See also: Body odor and List of neurosteroids § Pheromones and pherines

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.[53] 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 reviewed study.[54][55][56][57] Experiments have focused on three classes of possible human pheromones: axillary steroids, vaginal aliphatic acids, and stimulators of the vomeronasal organ, including a 2018 study claiming pheromones affect men's sexual cognition.[58]

Axillary steroids

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Axillary steroids are produced by the testes, ovaries, apocrine glands, and adrenal glands.[59] These chemicals are not biologically active until puberty when sex steroids influence their activity.[60] The change in activity during puberty suggest that humans may communicate through odors.[59] Several axillary steroids have been described as possible human pheromones: androstadienol, androstadienone, androstenol, androstenone, and androsterone.

  • Androstenol is the putative female pheromone.[60] 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.[61] Individuals with their masks treated with androstenol rated their photographs as being "warmer" and "more friendly".[61] The best-known case study involves the synchronization of menstrual cycles among women based on unconscious odor cues, the McClintock effect, named after the primary investigator, Martha McClintock, of the University of Chicago.[62][63] 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 and existence of menstrual synchronization into question.[64][65]
  • 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.[60] In 1983, study participants exposed to androstenone were shown to undergo changes in skin conductance.[66] Androstenone has been found to be perceived as more pleasant to women during their time of ovulation.[53]
  • Androstadienone seems to affect the limbic system and causes a positive reaction in women, improving mood.[59] Responses to androstadienone depend on the individual and the environment they are in.[67] Androstadienone negatively influences[how?] the perception of pain in women.[67] 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.[67]

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, false positives, and poor methodology.[68]

Vaginal aliphatic acids

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A class of aliphatic acids (volatile fatty acids as a kind of carboxylic acid) was found in female rhesus monkeys that produced six types in the vaginal fluids.[69] 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.[69] Even in humans, one-third of women have all six types of copulins, which increase in quantity before ovulation.[69] 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.[59]

Stimulators of the vomeronasal organ

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The human vomeronasal organ has epithelia that may be able to serve as a chemical sensory organ; however, the genes that encode the VNO receptors are nonfunctional pseudogenes in humans.[53] 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.[53] 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.[59]

In 2006, it was shown that a second mouse receptor sub-class is found in the olfactory epithelium. Called the trace amine-associated receptors (TAAR), some are activated by volatile amines found in mouse urine, including one putative mouse pheromone.[70] Orthologous receptors exist in humans providing, the authors propose, evidence for a mechanism of human pheromone detection.[71]

Although there are disputes about the mechanisms by which pheromones function, there is evidence that pheromones do affect humans.[72] 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.[59]

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 complexes (MHC) combination.[73] 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.[53] Claus Wedekind and colleagues found that both men and women prefer the axillary odors of people whose MHC is different from their own.[74]

Some body spray advertisers claim that their products contain human sexual pheromones that act as an aphrodisiac. Despite these claims, no pheromonal substance has ever been demonstrated to directly influence human behavior in a peer reviewed study.[59][56][disputed – discuss] Thus, the role of pheromones in human behavior remains speculative and controversial.[75]

See also

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References

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Further reading

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Pheromone

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A pheromone is a produced and released by an individual into the external environment, which elicits a specific al or physiological response in a conspecific—another member of the same —such as a stereotyped or developmental process. These substances are detected primarily through olfactory or gustatory systems and must meet rigorous criteria for identification, including at natural concentrations and an evolutionary basis for their signaling function. Pheromones play crucial roles in intraspecific communication across the animal kingdom, influencing processes like , alarm signaling, , and , and they are produced by diverse taxa including , mammals, , and reptiles. The concept of pheromones was first formalized in 1959 by German chemist Peter Karlson and Swiss entomologist Martin Lüscher, drawing on earlier observations by of chemical communication in animals, though the term itself evokes ancient notions of attraction and influence. The inaugural pheromone, bombykol, was isolated that same year from the silk moth () by Adolf Butenandt's team after processing extracts from approximately half a million female moths; this sex attractant triggers precise upwind flight in males over long distances. Subsequent discoveries expanded the field, revealing pheromones in non-insect species, such as the mammary pheromone in rabbits (2-methylbut-2-enal), which elicits nipple-searching and suckling in newborns, and urinary signals in male fish that synchronize female . Pheromones are classified into several functional types based on their effects: releaser pheromones provoke immediate behavioral responses, like the alarm pheromone in honeybees () that induces defensive stinging; primer pheromones trigger slower physiological changes, such as pheromones that suppress ovarian development in workers; and signaler or modulator pheromones convey social information or subtly influence mood and emotion. Many pheromones function as complex multicomponent blends, as seen in the sex pheromones of Ostrinia moths, where variations in ratios ensure species-specific attraction and . In mammals, examples include the (MHC)-linked odors in mice that guide to promote . Biologically, pheromones facilitate essential survival and reproductive strategies, from trail-following in ants to territory marking in wolves, and their study has practical applications, including integrated pest management where synthetic pheromones disrupt insect mating on over 10 million hectares annually as of 2017. In social insects like termites and bees, pheromones underpin caste differentiation and colony cohesion, while in vertebrates, they integrate with other sensory cues to modulate aggression or affiliation. Evolutionarily, pheromones often arise from repurposed metabolic byproducts, evolving into reliable signals through sensory tuning and behavioral adaptation. In humans, the existence of pheromones remains controversial and lacks robust evidence compared to other mammals, with no dedicated functional in adults and candidate molecules like showing only inconsistent, context-dependent effects on mood or rather than specific, stereotyped responses. One potential exception is a nipple-secreted in lactating women that stimulates suckling in infants, but broader claims—such as or sex attractants—have been undermined by methodological flaws, small sample sizes, and failure to replicate under controlled bioassays. Ongoing emphasizes the need for rigorous, hypothesis-driven approaches to distinguish true pheromones from general olfactory cues.

Definition and History

Definition and Characteristics

Pheromones are defined as substances that are secreted or excreted by an individual and received by a second individual of the same , in which they release a specific reaction such as a definite behavioral or developmental process. This definition, coined in 1959, emphasizes their role in intraspecific chemical communication, distinguishing them from hormones that act internally within the same organism. Key characteristics of pheromones include their species-specificity, meaning they typically elicit responses only in conspecifics due to specialized sensory adaptations in the receiving organism. They function primarily for intraspecific signaling, facilitating coordination within a species without benefiting or harming other species directly. For transmission, pheromones rely on volatility for airborne dispersal or solubility in water or on surfaces for contact-based delivery, allowing them to propagate through the environment at low concentrations to trigger responses. Pheromones differ from interspecific chemical signals such as allomones, which benefit the emitting species by eliciting advantageous responses in receivers of different species, and kairomones, which benefit the receiving species at the expense or neutrality of the emitter. These distinctions highlight pheromones' exclusive focus on same-species interactions. Pheromones exert effects through two primary modes: immediate behavioral responses, often termed releaser effects, which prompt rapid actions like attraction or ; and delayed physiological responses, known as primer effects, which influence longer-term changes such as developmental or reproductive states via neuroendocrine pathways. These modes underscore pheromones' versatility in modulating both acute and chronic intraspecific interactions.

