Pheromones are chemicals emitted by living organisms to send messages to individuals of the same species, in order to trigger a behavioural response. There are lot´s of pheromones, and they are classified according to their function: 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.
The term “pheromone” was introduced by Peter Karlson and Martin Lüscher in 1959, based on the Greek pherein (to transport) and hormon (to stimulate).

History of pheromones

The first pheromone ever identified (in 1956) was a powerful sex attractant for silkworm moths. A team of German researchers worked 20 years to isolate it. After removing certain glands at the tip of the abdomen of 500,000 female moths, they extracted a curious compound. The minutest amount of it made male moths beat their wings madly in a “flutter dance.” This clear sign that the males had sensed the attractant enabled the scientists to purify the pheromone. Step by step, they removed extraneous matter and sharply reduced the amount of attractant needed to provoke the flutter dance.

When at last they obtained a chemically pure pheromone, they named it “bombykol” for the silkworm moth, “Bombyx mori” from which it was extracted. It signaled, “come to me!” from great distances. “It has been soberly calculated that if a single female moth were to release all the bombykol in her sac in a single spray, all at once, she could theoretically attract a trillion males in the instant,” wrote Lewis Thomas in The Lives of a Cell.

Chemistry of pheromones

During the past 40 years, pheromones of hundreds of insect species have been chemically elucidated, including the sex pheromone of the codling moth. Its main component is (E,E)-8,10-dodecadien-1-ol, a primary alcohol containing a straight chain of 12 carbons and two conjugated double bonds. Other moth pheromones are hydrocarbons, epoxides, acetates or aldehydes. These molecules all vaguely resemble fatty acids, from which they are indeed biogenetically derived.

Most pheromones consist of blends of two or more chemicals which need to be emitted at exactly the right proportions to be biologically active. The female effluvia or sex gland can contain additional compounds which are related to the pheromone components and whose biological function is often unclear. On the other hand, many attractants of male moths have been discovered simply by field screening. In several cases it could later be shown that the attractant found with this technique was identical to the natural pheromone produced by the female. In most others, the composition of the true pheromone is still unknown. You can click the structural formula above to see a list of all the species for which (E,E)-8,10-dodecadien-1-ol has been reported as a pheromone or attractant component.

Types of pheromones

Aggregation pheromones

Produced by one or the other sex, these pheromones attract individuals of both sexes.

Alarm pheromones

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.

Epideictic pheromones

Recognized in insects, these pheromones are different than 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

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.

Primer pheromones

These pheromones trigger a change of developmental events.

Territorial pheromones

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.

Trail pheromones

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.

Sex pheromones

In animals, sex pheromones indicate the availability of the female for breeding. 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.

Male animals also emit pheromones that convey information about what species they are, and their genotype. The purpose of pheromones giving information about genotype is a mechanism to avoid inbreeding. Females are attracted to males with the least similar genotype, which means they are attracted to males who are the least likely to be related to them. An exception to this is when the female is pregnant. Then they are most drawn to individuals with the most similar pheromones (and therefore genotype) because they want to keep family close by to aid with the raising of their young and to take advantage of protection.[citation needed]

Other pheromones (not yet classified)

This classification, based on the effects on behavior, remains artificial. Pheromones fill many additional functions.

  • Nasonov pheromones (worker bees)
  • Royal pheromones (bees)
  • Calming (appeasement) pheromones (mammals)

Pheromones in mamals

In dealing with mammals, however, scientists faced an entirely different problem than when they studied insects, whose behavior is stereotyped and highly predictable. Mammals are independent, ornery, complex creatures. Their behavior varies greatly, and its meaning is not always clear.

Some studies showed what appears to be a pheromone like activity in Hamsters, with hamsters—but the experiment would be hard to repeat with larger mammals.

Human pheromones

Some commercially-available substances are advertised using claims that the products contain human sexual pheromones and can act as an aphrodisiac. These often lack credibility due to an excessive marketing of pheromones by unsolicited e-mail. Despite claims to the contrary, no defined pheromonal substance has ever been demonstrated to directly influence human behavior in a peer reviewed, published study.

A few well-controlled scientific studies have been published suggesting the possibility of pheromones in humans, however. The best-studied case involves the synchronization of menstrual cycles among women based on unconscious odor cues (the so called McClintock effect, named after the primary investigator). 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. Using a brain imaging technique, Swedish researchers have shown that homosexual and heterosexual males’ brains respond differently to two odours that may be involved in sexual arousal, and that the homosexual men respond in the same way as heterosexual women. 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 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 compounds found in mouse urine, including one putative pheromone. Orthologous receptors exist in humans providing, the authors propose, evidence for a mechanism of human pheromone detection.

Use for pest control

When used in combination with traps, sex pheromones can be used to determine what insect pests are present in a crop and what plant protection measures or further investigations might be in order to assure that there will be no excessive damage to the crop. If the synthetic attractant is exceptionally seducing and the population level is very low, some control can be achieved with pheromone traps or with a technique called “attract and kill”.

Generally, however, a technique called mating disruption is more effective: Synthetic pheromone is released from numerous sources placed throughout the crop to be protected; the males are then unable to locate the females and the number of matings and offsprings is reduced.

Mating disruption has been successful in controlling a number of insect pests. More than 20% of the grape growers in Germany and Switzerland employ this technique and produce their wine without using insecticides. If you are interested in this technique, you could look at the proceedings of conferences held by the Working Group “Use of pheromones and other semiochemicals in integrated control” of the International Organization of Biological Control (IOBC).

In this database we have included, species by species, any reports on sex pheromones and related chemicals that were thought to be of value to scientists interested in identifying Lepidoptera pheromones, studying biosynthetic pathways or using pheromones for insect monitoring and control. Not included are pheromones produced by males, such as aphrodisiacs and arrestants.

Sources: Howard Hughes Medical Institute, Wikipedia

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