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A pheromone (from Ancient Greek φέρω phero "to bear" and hormone, from Ancient Greek ὁρμή "impetus") is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual. There are alarm pheromones, food trail pheromones, sex pheromones, and many others that affect behavior or physiology. Pheromones are used from basic unicellular prokaryotes to complex multicellular eukaryotes. Their use among insects has been particularly well documented. In addition, some vertebrates and plants communicate by using pheromones.
The term "pheromone" was introduced by Peter Karlson and Martin Lüscher in 1959, based on the Greek word pherein (to transport) and hormone (to stimulate). They are also sometimes classified as ecto-hormones. 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 like "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. 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.
There are physical limits on the practical size of organisms employing pheromones, because at small sizes pheromone diffuses away from the source organism faster than it can be produced, and a sensible concentration accumulates too slowly to be useful. For this reason, bacteria are too small to use pheromones as sex attractants on an individual basis. However, they do use them to determine the local population density of similar organisms and control behaviors that take more time to execute (e.g. pheromones are used in quorum sensing or to promote natural competence for transformation, i.e. sexual gene transfer). In similar manner, the simple animals rotifers are, it appears, also too small for females to lay down a useful trail, but in the slightly larger copepods the female leaves a trail that the male can follow.
Aggregation pheromones function in mate selection, 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, Diptera, Hemiptera, Dictyoptera, and Orthoptera. In recent decades, the importance of applying aggregation pheromones in the management of the boll weevil (Anthonomus grandis), stored product weevils (Sitophilus zeamais), Sitophilus granarius, Sitophilus oryzae, and pea and bean weevil (Sitona lineatus) has been demonstrated. Aggregation pheromones are among the most ecologically selective pest suppression methods. They are nontoxic and effective at very low concentrations.
Some species release a volatile substance when attacked by a predator that can trigger flight (in aphids) or aggression (in ants, bees, termites) in members of the same species. For example, Vespula squamosa use alarm pheromones to alert others to a threat. 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 for the herbivore.
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."
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.
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.
Primer pheromones trigger a change of developmental events (in which they differ from all the other pheromones, which trigger a change in behavior).
Laid down in the environment, territorial pheromones mark the boundaries and identity of an organism's territory. In cats and dogs, these hormones are present in the urine, which they deposit on landmarks serving to mark the perimeter of the claimed territory. In social seabirds, the preen gland is used to mark nests, nuptial gifts, and territory boundaries with behavior formerly described as 'displacement activity'.
Trail pheromones are common in social insects. For example, ants mark their paths with these pheromones, which are 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. As long as the food source remains, the pheromone trail will be continuously renewed. The pheromone must be continuously renewed because it evaporates quickly. When the supply begins to dwindle, the trail making ceases. In at least one species of ant, trails that no longer lead to food are also marked with a repellent pheromone. An example of pheromones used to create and maintain foraging paths is shown in the Eciton burchellii species. 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.
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. 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 (see Natural competence).
Among eukaryotic microorganisms, pheromones promote sexual interaction in numerous species. 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.
Many if not all insect species, such as the ant Leptothorax acervorum, the moth Helicoverpa zea, and the butterfly Edith's Checkerspot release sex pheromones to attract a mate, and many lepidopterans (moths and butterflies) can detect a potential mate from as far away as 10 km (6.2 mi). Traps containing pheromones are used by farmers to detect and monitor insect populations in orchards. In addition, Colias eurytheme butterflies release pheromones, an olfactory cue important for mate selection.
The effect of Hz-2V virus infection on the reproductive physiology and behavior of female 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 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. 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.
This classification, based on the effects on behavior, remains artificial. Pheromones fill many additional functions.
Processing chemosignals like pheromones has evolved in all animal phyla and thus is the oldest phylogenetic receptive system shared by all organisms including bacteria. 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 prokaryotes to multicellular eukaryotes, 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.
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. While the VNO is present in most amphibia, reptiles, and non-primate mammals, it is absent in birds, adult catarrhine monkeys (downward facing nostrils, as opposed to sideways), and apes. 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.
Pheromones of certain pest insect species, such as the Japanese beetle, acrobat ant, and the gypsy 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.
Mice can distinguish close relatives from more distantly related individuals on the basis of scent signals, which enables them to avoid mating with close relatives and minimizes deleterious inbreeding. Jiménez et al. showed that inbred mice had significantly reduced survival when they were reintroduced into a natural habitat.
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. Experiments have focused on three classes of putative human pheromones: axillary steroids, vaginal aliphatic acids, and stimulators of the vomeronasal organ.
Axillary steroids are produced by the testes, ovaries, apocrine glands and adrenal glands. These chemicals are not biologically active until puberty when sex steroids influence their activity. The activity change during puberty provides evidence for the idea that human beings communicate through odors. Three axillary steroids have been described as human pheromones: androstenol, androstenone and androstadienone.
Further evidence of a role for pheromones in sociosexual behavior comes from two double blind, placebo-controlled experiments. The first from 1998, by Cutler, had 38 male volunteers apply either a "male pheromone" or control odor and record six different sociosexual behaviors over two weeks. This study found an increase in sexual behavior in the pheromone users compared to the control group. The study 2002 by McCoy and Pitino was similar, except that participants were women, not men. Females treated with "female pheromone" reported significant increases in many of the behaviors including "sexual intercourse", "sleeping next to a partner", "formal dates", and "petting/affection/kissing". The researchers believed that pheromones had a positive sexual attractant effect.
A class of aliphatic acids was found in female rhesus monkeys that produced six types in the vaginal fluids. The combination of these acids is referred to as "copulins". One of the acids, acetic acid, was found in all of the sampled female’s vaginal fluid. Even in humans one-third have all six types of copulins, which increase in quantity prior to 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.
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. Also, while there are sensory neurons in the human VNO there seem to be no connections between the VNO and the central nervous system. The associated olfactory bulb is present in the fetus, but regresses and vanishes in the adult brain. There have been some reports that the human VNO does function, but only responds to hormones in a "sex-specific manner". There also have been pheromone receptor genes found in olfactory mucosa. Unfortunately, there have been no experiments that compare people lacking the VNO, and people that have it. It is disputed on whether the chemicals are reaching the brain through the VNO or other tissues.
In 2006, it was shown that a second mouse receptor sub-class is found in the olfactory epithelium. Called the trace amine-associated receptors (TAAR), some are activated by volatile amines found in mouse urine, including one putative mouse pheromone. Orthologous receptors exist in humans providing, the authors propose, evidence for a mechanism of human pheromone detection.
Even though there are disputes about the mechanisms by which pheromones function there is evidence that pheromones do affect humans. Even with all of this evidence, nothing is conclusive on whether or not humans have functional pheromones. Even if there are experiments that suggest that certain pheromones have a positive effect on human, there are just as many that state the opposite or that they have no effect whatsoever.
A possible theory being studied now is that these axillary odors are being used to provide information about the immune system. Milinski and colleagues found that the artificial odors that people chose are determined in part by their major histocompatibility complexes (MHC) combination. Information about an individual’s immune system could be used as a way of "sexual selection" so that the female could obtain good genes for her offspring. 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 peer reviewed study. Thus, the role of pheromones in human behavior remains speculative and controversial.