Discovery and Early Research

The concept of chemical signaling among insects began to emerge in the 19th century through observational studies by naturalists. French entomologist Jean-Henri Fabre documented trail-following behavior in ants, noting how workers precisely adhered to established paths during foraging and raids, suggesting an invisible guide that persisted even after disruptions. In the 1870s, Fabre conducted experiments with the Great Peacock Moth (Saturnia pyri), where a single female confined behind wire gauze attracted over 40 males from distances up to several kilometers, despite visual and auditory barriers, implying a potent airborne attractant. These early insights laid groundwork for 20th-century biochemical investigations, culminating in the formalization of the term "pheromone" in by German entomologist Peter Karlson and Swiss zoologist Martin Lüscher. They defined it as substances secreted externally to elicit specific responses in conspecifics, deriving the word from the Greek pherein (to carry) and hormon (to excite or stimulate), distinguishing it from internal hormones. This nomenclature encapsulated prior observations and advanced the field toward chemical identification. A pivotal breakthrough came in 1959 when Adolf Butenandt's team at the Max Planck Institute isolated and synthesized bombykol, the first chemically characterized sex pheromone, from the silk moth (). After nearly two decades of effort starting in the late 1930s, they extracted it from the abdominal glands of approximately 500,000 females, using bioassays like male wing-fluttering to track activity during fractionation. Concurrently, in the 1950s, American biologist pioneered studies on ant alarm pheromones, identifying as a releaser of defensive behaviors in species like Pogonomyrmex badius through gland extractions and behavioral assays. Early isolation efforts faced significant hurdles due to pheromones' low concentrations—often in the nanogram range per individual—and high volatility, which complicated extraction and purification without degrading the compounds. Prior to the 1960s, research remained centered on insects, particularly social species like ants, bees, and moths, solidifying pheromones as a unique class of ectohormones distinct from vertebrate signaling.

Chemical Properties

Molecular Structure and Biosynthesis

Pheromones encompass a diverse array of chemical structures, primarily consisting of small, volatile molecules such as hydrocarbons (including alkenes and alcohols), esters, and aldehydes in , while often utilize proteins and . In , these structures frequently derive from long-chain fatty acids, featuring functional groups like alcohols, acetates, and hydrocarbons that enable specific signaling. pheromones, such as those in mammals, include steroidal compounds and peptide sequences that facilitate social and reproductive communication. Biosynthesis of pheromones in insects typically begins with the de novo synthesis of saturated fatty acids, such as , followed by chain elongation and modification through desaturation and reduction steps. Key enzymes include fatty desaturases (FADs), which introduce double bonds to create unsaturated precursors, and fatty reductases (FARs), which convert to alcohols; for instance, sex pheromone bombykol is produced from via sequential desaturations—first Δ11 to (Z)-11-hexadecenoic acid, then Δ10 to (10E,12Z)-10,12-hexadecadienoic acid—followed by reduction to the corresponding alcohol. In mammals, pheromone often involves the modification of steroidal precursors in gonadal and adrenal tissues, with compounds like derived from androstadienol through oxidation by , contributing to volatile signals in sweat and urine. Structural variations in pheromones are influenced by evolutionary pressures, balancing conservation of core biosynthetic pathways across taxa for functional reliability with species-specific modifications to prevent cross-attraction. Intraspecific and interspecific diversity arises from genetic changes in desaturase specificity and chain length, leading to unique blends; additionally, volatility is a critical factor, with short-chain, low-molecular-weight structures (e.g., aldehydes) suited for aerial dispersion over distances, whereas longer-chain hydrocarbons serve as contact pheromones on surfaces. Identification of pheromone structures relies heavily on gas chromatography-mass spectrometry (GC-MS), which separates volatile compounds by volatility and polarity before fragmenting them for structural elucidation based on mass spectra. This technique, often coupled with electroantennography for bioactivity confirmation, has enabled the precise characterization of complex blends from glandular extracts or headspace collections. Recent advances in laboratory synthesis have enhanced the efficiency of producing insect pheromones, incorporating methods like carbon-carbon coupling reactions (e.g., Negishi and couplings) for chain assembly, olefin metathesis for stereoselective double-bond formation, and asymmetric epoxidations for chiral center control, as detailed in a 2023 review of syntheses from 2013 to 2022. These approaches have facilitated scalable production for (IPM), including the development of carriers that control release rates and minimize environmental persistence.

Classification by Chemical Composition

Pheromones are broadly classified by their chemical composition into three main categories: hydrocarbons, oxygenated derivatives, and proteinaceous compounds, each exhibiting distinct molecular structures and physicochemical properties that influence their solubility, volatility, and persistence in the environment. This taxonomy highlights the diversity in pheromone chemistry across taxa, with hydrocarbons predominant in insects for contact-based signaling and oxygenated forms often volatile for aerial dispersion, while proteinaceous types are more common in vertebrates for binding and transporting smaller ligands. Hydrocarbon pheromones consist primarily of straight-chain, branched, or unsaturated aliphatic compounds, such as , , and methyl-branched variants, typically ranging from C21 to C35 in chain length. These are exemplified by cuticular hydrocarbons (CHCs) in , where n- like pentacosane (C25) and branched isomers such as 3-methylhentriacontane serve as recognition cues. Their high renders them poorly soluble in but highly compatible with lipid-rich cuticles, contributing to low volatility and long-term stability on surfaces, with degradation primarily through microbial oxidation rather than rapid . In moths, Type II and Type III hydrocarbons include polyunsaturated straight-chain forms (C17–C23) and functionalized branched structures, respectively, which maintain similar apolar characteristics but incorporate epoxides or methyl groups for structural diversity. Oxygenated derivatives introduce polar functional groups to hydrocarbon backbones, enhancing reactivity and volatility while retaining moderate ; these are subclassified into alcohols, aldehydes, and ketones. Alcohols, such as the monounsaturated (E,Z)-10,12-hexadecadien-1-ol (bombykol) in moths, feature hydroxyl groups on C10–C18 chains, making them slightly more polar and water-miscible than pure s, with stability affected by esterification to prolong persistence. Aldehydes, like (Z)-9-tetradecenal in the cotton bollworm moth (), contain carbonyl groups at chain ends, increasing susceptibility to oxidation and , thus exhibiting shorter environmental half-lives compared to alcohols but higher for dispersal. Ketones, including short-chain methyl ketones in primitive moths (e.g., Type 0 pheromones), possess internal carbonyls that confer polarity similar to aldehydes, with points elevated by hydrogen bonding but lower solubility in non-polar solvents; their degradation often involves enzymatic reduction . Overall, these derivatives derive biosynthetically from elongation and desaturation, yielding blends that combine single components for simplicity or multi-component mixtures for enhanced specificity. Proteinaceous pheromones, prevalent in mammals, are high-molecular-weight polypeptides that function as carriers for volatile ligands rather than direct signals, exemplified by major urinary proteins (MUPs) in . These ~18–19 kDa lipocalins feature a conserved β-barrel structure with eight antiparallel β-sheets forming a central hydrophobic pocket that binds small lipophilic volatiles, such as , with affinities varying by isoform (e.g., MUP4 shows 23-fold higher binding than MUP1). Their solubility (isoelectric points 4.6–5.3) contrasts with the of bound ligands, enabling aqueous in bodily fluids while protecting against rapid or oxidation, with proteolytic resistance extending ligand release over days from scent marks. Unlike smaller pheromones, MUPs exhibit low volatility themselves but enhance the stability and controlled diffusion of multi-component blends, underscoring their role in complex chemical profiles.
Chemical ClassKey Structural FeaturesRepresentative ExamplesPhysicochemical Properties
HydrocarbonsStraight/branched alkanes/alkenes (C21–C35)Cuticular hydrocarbons in (e.g., pentacosane); Type II in moths (C17–C23 polyunsaturates)Highly lipophilic, low water , high stability, low volatility, microbial degradation dominant
Oxygenated Derivatives - AlcoholsHydroxyl on C10–C18 unsaturated chainsBombykol in silkworm mothsModerately polar, esterifiable for stability, higher vapor pressure than hydrocarbons
Oxygenated Derivatives - AldehydesTerminal carbonyl on C10–C18 chains(Z)-9-tetradecenal in cotton bollworm mothPolar, prone to photo/oxidative degradation, moderate , short
Oxygenated Derivatives - KetonesInternal carbonyl, often short-chainMethyl ketones in primitive mothsPolar like aldehydes, reducible, balanced in polar/non-polar media
Proteinaceousβ-barrel lipocalins (~18 kDa) binding volatilesMajor urinary proteins in Water-soluble, hydrophobic binding pocket, proteolytically stable, slow ligand release

