A pheromone (from Greek φέρω phero "to bear" + 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. 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. 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 which elicit innate behaviours 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 but do use them to determine the local population density of similar organisms and control behaviors that take more time to execute (quorum sensing). Similarly, the simple animals rotifers are apparently 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 are referred as aggregation, whether consisting of one sex or both sexes. Male-produced sex attractant have been called aggregation pheromones, because they usually result in the arrival of both sexes at a calling site and increase in density of conspecifics surrounding of 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, 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 not toxic and they are 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. 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. Ex. Rabbit (mothers) release mammary pheromones that trigger immediate nursing behavior by their babies.
Signal pheromones cause short term changes; such as, the neurotransmitter release which 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 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 continually renewed. The pheromone must be continually 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.So that the species will know of who owns that area or who that territory is being occupied by.
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, 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 release sex pheromones to attract a mate, and many lepidopterans (moths and butterflies) can detect a potential mate from as far away as 10 kilometers (6.25 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 which 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 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.
Few well-controlled scientific studies have ever been published suggesting the possibility of pheromones in humans.
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.
It has been suggested that women with irregular menstrual cycles became regular when exposed to male underarm extracts. They hypothesized that male sweat contains pheromones, which mirror how pheromones affect other mammals.
Other studies have demonstrated that the smell of androstadienone, a chemical component of male sweat, maintains higher levels of cortisol in females, and that the compound is detected via the olfactory mucosa. 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 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 olfactorily mediated. This and a previous study by the same authors with the still undisclosed "pheromone" preparation has been heavily criticized for having methodological flaws and that upon re-analyzing there was no effect seen.
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, though it could not be determined whether this was cause or effect. 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, while their response to female cues was similar to that of heterosexual males. According to the researchers, this research suggests a possible role for human pheromones in the biological basis of sexual orientation. In 2008, it was found using functional magnetic resonance imaging that the right orbitofrontal cortex, right fusiform cortex, and right hypothalamus respond to airborne natural human sexual sweat. 
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 1970s, "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.