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A pheromone is a chemical that triggers a natural behavioral response in another member of the same species. There are alarm pheromones, food trail pheromones, sex pheromones, and many others that affect behavior or physiology. Their use among insects has been particularly well documented, although many vertebrates and plants also communicate using pheromones.
Pheromones of the pest insect species, such as the Japanese beetle and the gypsy moth, can be used to induce many behaviors. This facilitates trapping for monitoring purposes and population control by creating confusion, disrupting mating and preventing them from laying eggs.
In mammals and reptiles, pheromones may be detected by the vomeronasal organ, 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.
The term "pheromone" was introduced by Peter Karlson and Martin Lüscher in 1959, based on the Greek pherein (to transport) and hormon (to stimulate). They proposed the term to describe chemical signals from conspecifics which elicit innate behaviours soon after Butenandt characterized the first such chemical, Bombykol (a chemically well-characterized pheromone released by the female silkworm to attract mates).
Types of pheromones
Produced by one or the other sex, these pheromones attract individuals of both sexes.
Some species release a volatile substance when attacked by a predator that can trigger flight (in aphids) or aggression (in bees) 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.
Recognized in insects, these pheromones are different from territory pheromones. According to Fabre (translated from French), "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 so that they will clutch elsewhere."
Powerful attractant molecules that some organisms may use to attract mates from a distance of 2 miles or more. This type of pheromone generally elicites rapid response but is quickly degraded. In contrast, a primer pheromone would have a slower onset but a longer duration.
These pheromones trigger a change of developmental events.
Laid down in the environment, these pheromones mark the boundaries of an organism's territory. In dogs, these hormones are present in the urine, which they deposit on landmarks serving to mark the perimeter of the claimed territory.
These pheromones are common in social insects. For example, ants mark their paths with these pheromones, which are non-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 continually renewed. The pheromone must be continually renewed because it evaporates quickly. When the supply begins to dwindle, the trailmaking ceases. In at least one species of ant, trails that no longer lead to food are also marked with a repellent pheromone.
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. Many insect species release sex pheromones to attract a mate and many lepidopterans can detect a potential mate from as far away as 10 km (6.2 miles). Pheromones can be used in gametes to trail the opposite sex's gametes for fertilization. Pheromones are also used in the detection of oestrus in sows. Boar pheromones are sprayed into the sty, and those sows which exhibit sexual arousal are known to be currently available for breeding.
Other pheromones (not yet classified)
This classification, based on the effects on behavior, remains artificial. Pheromones fill many additional functions.
A few well-controlled scientific studies have been published suggesting the possibility of pheromones in humans. The best-studied 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 proposes 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". This is analogous to the Whitten effect, a male pheromone mediated modulation of estrus observed in mice.
Other studies have suggested that people might be using odor cues associated with the immune system to select mates who are not closely related to themselves. (See Disassortative sexual selection) Using a brain imaging technique, Swedish researchers have shown that homosexual and heterosexual males' brains respond differently to two odors that may be involved in sexual arousal, and that the homosexual men respond in the same way as heterosexual women. The study was expanded to include lesbian women and the results were consistent with previous findings meaning that homosexual women were not as responsive to male identified odors but their response to female cues was similar to heterosexual males. According to the researchers, this research suggests a possible role for human pheromones in the biological basis of sexual orientation. Another study demonstrated that the smell of androstadienone, a chemical component of male sweat, maintains higher levels of cortisol in females. The scientists suggest that the ability of this compound to influence the endocrine balance of the opposite sex makes it a human pheromonal chemosignal. in 2002 a study published in the quarterly journal Physiology and Behavior showed an unnamed synthetic chemical in women's perfume appeared to increase intimate contact with men. The authors hypothesize, but do not demonstrate, that the observed behavioural differences are olfactory mediated. 
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.
Some body spray advertisers claim that their products contain human sexual pheromones which act as an aphrodisiac. In the 1970's "copulins" were patented as products which release human pheromones, based on research on rhesus monkeys. Subsequent to that androstenone, axillary sweat, and "vomodors" have been claimed to act as human pheromones.  Despite these claims, no pheromonal substance has ever been demonstrated to directly influence human behavior in a peer reviewed study.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Pheromone". A list of authors is available in Wikipedia.|