Pheromones

A pheromone is any chemical produced by a living organism that transmits a message to other members of the same species. There are alarm pheromones, food trail pheromones, sex pheromones, and many others. Their use among insects has been particularly well documented, although many vertebrates and plants also communicate using pheromones. The existence of human pheromones is debated.

Insect pheromones of pest species, such as the Japanese beetle and the gypsy moth, can be used to trap them for monitoring purposes or for control by creating confusion, disrupting mating and preventing them from laying eggs. Bombykol is a chemically well characterized pheromone released by the female silkworm to attract mates.

In mammals and reptiles, pheromones may be detected by the vomeronasal organ, or Jacobson's organ, which lies between the nose and mouth, although some are detected by regular olfactory membranes.

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.

Alarm pheromones
Volatile substances released by an organism when attacked by a predator, that 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.

Sex pheromones
In animals, sex pheromones indicate the availability of the female for breeding. Certain butterflies can detect a potential mate from as much as 10km away. Male animals also emit pheromones that convey information about what species they are and their genotype.

Epideictic pheromones
Recognized in insects, these pheromones are different than territory pheromones. According to H. 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."

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

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)

Human pheromones
Some commercially-available substances are advertised using claims that the products contain sexual pheromones and can act as an aphrodisiac. These claims often lack credence due to an excessive marketing of pheromones by unsolicited e-mail, and their effectiveness has not been demonstrated through scientific means.

Nevertheless, a few well-controlled scientific studies have been published demonstrating the possibility of pheromones in humans. The best-studied case involves the synchronization of menstrual cycles among women based on unconscious odor cues (by Martha McClintock, Professor of Psychology at the University of Chicago). This study states 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". 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)

Pheromones in humans are believed to be produced by the apocrine glands. These glands become functional after reaching puberty, which could explain why most people develop an attraction for others at that time. Pheromones could also be the reason why a person can sense "chemistry", or feel an instant attraction or dislike when first meeting someone.

"Using a brain imaging technique, Swedish researchers have shown that homosexual and heterosexual men respond differently to two odors that may be involved in sexual arousal, and that the gay men respond in the same way as women. The new research may open the way to studying human pheromones, as well as the biological basis of sexual preference. Pheromones, chemicals emitted by one individual to evoke some behavior in another of the same species, are known to govern sexual activity in animals, but experts differ as to what role, if any, they play in making humans sexually attractive to one another." New York Times

Pheromones in fiction
Pheromones are a popular device in fiction, including the novel Jitterbug Perfume by Tom Robbins and the film Love Potion No. 9. They were also mentioned in an episode of Wolfgang Petersen's television series The Agency. In the Star Trek universe, female Orions are said to have powerful pheromones which men find hard to resist.

Journal articles

 * Karlson, P., Lüscher, M., 1959. Pheromones: a new term for a class of biologically active substances. Nature 183, 55-56.
 * Wilson, E. O., Bossert, W. H., 1963: Chemical communication among animals. Recent Progress in Hormone Research 19, 673-716.
 * McClintock, M.K., 1984. Estrous synchrony: modulation of ovarian cycle length by female pheromones. Physiological Behavior 32, 701-705

Books

 * Wyatt, Tristram D. 2003. Pheromones and Animal Behaviour: Communication by Smell and Taste. Cambridge: Cambridge University Press. ISBN 0521485266.