Any chemical compound secreted by an organism in minute amounts to elicit a particular reaction from other organisms of the same species. Pheromones are widespread among insects and vertebrates (except birds) and are present in some fungi, slime molds, and algae. The chemicals may be secreted by special glands or incorporated into other substances (e.g., urine), shed freely, or deposited in selected locations. Pheromones are used to bring creatures together (e.g., in termite, bee, and ant colonies), lead them to food (e.g., in scent trails laid by ants), signal danger (e.g., when released by wounded fish to alert others), attract a mate and elicit sexual behaviour (numerous examples, possibly including humans), and influence sexual development (in many mammals and certain insects). Alarm pheromones often last a shorter time and travel a shorter distance than other types. In vertebrates, chemical stimuli often influence parent-young responses. Sex-attractant pheromones are used in certain products to lure and trap unwanted or harmful insects.
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Produced by one or the other sex, aggregation 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.. Examples are:
Recognized in insects, epideictic 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."
Releaser pheromones are powerful attractant molecules that some organisms may use to attract mates from a distance of two miles or more. This type of pheromone generally elicits a rapid response but is quickly degraded. In contrast, a primer pheromone has a slower onset and a longer duration.
Primer pheromones trigger a change of developmental events.
Laid down in the environment, territorial 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.
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 (e.g., moths and butterflies) can detect a potential mate from as far away as 10 kilometers (6 mi). 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.
This classification, based on the effects on behavior, remains artificial. Pheromones fill many additional functions.
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.
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. 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 homosexual women; 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. A study in urinary odour preferences of mice (2001) concluded that the pheromones associated with urinary odor assisted in deterring them from selecting mates from a related species. Interbreeding within mammals has shown to lead to a decrease in the overall fitness of that species, thus pheromones that decrease the probability of interbreeding are selected for. 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. Subsequently, 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.