Functional Categories

Alarm and Defensive Pheromones

Alarm and defensive pheromones are chemical signals released rapidly by animals in response to immediate threats, such as predation or , to elicit defensive behaviors in nearby conspecifics, including flight, , or for defense. These pheromones typically consist of volatile compounds that diffuse quickly through air or water, allowing for swift communication without physical contact, and their release is often triggered by mechanical disturbance or attack. In social species, this mechanism enhances survival by coordinating group responses, where the sender may sacrifice itself to warn others. In , alarm pheromones are particularly well-studied, with formic acid serving as a key component in many species, such as those in the genus Camponotus, where it is ejected from the poison to provoke avoidance, , or stinging in nestmates. For example, in carpenter ants (Camponotus obscuripes), combined with hydrocarbons like n-undecane elicits aggressive recruitment toward the threat source. In , the (E)-β-farnesene acts as the primary alarm pheromone, released from the cornicles upon attack to induce dispersal or freezing behaviors in nearby individuals, thereby reducing predation risk across the colony. Among vertebrates, defensive pheromones include skin secretions in salamanders, such as those of the (Plethodon cinereus), where stressed individuals release compounds that signal danger and cause avoidance in conspecifics for up to several minutes, functioning as an alarm cue. In fish, alarm substances—often hypoxanthine-3-N-oxide from epidermal club cells—are released upon skin injury to trigger antipredator responses like increased shoaling and darting in species such as (Danio rerio), though their status as true pheromones remains debated due to their non-species-specific nature and broad elicitation of fear across taxa. Many alarm pheromones are multi-component blends that enhance response specificity, preventing unintended reactions from heterospecifics; for instance, in , combinations of with hydrocarbons or aldehydes produce graded behaviors—from mild alerting to full attack—tailored to threat intensity, while ensuring only members respond appropriately. Ecologically, these pheromones play a crucial role in group protection by enabling rapid, non-contact coordination that dilutes individual risk and disrupts predator attacks, as seen in colonies where (E)-β-farnesene emission leads to synchronized escape that benefits the group's overall survival without requiring visual or auditory cues.

Sex and Reproductive Pheromones

Sex and reproductive pheromones play a crucial role in coordinating behaviors across , primarily by facilitating mate attraction, , and reproductive timing. These pheromones are categorized into attractants, which operate over long distances to draw potential mates closer; aphrodisiacs, which elicit close-range sexual excitation and copulatory responses; and synchronizers, which align reproductive cycles among individuals to optimize breeding opportunities. In , sex pheromones often exemplify long-range attractants, with bombykol serving as a seminal example in the silkworm moth . Female moths release bombykol, a 16-carbon alcohol, to lure males from kilometers away, triggering oriented flight and upwind navigation behaviors essential for locating the source. In contrast, species employ multi-component blends of cuticular hydrocarbons as sex pheromones, where specific ratios of compounds like 7-tricosene and 7,11-heptacosadiene in females stimulate male courtship behaviors such as wing vibration and licking. Among vertebrates, reproductive pheromones frequently involve volatile steroids or urinary volatiles that mediate attraction and . In pigs, androstenol, a steroidal compound in boar , acts as an to induce the "standing estrus" response in sows, promoting immobility for mounting and facilitating copulation. In , such as house mice, urinary pheromones like major urinary proteins (MUPs) convey male attractiveness, influencing female by eliciting approach and investigation behaviors that correlate with higher mating success. The specificity of these pheromones is maintained through molecular features like chirality and precise blend ratios, which prevent cross-attraction between species or sexes. For instance, enantiomeric forms of pheromone alcohols in moths ensure that only conspecific males respond, as the wrong chirality elicits no behavioral activation. Similarly, in Drosophila, deviations in hydrocarbon ratios reduce male arousal, thereby reinforcing species barriers during mate selection. Overall, these pheromones drive key behavioral outcomes, including the initiation of sequences—such as male song production in or precopulatory displays in —and informed , where females preferentially approach males signaling high genetic quality or fertility.

Aggregation and Trail Pheromones

Aggregation pheromones are chemical signals that induce non-sexual clustering of individuals, facilitating coordinated activities such as resource exploitation in social . In bark beetles of the Ips, such as Ips pini, males produce ipsenol, a key aggregation pheromone component, to attract conspecifics to suitable host trees, enabling mass attacks that overwhelm tree defenses. This semiochemical, first identified in 1966 alongside ipsdienol and cis-verbenol, is biosynthesized de novo from host tree precursors like , promoting rapid congregation of beetles for reproduction and feeding. Trail pheromones, in contrast, serve as volatile or contact chemical markers that guide or relocation paths in eusocial insects, enhancing colony efficiency in locating food or new nests. A prominent example is (Z)-9-hexadecenal, the primary of the (Linepithema humile, formerly Iridomyrmex humilis), which workers deposit from the pygidial gland to form persistent trails leading to resources. Behavioral assays confirm that this unsaturated elicits strong trail-following responses at concentrations as low as 0.001 pg/cm, allowing ants to navigate complex environments collectively. Some pheromones exhibit dual roles, combining trail guidance with functions to integrate and defense in societies. In species like the (Monomorium pharaonis), glandular secretions can integrate and defensive signaling, with pheromones like faranal from the Dufour's marking paths while separate signals from other glands prompt recruits to mobilize against threats. In some ant species, secretions from various glands enable rapid shifts from to defensive . The persistence of these pheromones is evolutionarily tuned to their ecological roles, with degradation rates balancing signal duration against environmental factors like volatility and microbial breakdown. Trail pheromones, such as (Z)-9-hexadecenal in Argentine , typically degrade within about 4 hours if unreplenished, preventing outdated paths from misleading foragers. Aggregation pheromones, however, exhibit longer persistence at sites, as seen in bark beetles where ipsenol remains effective for days to sustain mass assemblages, aided by lower rates in confined tree galleries. While aggregation and trail pheromones are predominantly studied in insects, limited evidence suggests analogous social scents in vertebrates promote grouping. In European rabbits (Oryctolagus cuniculus), chin gland secretions containing volatile fatty acids and steroids function as aggregation signals, fostering burrow-sharing and social cohesion among kin groups without reproductive intent. These vertebrate examples highlight conserved chemical mechanisms for non-sexual coordination, though they are far less specialized than in arthropods.

