Pheromone

A fanning honeybee exposes its Nasonov gland (located at the tip of the abdomen), releasing a pheromone to entice the swarm into an empty hive.

A pheromone is an endogenous (internally-produced) chemical secreted by an organism that triggers an innate behavioral response in another member of the same species. Pheromones are a subclass of semiochemicals (chemicals involved in animal communication) used for communication within the species

The use of pheromones among insects has been particularly well documented, although many vertebrates and plants also communicate using pheromones. (unknown in birds, also crustaceans, used by some fungi, slime molds, and algae in reproduction; role/presence in humans difficult to ascertain)

The term ‘’pheromone’’ was introduced by Peter Karlson and Martin Lüscher in 1959, based on the Greek pherein (to carry or transfer) and hormon (to excite or stimulate). Karlson and Lüscher proposed the term to describe chemical signals from conspecifics (members of the same species) that elicit innate behaviors.

Compare to hormones (internal signals within an individual organism).

Pheromones may be secreted by special glands or incorporated into other substances like urine.

Shared sex pheromone of elephants and moths: illustrates ubiquity of pheromones; not precise enough for moths (for whom pheromones are multi-component) and released in amounts to small for male elephants to detect (4-5)

Honeybees have one of the most complex pheromonal communication systems found in nature, possessing 15 known glands that produce an array of compounds.

The detection of pheromones

Across the animal kingdom, pheromones are detected by olfactory systems, which have a remarkable degree of similarity. These systems for detecting pheromones and other odors include olfactory sensory neurons (OSNs), which are nerve cells with one end exposed to the external environment, often imbedded in an otherwise impermeable skin or cuticle. Pheromones are converted into signals by first binding to a receptor protein in the cell membrane of the OSN. This activities a G-protein, triggering a cascade of reactions leading to the transmittal of electrical impulses down the axon of the OSN to the brain.

Most amphibians, reptiles, and mammals have a dual olfactory system, which includes the main olfactory epithelium (MOE) and the vomeronasal organ (VNO), or Jacobson's organ. Located between the nose and mouth, the VNO sends signals to an accessory olfactory bulb. Depending on the species, pheromones and other olfactory recognition cues may be detected by the MOE or the VNO, or both.

Hormones (signal molecules within the individual organism) also help to orchestrate the appropriate responses to pheromone signals.

Types of pheromones

Pheromones may be divided into two broad categories:

• Releaser pheromones, which typically have immediate effects on the behavior of the receiver and are quickly degraded.
• Primer pheromones, which trigger long-term physiological effects. Primer pheromones have a slower onset but longer duration than releaser pheromones.

These divisions are not strict, however, as many pheromones can play both roles.

Pheromones are typically classified by function. The divisions below represent only a sampling of the diverse activities coordinated by pheromones:

Sex pheromones

The male silkworm (above) possesses elaborate, feathery antennae to detect a sex pheromone emitted by the female (below).

One of the best-known sex pheromones (and the first pheromone to be characterized) is a polyalcohol called bombykol, which is released by the female silkworm (bombyx mori) to attract mates. The male's antennae are so sensitive to bymbykol that a female has simply to emit a small quantity of the substance and sit tight to secure a reproductive partner. The male needs a mere 200 molecules to strike his antennae within a second for him to be able to orient himself toward the waiting female and home in on her.

In crustaceans and mammals, sex pheromones also indicate the female’s availability for breeding. During these times of female fertility, dominant male kangaroos or African elephants, may respond to these chemical cues, often advertised through the urine, by attempting to monopolize access to the female through precopulatory mate guarding.

The emission of sex pheromones is not limited to females, however; males secrete pheromones that play a role in mate choice and sexual competition. A particular pheromone might indicate sexually desirable traits to a selecting female. For example, female tobacco moths demonstrate a preference for large males, which produce more than twice as much wing-gland pheromone as smaller ones.

Male animals also emit pheromones that convey information about their species genotype as a mechanism to avoid inbreeding with kin. Female mice, for example, 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. The same receptors that can be used to avoid mating with kin can also be used for recognizing and cooperating with kin. In this case, hormonal changes during pregnancy may draw mice to individuals with the most similar pheromones because they want to keep family close by to aid with the raising of their young and to take advantage of protection (Wyatt, 2003).

Animals that participate in external fertilization, such as marine worms and sea urchins, use pheromones to precisely synchronize their release of gametes (the egg and sperm).

Alarm pheromones

After sex pheromones, alarm pheromones are the most commonly produced class of chemical signals in social insects ( a group that includes ants, bees, and some species of wasp and termite), and alarm hormones have evolved independently within all major taxa. This altruistic signaling probably developed as a means of warning kin of danger, but the benefits may extend to unrelated members of the species.

