A pheromone () is a chemical that is secreted or excreted by an organism, which triggers a social response in members of the same species.
Background
The
portmanteau word "pheromone" was coined by
Peter Karlson and Martin Lüscher in 1959, based on the
Greek language φέρω () and ὁρμων ().
Pheromones are also sometimes classified as ecto-hormones ("ecto-" meaning "outside"
). They were researched earlier by various scientists, including
Jean-Henri Fabre, Joseph A. Lintner,
Adolf Butenandt, and
ethology Karl von Frisch who called them various names, such as "alarm substances". These chemical messengers are transported outside of the body and affect
nervous system, including the autonomous nervous system with
hormone or
cytokine mediated
physiology changes,
inflammation signaling,
immune system changes and/or behavioral change in the recipient.
They proposed the term to describe chemical signals from
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.
Categorization by function
Aggregation
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.
Aggregation pheromones have been found in members of the
Coleoptera,
Springtail,
,
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.
Alarm
Some species release a volatile substance when attacked by a predator that can trigger flight (in
) or aggression (in
,
,
, and
)
in members of the same species. For example,
Vespula squamosa use alarm pheromones to alert others to a threat.
[Landoldt, P. J., Reed, H. C., and Heath, R. R. "An Alarm Pheromone from Heads of Worker Vespula squamosa (Hymenoptera: Vespidae)", "Florida Entomologist", June 1999.] In
Polistes exclamans, alarm pheromones are also used as an alert to incoming predators.
Pheromones also exist in plants: Certain plants emit alarm pheromones when grazed upon, resulting in
tannin production in neighboring plants.
These tannins make the plants less appetizing to
.
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.
Epideictic
Epideictic pheromones are different from territory pheromones, when it comes to insects. Fabre observed and noted how "females who lay their eggs in these fruits deposit these mysterious substances in the vicinity of their clutch to signal to other females of the same species they should clutch elsewhere." It may be helpful to note that the word
epideictic, having to do with display or show (from the Greek 'deixis'), has a different but related meaning in rhetoric, the human art of persuasion by means of words.
Territorial
Laid down in the environment, territorial pheromones mark the boundaries and identity of an organism's territory. 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'.
Trail
Social insects commonly use trail pheromones. For example,
mark their paths with pheromones consisting of volatile
. Certain ants lay down an initial trail of pheromones as they return to the nest with food. This trail attracts other ants and serves as a guide.
As long as the food source remains available, visiting ants will continuously renew the pheromone trail. The pheromone requires continuous renewal because it evaporates quickly. When the food supply begins to dwindle, the trail-making ceases. Pharaoh ants (
Monomorium pharaonis) mark trails that no longer lead to food with a repellent pheromone, which causes avoidance behaviour in ants.
Repellent trail markers may help ants to undertake more efficient collective exploration.
[
]
The army ant
Eciton burchellii provides an example of using pheromones to mark and maintain foraging paths. When species of wasps such as
Polybia sericea found new nests, they use pheromones to lead the rest of the colony to the new nesting site.
Gregarious caterpillars, such as the forest tent caterpillar, lay down pheromone trails that are used to achieve group movement.[
]
Sex
In animals, sex pheromones indicate the availability of the female for breeding. Male animals may also emit pheromones that convey information about their species and
genotype.
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.
Among eukaryotic microorganisms, pheromones promote sexual interaction in numerous species.[Danton H. O'Day, Paul A. Horgen (1981) Sexual Interactions in Eukaryotic Microbes Academic Press, New York. ] 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 can follow a three-dimensional pheromone trail left by a swimming female, and male of many animals use a pheromone to help find a female gamete for fertilization.[Dusenbery, David B. (2009). Living at Micro Scale, Chapters 19 & 20. Harvard University Press, Cambridge, Massachusetts .]
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 (moths and butterflies) can detect a potential mate from as far away as .
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.
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.["Alpinobombus". Natural History Museum. Retrieved 26 September 2015] In particular, pheromones for the Bombus hyperboreus, include Oleyl alcohol, 2,3-dihydro-6-transfarnesol, citronellol, and geranylcitronellol.
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
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 acid and , they allow crustaceans and hexapods to identify the presence of dead Conspecificity.
-
Suckling: TAA is present in rabbit milk and seems to play a role of pheromone inducing suckling in the newborn rabbit.
Categorization by type
Releaser
Releaser pheromones are pheromones that cause an alteration in the behavior of the recipient. For example, some organisms use powerful attractant molecules to attract mates from a distance of two miles or more. In general, this type of pheromone elicits a rapid response, but is quickly degraded. In contrast, a primer pheromone has a slower onset and a longer duration. For example, rabbit (mothers) release mammary pheromones that trigger immediate nursing behavior by their babies.
Primer
Primer pheromones trigger a change of developmental events (in which they differ from all the other pheromones, which trigger a change in behavior). They were first described in
Desert locust by
Maud Norris in 1954.
Signal
Signal pheromones cause short-term changes, such as the neurotransmitter release that activates a response. For instance, GnRH molecule functions as a neurotransmitter in rats to elicit lordosis behavior.
[
]
Pheromone receptors
In the olfactory epithelium
The human trace amine-associated 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.
A review of studies involving non-human animals indicated that TAARs in the olfactory epithelium can mediate reward system or aversives behavioral responses to a receptor agonist.
Figure 2: Table of ligands, expression patterns, and species-specific behavioral responses for each TAAR
In the vomeronasal organ
In , 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.
Evolution
Olfactory processing of chemical signals like pheromones exists in all animal phyla and is thus the oldest of the senses. It has been suggested that it serves survival by generating appropriate behavioral responses to the signals of threat, sex and dominance status among members of the same species.