Territorial and Epideictic Pheromones

Territorial pheromones are chemical signals used by animals to mark and defend specific areas against intruders, thereby asserting dominance and exclusive access to resources such as food, mates, or shelter. In mammals, these pheromones are often deposited via , which contains volatile compounds bound to carrier proteins that prolong their detectability. For instance, male house mice (Mus musculus) release major urinary proteins (MUPs) in their urine marks, which bind small pheromones like 2-sec-butyl-4,5-dihydrothiazole (SBT) and dehydro-exo-brevicomin (DB), signaling individual identity and deterring rival males from encroaching on the territory. Similarly, coyotes (Canis latrans) employ scents to delineate territorial boundaries, informing other individuals of the marker's presence and status to avoid direct confrontations. In , territorial pheromones can involve cuticular hydrocarbons, waxy compounds on the that convey status in dominance interactions. Male Nauphoeta cinerea exhibit distinct hydrocarbon profiles, with dominants showing higher levels of monomethylalkanes such as 11- and 13-methyltriacontane, which signal their superior status and help maintain territorial control within hierarchies. These marks are typically applied through physical contact or glandular secretions, creating semi-permanent boundaries that persist for days or longer due to the stability of the compounds. Epideictic pheromones, a subset of territorial signals, specifically indicate resource saturation or depletion to prevent overcrowding and promote even distribution among individuals. In parasitic wasps, such as Trissolcus basalis, females deposit host-marking pheromones from the Dufour's gland after oviposition on egg masses, alerting conspecifics to avoid superparasitism and seek unexploited hosts elsewhere. This lipid-based secretion acts as an cue, triggering dispersal when host availability is low, thereby optimizing oviposition sites and reducing wasteful competition. Territorial and pheromones often overlap with -releaser functions, where detection of a rival's mark elicits defensive behaviors to reinforce boundaries. In male mice, exposure to conspecific MUPs or exocrine gland-secreting peptide 1 (ESP1) via the activates neural circuits in the medial and ventromedial , promoting territorial to establish dominance and limit intrusions. These semi-permanent marks, derived from glandular secretions like flank or anal glands in mammals, endure environmental exposure, allowing prolonged signaling without constant reapplication. Ecologically, these pheromones minimize intraspecific conflict by enabling non-violent resolution of territorial disputes, as recipients assess the marker's fitness and opt for avoidance rather than confrontation, thus conserving energy and optimizing across populations. In parasitoid systems, epideictic signals further enhance efficiency by spacing oviposition, reducing host overexploitation and supporting dynamics.

Effect-Based Categories

Releaser Pheromones

Releaser pheromones are chemical signals secreted by an individual that elicit immediate and specific behavioral responses in conspecifics, such as freezing, attacking, or behaviors. These responses occur with short latency, typically within seconds to minutes, them from slower-acting pheromone types. The neurobiological basis of releaser pheromones involves rapid detection and processing through olfactory pathways, primarily the main and the . In the , specialized sensory neurons express G protein-coupled receptors like V1Rs for volatile ligands and V2Rs for or protein signals, leading to fast synaptic activation via intracellular cascades such as IP₃ production and TRPC2 channel opening. This direct neural wiring to hypothalamic and limbic centers enables instinctive behavioral outputs without requiring higher cognitive processing. Releaser pheromones encompass a range of functional types, including those that trigger responses for predator avoidance, pheromones that promote reproductive behaviors, and pheromones that guide group movement or . These examples illustrate how releaser pheromones facilitate essential social and survival interactions across , from to mammals. One key advantage of releaser pheromones is their role in energy-efficient communication, allowing organisms to convey critical information with minimal metabolic cost compared to visual or auditory signals. This efficiency supports rapid coordination in dynamic environments, enhancing and group cohesion. However, their effects are often limited to short ranges due to the volatility of the chemical carriers, and responses can be highly context-dependent, varying with environmental factors, individual state, or species-specific adaptations.

Primer Pheromones

Primer pheromones are chemical signals that trigger prolonged physiological or developmental alterations in the receiving organism, primarily through the activation of endocrine pathways rather than immediate behavioral responses. These effects often involve hormonal cascades that influence reproductive maturation, gonadal function, or social regulation within a group, distinguishing them from pheromones that elicit rapid neural-mediated actions. Unlike short-term signals, primer pheromones operate on timescales ranging from hours to days, leading to changes in and sustained modifications in the receiver's internal state. A prominent example of primer pheromone action is the acceleration of in female mice exposed to urinary proteins from adult males. These proteins, detected in male urine, stimulate the hypothalamic-pituitary-gonadal axis, advancing the onset of first estrus by 4–6 days compared to unexposed females, thereby synchronizing reproductive readiness in social groups. In , another key effect is the pregnancy block known as the , where exposure to scents from an unfamiliar male shortly after mating disrupts implantation through suppression of surges and , preventing the gestation of offspring sired by a previous mate. This is mediated by specific male pheromones, such as exocrine gland-secreting 1 (ESP1), which alters the female's hormonal profile to favor potential future reproduction with the new male. In social , primer pheromones play a critical role in regulating . For instance, the mandibular gland secretions of queen honeybees, containing compounds like (E)-9-oxodec-2-enoic acid, suppress ovarian development in workers, ensuring the queen's monopoly on and maintaining harmony over extended periods. These pheromones inhibit vitellogenin synthesis and alter levels in workers, effects that persist as long as the queen is present. Detection of primer pheromones frequently occurs via the (VNO), a specialized chemosensory structure that processes these signals through slower, modulatory pathways compared to the main . VNO neurons, expressing G-protein-coupled receptors, relay information to the accessory olfactory bulb and , initiating the downstream endocrine responses without evoking instantaneous behaviors. This mechanism underscores the pheromones' role in subtle, long-term physiological tuning. Despite extensive study in social mammals and , primer pheromones remain understudied in non-social , where their potential roles in reproductive timing or environmental are less clear, highlighting a gap in understanding broader evolutionary applications.

