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A pheromone (from Greek φέρω phero "to bear" and hormone, from 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, Adolph Butenandt, and the prominent ethologist Karl von Frisch who called them various names like "alarm substances." These chemical messengers are transported outside of the body and result in a direct developmental effect on hormone levels or behavioral change. They proposed the term to describe chemical signals from conspecifics that elicit innate behaviors soon after the German Biochemist Adolf Butenandt 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. So bacteria are too small to use pheromones as sex attractants on an individual basis but do use them to determine the local population density of similar organisms and control behaviors that take more time to execute (quorum sensing or to promote Natural competence for Transformation (genetics), 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 defense against predators, mate selection, and overcoming host resistance by mass attack. 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 and 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. 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 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'.
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.
Information pheromones are indicative of an animal's identity or territory. For example, dogs and cats deposit chemicals in and around their territory, which then serve as an indicator for other members of the species about the presence of the occupant in that territory.
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 insect species, such as the ant Leptothorax acervorum, 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.
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.
Pheromones have evolved in all animal phyla, to signal sex and dominance status, and are responsible for stereotypical social and sexual behaviour among members of the same species. In mammals, these chemical signals are believed to be detected primarily by the vomeronasal organ (VNO), a chemosensory organ located at the base of the nasal septum. The VNO is present in most amphibia, reptiles, and non-primate mammals but is absent in birds, adult catarrhine monkeys, and apes. An active role for the human VNO in the detection of pheromones is disputed; the VNO is clearly present in the foetus but appears to be atrophied or absent in adults. Three distinct families of putative pheromone receptors have been identified in the vomeronasal organ (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.
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.
Pheromones of pest insect species, such as the Japanese beetle and the gypsy moth, can be used to induce many behaviors. As a result, a pheromone trap can be used to trap pests for monitoring purposes, to control the population by creating confusion, to disrupt mating, as well as to prevent further egg laying.
In mammals and reptiles, pheromones may be detected by the vomeronasal organ (VNO), or Jacobson's organ, which lies between the nose and mouth and is the first stage of the accessory olfactory system. Some pheromones in these animals are detected by regular olfactory membranes.
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 to minimize deleterious inbreeding. showed that the inbreeding of mice derived from wild populations significantly reduced survival when such mice were reintroduced into a natural habitat.
While humans are highly dependent upon visual cues, when in close proximity smells also play a big role in sociosexual behaviors. There is an inherent difficulty in studying human pheromones because of the need for cleanliness and odorlessness in human participants. The focus of the experiments on human pheromones has been on three classes of putative pheromones: axillary steroids, vaginal aliphatic acids, and stimulators of the vomeronasal organ.
There are three axillary steroids that have been described as human pheromones: androstenone, androstenol and androstadienone. The axillary steroids are produced by the testes, ovaries, apocrine glands and adrenal glands. These chemicals are not biologically active until puberty when the sex steroids influence their activity. This change in activity associated with puberty is some of the best evidence that our species do communicate through odors.
Androstenol is the putative female pheromone. In a study by Kirk-Smith, people wearing surgical masks treated with androstenol or untreated were shown pictures of people, animals and buildings and asked to rate the pictures on attractiveness. Individuals with their masks treated with androstenol rated their photographs as being "warmer" and "more friendly".
The best-known case 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). This study exposed a group of women to a whiff of perspiration from other women. It was found that it caused their menstrual cycles to speed up or slow down depending on the time in the month the sweat was collected: before, during, or after ovulation. Therefore, this study proposed that there are 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 McClintock methodology have called into question the validity of her results.
Van Toller and colleagues showed that people exposed to androstenone undergo physiological changes in skin conductance. Further, androstenone has also been found to be perceived as more pleasant to men at a woman’s time of ovulation. It is hypothesized that this may be a way for a male to detect an ovulating female who would be more willingly to be involved in sexual interaction. Females are also most sensitive to this pheromone while ovulating. This pheromone is said to be only secreted by males as an attractant for women and is also thought to be a positive effector for their mood. Depending on where a female is in her menstrual cycle, the pheromones seem to have different effects on women.
Another putative pheromone is androstadienone. This steroid seems to affect the limbic system and causes a positive reaction in women, often improving their moods. Responses to androstadienone are dependent on the individual and the environment they are in. Androstadienone negatively influences the perception of pain in women. Women tend to react positively after androstadienone presentation while men are more negative. In an experiment by Hummer and McClintock, androstandienone or a control odor was put on the upper lips of fifty males and females and they were tested for four different effects of the pheromone: 1) automatic attention towards positive and negative facial expressions, 2) the strength of cognitive and emotional information as distracters 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. The androstadienone was found to draw attention towards emotional facial expressions. Those treated with androstadienone drew more attention to emotional words while it did not increase attention to neutral faces. These data suggest that androstandienone increases attention to emotional information resulting a feeling of being more focused. It is thought that androstadienone is a modulator on how the mind attends and processes information instead of being a mood-alerter.
Further evidence of a role for pheromones in the modulation of sociosexual behavior comes from two double blind, placebo-controlled experiments. The first, 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 that there is an increase in sexual behavior in the pheromone users compared to the control group. The study by McCoy and Pitino was similar to the Cutler study, only females instead of males were subjects. Females treated with female pheromones 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 the 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 has 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 still 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.