Furthermore, it has been suggested that in the evolution of unicellular to multicellular , primordial pheromone signaling between individuals may have evolved to paracrine and endocrine signaling within individual organisms.
Some authors assume that approach-avoidance reactions in animals, elicited by chemical cues, form the phylogenetic basis for the experience of emotions in humans.[R.S. Herz, T. Engen, Odor memory: review and analysis, Psychon. Bull.
Rev. 3 (1996) 300–313.]
Evolution of sex pheromones
Avoidance of inbreeding
Mice can distinguish close relatives from more distantly related individuals on the basis of scent signals,
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.
Applications
Pheromone trapping
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
Pheromones are used in the detection of oestrus in Pig. Boar pheromones are sprayed into the , and those sows that exhibit sexual arousal are known to be currently available for breeding.
Human sex pheromone controversies
While humans are highly dependent upon visual cues, when in close proximity smells also play a role in sociosexual behaviors. An inherent difficulty in studying human pheromones is the need for cleanliness and odorlessness in human participants.[Wyatt, Tristram D. (2003). Pheromones and Animal Behaviour: Communication by Smell and Taste. Cambridge: Cambridge University Press. . p. 298 Quoting Preti & Weski (1999) "No peer reviewed data supporting the presences of ... human ... pheromones that cause rapid behavioral changes, such as attraction and/or copulation have been documented."][ p. 264 ...there has not yet been any hard evidence for human pheromones that might change sexual attraction (for members of either sex) naturally]
Axillary steroids
Axillary steroids are produced by the Testicle, Ovary, apocrine glands, and .[ Several axillary steroids have been described as possible human pheromones: androstadienol, androstadienone, androstenol, androstenone, and androsterone.
]
-
Androstenol is the putative female pheromone.
[ In a 1978 study by Kirk-Smith, people wearing surgical masks treated with androstenol or untreated were shown pictures of people, animals and buildings and asked to rate the pictures on attractiveness.][ The best-known case study involves the synchronization of among women based on unconscious odor cues, the McClintock effect, named after the primary investigator, Martha McClintock, of the University of Chicago.][.] A group of women were exposed to a whiff of perspiration from other women. Depending on the time in the month the sweat was collected (before, during, or after ovulation) there was an association with the recipient woman's menstrual cycle to speed up or slow down. The 1971 study proposed two types of pheromone involved: "One, produced prior to ovulation, shortens the ovarian cycle; and the second, produced just at ovulation, lengthens the cycle". However, recent studies and reviews of the methodology have called the validity of her results and existence of menstrual synchronization into question.
-
Androstenone is postulated to be secreted only by males as an attractant for women, and thought to be a positive effector for their mood. It seems to have different effects on women, depending on where a female is in her menstrual cycle, with the highest sensitivity to it during ovulation.
[ In 1983, study participants exposed to androstenone were shown to undergo changes in skin conductance.][
]
-
Androstadienone seems to affect the limbic system and causes a positive reaction in women, improving mood.
[ Responses to androstadienone depend on the individual and the environment they are in.][ Women tend to react positively after androstadienone presentation, while men react more negatively. In an experiment by Hummer and McClintock, androstadienone or a control odor was put on the upper lips of fifty males and females and they were tested for four effects of the pheromone: 1) automatic attention towards positive and negative facial expressions, 2) the strength of cognitive and emotional information as distractors in a simple reaction time task, 3) relative attention to social and nonsocial stimuli (i.e. neutral faces), and 4) mood and attentiveness in the absence of social interaction. Those treated with androstadienone drew more attention to towards emotional facial expressions and emotional words but no increased attention to neutral faces. These data suggest that androstadienone may increase attention to emotional information causing the individual to feel more focused. It is thought that androstadienone modulates on how the mind attends and processes information.][
]
While it may be expected on evolutionary grounds that humans have pheromones, these three molecules have yet to be rigorously proven to act as such. Research in this field has suffered from small sample sizes, publication bias, false positives, and poor methodology.
Vaginal aliphatic acids
A class of aliphatic acids (volatile fatty acids as a kind of carboxylic acid) was found in female that produced six types in the vaginal fluids.[ Even in humans, one-third of women have all six types of copulins, which increase in quantity before ovulation.][ Copulins are used to signal ovulation; however, as human ovulation is concealed it is thought that they may be used for reasons other than sexual communication.][
]
Stimulators of the vomeronasal organ
The human 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 in humans.[ Also, while there are sensory neurons in the human VNO there seem to be no connections between the VNO and the central nervous system. The associated olfactory bulb is present in the fetus, but regresses and vanishes in the adult brain. There have been some reports that the human VNO does function, but only responds to hormones in a "sex-specific manner". There also have been pheromone receptor genes found in olfactory mucosa.][ There have been no experiments that compare people lacking the VNO, and people that have it. It is disputed on whether the chemicals are reaching the brain through the VNO or other tissues.][
]
In 2006, it was shown that a second mouse odorant receptor sub-class is found in the olfactory epithelium. Called the trace amine-associated receptors (TAAR), some are activated by volatile found in mouse urine, including one putative mouse pheromone.
Although there are disputes about the mechanisms by which pheromones function, there is evidence that pheromones do affect humans.[
]
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.[ Claus Wedekind and colleagues found that both men and women prefer the axillary odors of people whose MHC is different from their own.]
Some body spray advertisers claim that their products contain human sexual pheromones that act as an aphrodisiac. Despite these claims, no pheromonal substance has ever been demonstrated to directly influence human behavior in a study.
See also
Further reading
External links