Signal Pheromones

Signal pheromones represent a specialized category within the effect-based of pheromones, primarily functioning to enhance or modulate the transmission and reception of other chemical signals in social contexts, without eliciting direct behavioral or physiological responses on their own. In social , these pheromones often provide contextual amplification, allowing for more nuanced communication within colonies by fine-tuning the impact of primary signals such as those involved in or aggregation. This modulatory role is crucial in complex societies where multiple pheromones interact to convey layered information, ensuring efficient coordination among . A key characteristic of signal pheromones is their integration into multi-component blends, where they act as enhancers rather than standalone agents. Unlike more direct-acting pheromones, signal types contribute subtly to the overall semiochemical profile, adjusting the sensitivity or specificity of receivers to accompanying signals. For instance, in colonies, cuticular hydrocarbons and volatile compounds often blend to modulate recognition cues, preventing miscommunication in diverse interactions. This blend-dependent nature underscores their role in maintaining social without independent triggering effects. Representative examples illustrate this modulatory function in , where signal pheromones enhance recognition. In Reticulitermes speratus, the queen pheromone, composed of butyl butyrate and 2-methyl-1-butanol in a 2:1 ratio, not only regulates differentiation but also amplifies workers' responses to egg recognition pheromones like and , thereby improving egg-carrying efficiency and colony cohesion. Similarly, soldier-specific volatiles such as (-)-β-elemene in the same species act as age-signaling modulators, subtly adjusting worker behaviors around aging soldiers without direct arrest or inhibition. These examples highlight how signal pheromones refine communication in eusocial hierarchies. The theoretical status of signal pheromones remains debated among researchers, with some classifying them as a distinct category due to their unique modulatory effects, while others regard them as accessory elements integrated into releaser or primer systems. This ambiguity arises from the multifunctional nature of many pheromones, where compounds serve multiple roles across contexts, complicating rigid categorization. For example, trail pheromones in like (Z,Z,E)-3,6,8-dodecatrien-1-ol can shift from primary signaling to modulatory enhancement in blends, challenging traditional boundaries. Recent theoretical advancements from 2023 to 2025 have increasingly linked the of signal pheromones to sensory mechanisms, positing that pre-existing sensory preferences in receivers predispose certain chemical modulators to become integrated into communication systems. In social insects, this may explain the rapid co-option of defensive or compounds into caste-enhancing signals, as seen in recognition pheromones derived from such precursors. A 2024 review on queen pheromones further supports this, suggesting sensory facilitated the modulatory of royal signals from attractants, a pattern potentially applicable to analogs.

Detection Mechanisms

Olfactory Epithelium Receptors

The serves as the primary site for detecting volatile chemical signals, including pheromones, through specialized sensory neurons. In vertebrates, this structure consists of olfactory receptor neurons (ORNs) embedded in the nasal , each featuring a dendritic knob from which multiple non-motile cilia extend into the layer. These cilia-bearing ORNs express G-protein-coupled receptors (GPCRs), known as (ORs), which are seven-transmembrane proteins tuned to bind specific odorants. Approximately 300–400 functional OR genes are present in humans, while mice possess around 1,000, enabling a broad repertoire for volatile detection. Activation of these receptors occurs when volatile pheromones diffuse through the nasal mucus and bind to ORs on the ciliary membrane, initiating a G-protein-mediated signaling cascade. In mammals, binding promotes GDP-to-GTP exchange on the stimulatory G-protein subunit Gαolf, which activates type III to produce (cAMP). Elevated cAMP levels open cyclic nucleotide-gated (CNG) ion channels, permitting influx of Na⁺ and Ca²⁺ ions, of the , and propagation of action potentials to the . This pathway ensures rapid transduction of volatile signals, with pheromones like those in eliciting responses in the main for certain social cues. In , the olfactory epithelium equivalent resides in the antennal sensilla, where pheromone detection dominates behavioral responses and projects to antennal lobes for processing. Here, ORNs housed in cuticular pores express a distinct class of olfactory receptors that form heteromeric complexes with the conserved co-receptor Orco, functioning primarily as ligand-gated cation channels rather than GPCRs. Binding of volatile pheromones, such as bombykol in silkmoths, directly opens these channels to generate depolarizing currents, though a secondary G-protein-coupled pathway may amplify signals in some cases. Antennal lobes feature specialized macroglomerular complexes dedicated to pheromone integration, underscoring the system's primacy in insect chemosensation. In mammals, the plays an accessory role, primarily handling non-social volatile scents while contributing to broader perception. Olfactory receptors exhibit remarkable sensitivity, with detection thresholds reaching picomolar concentrations for certain pheromones, as observed in teleost fish where sex pheromone-responsive ORNs respond to near-picomolar levels of bile salts. This high affinity enables ecological at trace environmental levels. Recent evolutionary studies highlight how shifts in receptor tuning drive ; for instance, substitutions in pheromone receptors like OR14b and OR16 alter binding specificity to distinct or alcohol components, promoting among Helicoverpa species. Unlike the , which processes non-volatile cues, the excels in volatile pheromone detection across taxa.

Vomeronasal Organ Receptors

The (VNO) is a paired, tubular structure located in the of many vertebrates, consisting of a sensory lined with bipolar vomeronasal sensory neurons (VSNs) that detect pheromones. These neurons are organized into apical and basal layers within the VNO , with apical neurons primarily expressing vomeronasal type-1 receptors (V1Rs) coupled to Gαi2 proteins, and basal neurons expressing vomeronasal type-2 receptors (V2Rs) coupled to Gαo proteins. V1Rs and V2Rs form distinct subfamilies of G-protein-coupled receptors specialized for chemosensory detection. Upon pheromone binding, V1Rs primarily detect volatile small molecules, while V2Rs sense non-volatile, water-soluble ligands such as peptides, peptide hormones, and (MHC)-related molecules. Signal transduction in VSNs involves the activation of , leading to the opening of transient receptor potential channel 2 (TRPC2), which generates depolarizing currents essential for pheromone-evoked responses. This pathway projects axons from VSNs to the accessory olfactory bulb, facilitating the processing of social and reproductive signals. The VNO and its receptors are prominent in reptiles and most mammals, where they mediate close-range chemosensory detection, but are absent in most birds and have been lost in primates, including humans. In reptiles like snakes and , the VNO supports prey detection and social behaviors through V1R and V2R expression. Mammals exhibit extensive V1R and V2R diversification, with possessing over 300 V1R genes, enabling fine-tuned pheromone discrimination. Functionally, VNO receptors process social and sexual cues, such as aggression-inducing predator odors or in . In mice, V2R-expressing neurons detect MHC class I ligands from conspecific urine, influencing and to avoid . These responses often trigger innate behaviors like mounting or avoidance without prior learning. In , the VNO is vestigial, developing embryonically but regressing postnatally into a non-functional pit lacking intact VSNs or TRPC2 expression, as confirmed by histological and genomic analyses in recent studies. A 2025 review of human VNO morphology highlights its rudimentary structure without operational receptors, underscoring evolutionary loss in . This absence correlates with reliance on the main for social chemosignals.

Other Sensory Pathways

In many , contact chemoreception serves as a key mechanism for detecting non-volatile pheromones at close range, complementing airborne olfactory cues. Specialized gustatory sensilla on the tarsi, mouthparts, and antennae house chemosensory neurons that directly sample chemical traces from surfaces or conspecifics. For instance, in , trail pheromones laid down during are perceived primarily through tarsal contact chemoreceptors, enabling precise path-following and recruitment of nestmates to resources. Similarly, in , tarsal sensilla detect cuticular hydrocarbons acting as pheromones, mediating male-male repulsion and male-female attraction during . Mouthpart chemoreceptors further contribute, as demonstrated in the Galerucella grisescens, where labellar sensilla process contact pheromones to facilitate sexual discrimination. Gustatory receptors in vertebrates enable oral and pharyngeal detection of chemical , including those with pheromonal functions, particularly in non-mammalian . These receptors, embedded in of the and , respond to soluble compounds ingested or encountered during social interactions like grooming or feeding. In the (Petromyzon marinus), pharyngeal cells exhibit robust activity to and acids that serve as social signals, integrating gustatory input for reproductive and feeding behaviors. Although less emphasized in mammals, where olfactory pathways predominate, preliminary evidence suggests oral gustatory involvement in processing conspecific cues during direct contact. Representative examples highlight diverse contact-based pheromone detection across taxa. In moths, such as , antennae bear both olfactory trichoid sensilla for volatile sex pheromones and shorter basiconic sensilla capable of contact chemoreception, allowing males to confirm female presence upon close approach. In amphibians, skin absorption provides a pathway for pheromone uptake, bypassing nasal routes; for example, in salamanders like Desmognathus ocoee, males deliver pheromones directly to female via jaw glands, where they are absorbed and elicit behavioral responses without olfactory mediation. These non-olfactory pathways often integrate with olfactory signals to form multimodal pheromone communication, enhancing behavioral precision. In , olfactory detection of distant cues transitions to contact chemoreception upon approach, with neural circuits in the converging inputs from both modalities to modulate vigor. This integration ensures robust responses to complex chemical landscapes, as seen in where volatile trail pheromones guide initial orientation, while tarsal contact refines path accuracy. Emerging research underscores the role of extra-oral taste receptors in mammals for detecting pheromone-like compounds beyond traditional oral sites. Recent studies (2023–2025) have identified ectopic expression of TAS2R bitter receptors in respiratory epithelia and , potentially sensing social volatiles or microbial cues with pheromonal implications, though direct links to reproductive signaling remain under investigation. These findings suggest broader gustatory contributions to chemosensory processing in mammals, expanding beyond canonical taste functions.

Evolutionary Aspects

Origins and General Evolution

Pheromone systems are believed to have originated from the of metabolic byproducts or compounds with prior non-communicative functions, rather than through sensory bias or exploitation of pre-existing preferences. This evolutionary pathway is supported by evidence from , where many pheromones derive from cuticular hydrocarbons or derivatives that initially served protective or physiological roles. Such precursors provided a readily available chemical foundation, allowing to repurpose them for intraspecific signaling without requiring novel biosynthetic pathways. In prokaryotes, the precursor to pheromone-like communication is evident in , where release diffusible autoinducers to coordinate population-level behaviors such as formation and . These systems, ancient and widespread across bacterial lineages, parallel eukaryotic pheromones by enabling density-dependent gene regulation, suggesting a deep evolutionary continuity in chemical signaling from unicellular to multicellular life. Transitioning to eukaryotes, pheromone use emerged early in fungi for and aggregation, with peptide-based signals facilitating partner recognition in species like yeasts and mushrooms as far back as 1 billion years ago. In arthropods, such as , aggregation and pheromones similarly arose for group cohesion, predating more specialized functions. The of pheromones in vertebrates occurred later, primarily linked to the development of social structures, with the enabling detection of signals for and group coordination in amphibians and mammals. Driving this progression was favoring traits that enhanced through kin and group benefits, such as alarm pheromones in social insects that protect related individuals and promote survival. Recent theories emphasize the role of gene duplications in pheromone receptor families, which generated diversity in ligand specificity and facilitated the rapid of novel signaling systems, as seen in odorant receptors where duplications led to specialized pheromone detection.

Role in Speciation and Adaptation

Pheromones play a crucial role in by acting as prezygotic barriers that prevent interbreeding between diverging populations through mismatches between emitted signals and receiver sensitivities. In moths, such as those in the genus Helicoverpa, evolutionary shifts in pheromone receptors have led to functional differentiation that contributes to . For instance, orthologous receptors OR14b and OR16 exhibit altered binding in different species, reducing cross-attraction and promoting by ensuring occurs only within compatible groups. These receptor changes, often involving substitutions, exemplify how sensory tuning evolves to reinforce species boundaries in sympatric or parapatric populations. Beyond , pheromones facilitate to environmental challenges by modifying volatile profiles to suit ecological conditions. In arid , like in the genus have evolved alterations in cuticular hydrocarbons—compounds that double as pheromones for nestmate recognition and communication—through changes in elongase enzymes, enhancing resistance while maintaining social cohesion. This allows trail pheromones to persist longer in dry conditions, improving foraging efficiency in desert environments without compromising signal specificity. Similarly, genetic mutations in desaturase enzymes underlie shifts in pheromone blends, enabling fine-tuned responses to habitat variations; in leafroller moths, regulatory changes in desaturase genes produce species-specific compositions that align with local ecological pressures, such as host plant availability. Illustrative examples highlight these processes across taxa. In , divergence in male wing pheromones, controlled by clustered genetic loci, drives and reinforces , particularly in mimetic complexes where pheromone differences complement visual signals to isolate hybridizing races. In , geographic variation in major urinary protein (MUP) pheromone blends creates "dialects" that promote local adaptation and subtle ; wild house populations exhibit combinatorial diversity in MUP profiles correlated with habitat differences, reducing cross-attraction between distant groups and aiding subpopulation persistence. Recent studies as of 2025 underscore pheromones' role in social adaptation, particularly in eusocial insects. In honeybees, the evolution of queen mandibular pheromone (QMP) production and detection has been linked to increasing social complexity, with receptor tuning in species like Apis mellifera enabling precise regulation of worker sterility and colony cohesion, a trait that likely facilitated transitions from solitary to advanced . This molecular sophistication highlights how pheromone systems adapt not only to abiotic environments but also to biotic interactions within societies, driving evolutionary innovations in group living.

Evolution of Sex Pheromones

Sex pheromones have evolved from simple chemical attractants derived from metabolic precursors to complex blends that facilitate and recognition in various taxa. In , this progression often involves modifications in biosynthetic pathways, where ancestral compounds are repurposed through duplications or in desaturase enzymes, leading to species-specific ratios of pheromone components that promote . For instance, via sensory exploitation has driven the diversification of these signals, allowing receivers to exploit preexisting biases for more elaborate cues that enhance mate attraction while minimizing cross-species interference. A notable example of evolutionary shifts in sex pheromones is observed in Ostrinia moths, where ancestral s have been reactivated to alter pheromone blends. In the Asian corn borer (Ostrinia furnacalis), a previously nonfunctional Δ14-desaturase was activated in pheromone glands, enabling the production of (Z)- and (E)-12-tetradecenyl acetate components, distinct from the (Z)- and (E)-11-tetradecenyl acetate blend used by the closely related (Ostrinia nubilalis). This shift, occurring approximately 1 million years ago, likely contributed to by establishing asymmetric tracking, where a subset of males from the ancestral population could respond to the novel blend, facilitating divergence without complete . In vertebrates, sex pheromones play a critical role in through (MHC)-correlated scents, which signal genetic compatibility to prevent close-kin mating. In house mice, individuals preferentially select mates with dissimilar MHC genotypes based on urinary and body odors, as MHC genes influence volatile compounds that convey immune profile information, thereby optimizing offspring heterozygosity and disease resistance. This mechanism is conserved across many mammals, where MHC-dissimilar odors trigger avoidance of familiar kin scents, reducing . Evolutionary trends in vertebrates show retention of functional sex pheromone systems in many lineages but progressive loss in . While and other mammals maintain intact vomeronasal organs (VNOs) with hundreds of functional vomeronasal receptor genes (V1Rs) for pheromone detection, catarrhine (including monkeys and hominoids) exhibit pseudogenization of key transduction components, such as the TRP2 , dating back about 23 million years. This deterioration correlates with the rise of visual cues like trichromatic vision and sexual swellings for mate signaling, rendering pheromone pathways vestigial. Recent reviews propose that vestigial VNO structures, present embryonically but nonfunctional in adults, represent an inheritance from mammalian ancestors where pheromones mediated innate reproductive behaviors, now supplanted by higher cognitive and visual modalities.

Practical Applications

Pest Management and Agriculture

Pheromones play a central role in (IPM) strategies for , particularly in controlling lepidopteran pests that damage crops such as fruits, vegetables, and . By exploiting ' natural chemical communication, pheromones enable targeted interventions that minimize broad-spectrum applications, thereby supporting sustainable farming practices. Two primary techniques dominate pheromone-based pest control: trapping and mating disruption. In trapping, synthetic sex pheromones are used as lures in sticky or bucket traps to monitor pest populations or achieve mass capture, allowing early detection of infestations and direct reduction of male insects before they mate. For instance, pheromone traps have been instrumental in the boll weevil eradication program across the , where they facilitated detection and monitoring, contributing to the near-complete elimination of this pest in most regions by combining lures with cultural practices and minimal insecticides over a 4- to 5-year period; ongoing surveillance continues in areas like the of as of 2025. Mating disruption, conversely, involves the aerial or hand-applied release of high concentrations of synthetic female sex pheromones to overwhelm male sensory systems, preventing them from locating mates and thus reducing offspring production; this method is deployed via dispensers or puffers in orchards and fields. A prominent example of mating disruption is the control of the codling moth (Cydia pomonella), a major pest of apples and pears, using products like CM-XL 2.0 dispensers. These hand-applied devices release codlemone, the species-specific pheromone, over an entire , reducing codling moth damage by 50% or more and hindering male moths' ability to find traps when integrated with monitoring and a comprehensive IPM program. Similar applications have been successful against the oriental fruit moth and peachtree borer in stone fruit crops, where dispensers disrupt cycles without leaving residues. Recent advances have enhanced the practicality and environmental profile of these technologies. Developments in more stable formulations and biodegradable materials for multi-pheromone dispensers have reduced plastic waste and extended , aligning with eco-friendly IPM goals. The global agricultural pheromones market, valued at approximately USD 5 billion in 2025, is projected to reach USD 11-18 billion by 2032, driven by demand for sustainable alternatives amid regulatory pressures on chemical pesticides. These approaches offer significant benefits, including reduced reliance on synthetic pesticides—potentially cutting applications by 50-70% in orchards—preservation of beneficial , and no harvest residues, which enhances and export compliance. However, challenges persist, such as the high species-specificity of pheromones, which requires tailored formulations for each pest and limits applicability to polyphagous , alongside initial costs for dispensers that can deter small-scale farmers. Global adoption is accelerating, particularly in the region, which is the fastest-growing market with a projected 22.3% share in 2025 due to expanding orchards in and and supportive policies for . According to MarketsandMarkets, this region's growth is fueled by rising pest pressures on high-value crops and a shift toward IPM, with pheromone use expanding from monitoring to full-scale disruption in , , and production.

Animal Husbandry and Behavior Modification

In animal husbandry, synthetic pheromones have been employed to synchronize estrus in livestock, enhancing breeding efficiency without relying solely on hormonal treatments. For instance, analogs of bovine urinary pheromones, derived from bull secretions, have been shown to hasten puberty onset in prepubertal heifers and aid in estrus synchronization compared to controls. Similarly, in swine production, synthetic boar saliva pheromones, such as a commercial analog containing androstenone, androstenol, and a novel component, stimulate standing estrus in sows, improving reproductive performance including litter size in multi-farm trials and enabling artificial insemination protocols that avoid the use of live boars, thereby mitigating boar taint—a malodorous compound linked to the same pheromones accumulating in pork. These applications support fixed-time artificial insemination, streamlining herd management and genetic improvement in cattle and pig operations. Calming pheromones, mimicking maternal or facial secretions, are widely used in veterinary and husbandry settings to reduce stress and in companion and farm animals. In cats, synthetic feline pheromone (FFP) analogs, applied via diffusers or wipes, may decrease signs of acute such as vocalization and hiding during veterinary visits, with studies showing potential reductions in stress behaviors. For dogs, dog-appeasing pheromone (DAP), an analog of the maternal pheromone, alleviates separation anxiety in and home environments, with some controlled trials reporting reductions in destructive behaviors and vocalizations, though varies with . In equine husbandry, appeasing pheromone gels like Confidence EQ, which replicate nursing mare signals, help calm during transport, reducing resistance to trailer loading and overall agitation. Recent field observations as of 2024 noted improved compliance in loading trials, with showing decreased elevations when pheromones were used alongside behavioral training. These interventions collectively enhance by curbing in group housing and facilitating smoother handling in farming routines. The benefits of pheromone use in include boosted reproductive outcomes, such as higher farrowing rates in pigs (up to 3.5% increase) and synchronized calving in , which optimize labor and resource allocation. Reduced stress responses also lower risks and improve weight gain, as seen in calmer pigs post-weaning with appeasing pheromones. However, limitations persist due to species-specific variability; for example, DAP shows inconsistent results across breeds, with only moderate evidence for anxiety reduction in systematic reviews. Regulatory hurdles further constrain adoption, as veterinary pheromone products require approval from bodies like the FDA or EMA for safety and efficacy, often delaying commercial rollout and limiting use to well-studied analogs. Ongoing emphasizes the need for tailored formulations to address these interspecies differences.

Emerging Uses in Medicine and Industry

In recent years, researchers have explored synthetic analogs of primer pheromones for potential therapeutic applications in fertility treatments, particularly in where these compounds mimic natural signals to synchronize reproductive cycles in . For instance, analogs derived from male mouse pheromones have been shown to accelerate female estrus and improve conception rates in models, suggesting scalable applications for enhancing breeding efficiency in mammals. These developments build on observations that primer pheromones trigger long-term physiological changes, such as hormonal shifts, without direct behavioral cues. Emerging human medical applications include synthetic pheromones, or pherines, designed as odorless neuroactive steroids to modulate anxiety via nasal chemosensory receptors. Clinical studies have demonstrated that intranasal administration of such compounds, like (PH94B), rapidly reduces symptoms of , with formulations showing efficacy in on-demand relief comparable to traditional anxiolytics but with fewer side effects. As of 2025, pherines are in Phase 3 clinical trials, including repeat-dose studies and topline data expected in Q4 2025 for social anxiety, targeting conditions like generalized anxiety by engaging vomeronasal-like pathways in humans, though large-scale trials continue to confirm long-term safety and efficacy. In industry, synthetic pheromones serve as mimics in fragrance formulations to enhance perceived attractiveness and social signaling. Perfumes incorporating these compounds, such as analogs of androstenol, aim to amplify natural chemical cues, with market analyses indicating their integration into premium scents by blending with notes like for subtle behavioral influence. Additionally, pheromone-inspired biocatalysts are being investigated for production, where enzymes mimicking pheromone synthesis convert fatty acids into long-chain alcohols suitable for , offering a sustainable alternative to processes. The synthetic pheromone market, valued at approximately USD 455 million in 2023, is projected to reach USD 740 million by 2030, driven by demand in non-agricultural sectors like and therapeutics. Recent advancements include multi-pest (IPM) technologies using blended synthetic pheromones, which disrupt mating across species in industrial storage and urban settings, reducing reliance on broad-spectrum chemicals. Despite these innovations, challenges persist, including ethical concerns over unintended behavioral manipulation in medical contexts and the need for rigorous proof of beyond animal models. Safety risks from synthetic biology-derived compounds, such as off-target physiological effects, underscore the importance of protocols in clinical translation. Looking ahead, offers promising solutions for controlled-release systems, enabling precise delivery of synthetic pheromones in medical and industrial applications. Nanocarriers, such as electrospun nanofibers, can sustain pheromone release for weeks, minimizing dosage frequency while targeting specific receptors for enhancement or anxiety modulation. These eco-efficient matrices also support industrial scalability, potentially revolutionizing therapeutic precision by 2030.

Human Pheromones

Research Evidence and Candidates

Scientific research on human pheromones has primarily focused on a few proposed chemical candidates derived from bodily secretions, with evidence drawn from controlled laboratory experiments assessing behavioral and physiological responses. Key candidates include (AND), a found in male axillary sweat, estratetraenol (EST), an estrogen derivative present in female urine, and copulins, a mixture of short-chain volatile fatty acids in vaginal secretions. Studies investigating these candidates have employed double-blind, placebo-controlled trials to evaluate effects on mood, , attraction, and physiological . For instance, exposure to AND has been shown to modulate in a sex-dependent manner, reducing both reactive and proactive in men while increasing reactive in women, as measured by behavioral tasks in a 2023 study. Similarly, EST has been linked to enhanced sexual cognition in men, with one experiment demonstrating improved recognition of ambiguous erotic stimuli following subliminal exposure. Copulins, however, have yielded inconsistent results; a 2017 double-blind trial found no significant impact on men's sexual motivation or testosterone levels after exposure to synthetic versions. Regarding , early research suggested that axillary odors from women could influence alignment in small groups, but a 2023 study of cohabiting female medical students reported evidence of synchrony, though the phenomenon remains controversial with methodological concerns in odor-specific research. Recent findings from 2023 to 2025 underscore the inconclusive nature of these candidates as true pheromones. A 2024 review highlighted subtle mood and attraction effects in lab settings for AND and EST), such as increased positive affect in women exposed to AND, but emphasized that these responses are context-dependent and not consistently replicable across populations. A January 2025 review in Physiology & Behavior stressed the importance of preregistered, large-scale studies to resolve replication challenges in AND and EST research, as smaller trials continue to yield variable results. No compound has been conclusively isolated as a human-specific pheromone signal, with 2025 analyses in Physiology & Behavior concluding that AND does not meet criteria for a due to variable behavioral outcomes and lack of species-specific signaling. Significant gaps persist in the evidence base, particularly concerning the functionality of the human vomeronasal organ (VNO), which in other mammals detects pheromones but appears vestigial and non-functional in adult humans. Neuroimaging and histological studies indicate that the human VNO lacks neural connections to the brain's olfactory pathways and expresses pseudogenized receptor genes, supporting theories of evolutionary loss during hominid development as visual and social cues became dominant. These anatomical limitations challenge the pheromone hypothesis, as any chemosensory effects may instead involve the main olfactory system or trigeminal nerve, rather than a dedicated pheromone pathway.

Controversies and Commercial Products

The existence of true pheromones in humans remains highly debated, with most experts concluding that while subtle chemosignals may influence mood or , no definitive pheromones—defined as species-specific chemical signals eliciting stereotyped responses—have been identified. A key point of contention is the human (VNO), a structure thought to detect pheromones in other mammals, which is vestigial and non-functional in adult s, rendering traditional pheromone detection mechanisms unlikely. Instead, some researchers propose that the main might process chemosensory cues, but these effects are inconsistent and context-dependent, often failing replication in controlled studies. Commercial products claiming to harness human pheromones, such as and colognes, proliferate in the market despite lacking scientific validation. Commercial pheromone colognes rarely contain effective real human pheromones, with studies showing little evidence of increased attraction beyond placebo effects; compounds like may subtly influence mood or perception but lack consensus for driving sexual attraction akin to that in animals, effects often attributable to pleasant scents or user confidence rather than true pheromonal signaling. Perfumes cannot directly elevate women's hormone levels such as estrogen, which are regulated by internal physiological processes influenced by factors like diet, exercise, and sleep. Commercial "pheromone" or "hormone" perfumes lack strong scientific evidence for physiological effects, with any perceived benefits likely stemming from psychological suggestion, increased confidence, or the pleasure of the scent itself. Brands like Institute's pheromone additives and Pure Instinct's body sprays market synthetic compounds (e.g., derivatives) as attraction enhancers, often citing anecdotal testimonials or outdated studies, but independent reviews find no evidence of increased romantic or sexual appeal. For instance, a 2024 analysis highlighted that while these products may boost wearer confidence through effects, they do not demonstrably alter others' perceptions or behaviors in double-blind trials. The pheromone industry, valued at millions annually, relies on via , yet rigorous testing consistently shows effects attributable to fragrance alone rather than pheromonal action. Ethical concerns arise from the of these products, which exploit cultural myths about invisible attraction signals to prey on insecurities around and . Advertisements often imply guaranteed results, such as "scientifically proven" allure, fostering unrealistic expectations and potentially discouraging genuine interpersonal efforts. Critics argue this constitutes deceptive practices, particularly targeting vulnerable consumers, and raises broader questions about the of unproven in personal care. Regulatory scrutiny has intensified, with the U.S. (FDA) viewing unsubstantiated pheromone claims as misleading under cosmetics labeling laws, similar to warnings issued for other unproven enhancement products. In 2025, a class-action against alleged for claiming "human-compatible" pheromones that attract others, citing that no such effects exist; the suit seeks damages for deceived purchasers and highlights the FDA's stance on deceptive health-related claims. Such actions underscore the gap between bold marketing and evidentiary standards. Recent studies from 2023 to 2025 further debunk commercial pheromone perfumes, with a 2025 review in Physiology & Behavior concluding that candidate compounds like fail to meet pheromone criteria due to inconsistent behavioral outcomes across populations. Echoing a 2020 report, researchers call for reproducible protocols—larger samples, preregistration, and replication—to address past flaws like and small effect sizes, emphasizing that without these, claims remain speculative and commercial exploitation persists.

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