Black-tailed deer use a variety of cues, including the release of alarm pheromones, to warn other deer of the species of a nearby predator.

Some species release a volatile substance when attacked by a predator that can trigger a flight response (in aphids, for example) or aggressive behavior (in bees) in members of the same species. When alarmed or pursued, black-tailed deer release a strong garlic-like odor as part of a larger system of signaling danger, which also includes visual and aural cues. The European minnow (phoxinus phoxinus) releases an alarm pheromone stored in specialized cells on its skin only when the skin is damaged, warning other minnows that a predator has attacked.

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. (confirm)

Aggregation pheromones

In contrast to sex hormones, aggregation pheromones attract individuals of both sexes. These hormones lead to the formation of animal groups near the pheromone source, and can be used as cues for settlement or as a signal to cluster together for defense. An example of the former: thousands of bark beetles can be attracted to a suitable tree within the hour upon the release of pheromones by pioneer beetles at the site.

Recruitment signals

Recruitment pheromones are common in social insects, which use them for a variety of tasks related to coordinating the activities of the group.

For example, the Nasonov (alternatively, Nasanov) pheromone is released by worker bees to orient returning forager bees back to the colony as well as to recruit other workers outside the hive. To broadcast this scent, bees raise their abdomens, which contain the Nasonov glands, and fan their wings vigorously.

Foragers, such as ants, mark their paths with trail 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 wand serves as a guide. (ref, year) As long as the food source remains, the pheromone trail will be continually renewed. The pheromone must be continually renewed because it degrades 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 (ref, year).

Recognition mechanisms

In the most complex animal societies (those of social insects and mammals), semiochemical signals function in societal action, including the chemical "signatures" associated with an individual. The ‘’saddleback tamarin’’, a South American primate, produces chemical signals that identify species, subspecies, individual, and gender, and may also contain information on social status.

Colony and kin recognition are central to social behavior. In social insects, eusociality is the phenomenon of reproductive specialization found in some animals. It generally involves the production of sterile members of the species, which carry out specialized tasks, effectively caring for the reproductive members. It most commonly manifests in the appearance of individuals within a group whose behavior (and sometimes anatomy) is modified for group defense, including self-sacrifice ("altruism").

Scent-marking and territorial pheromones

Scent-marking pheromones mark the boundaries of an organism's territory; they are particularly important in the territorial behavior of mammals and other terrestrial vertebrates. In dogs, a well-known-example, these pheromones are present in the urine, which they deposit on landmarks serving to mark the perimeter of the claimed territory.

Host-marking pheromones

Many species of parasitic insects leave a pheromone mark on or inside of the host, such as a small fruit or a caterpillar, after laying an egg inside the host. Other females of the species usually avoid laying eggs in these marked hosts, which are of limited size and can only successfully support the development of a limited number of larvae.

Intercepted signals: the role of pheromones in interactions between species

’’Sesiidae’’ (clearwing moth) on a pheromone trap.

The signals given by pheromones can be intercepted or replicated by other species. Spiders “eavesdrop” on the alarm pheromone emitted by fighting ants, drawing them to their prey. Other species actively produce chemicals that mimic the pheromones of their prey; for example, bolas spiders produce moth sex pheromones to lure male moths within striking distance. Sometimes the communication can involve three species: researchers have noted certain plant species’ use of semiochemicals to attract invertebrate predators when under attack by insects or mites. Not all relationships are exploitative, however: some mutually beneficial relationships also involve chemical cues.

Human knowledge of pheromones can also be applied in our interaction with other species, most notably in the development of more environmentally safe pesticides. Insect pheromones of species deemed pests, 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 patterns, and preventing them from laying eggs.

Pheromones are also used in managing the reproduction of farm animals. Farmers may use pheromones to detect estrus in sows: boar pheromones are sprayed into the sty, and those sows that exhibit sexual arousal are known to be currently available for breeding.

The case for human pheromones

It is likely that odors are an important means of communication for humans, given the significance of pheromones in the behavior of many other mammal species. As yet, however, no defined pheromonal substance has been demonstrated to be a direct influence on human behavior in a peer reviewed, published study. A few well-controlled scientific studies have been published suggesting the possible action of pheromones in humans:

• 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, in which a pheromone produced by male mice induce estrus in adult females (Whitten, 1957; Gangrade, Dominic, 1984).
• Other studies have suggested that humans might use 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. According to the researchers, this finding suggests a possible role for human pheromones in the biological basis of sexual orientation (Wade, 2005).
• 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 signal (Wyart et al, 2007).
• In 2006, it was shown that a second mouse receptor sub-class is located in the olfactory epithelium. Some of these molecules, called trace amine-associated receptors (TAAR), 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 (Liberles and Buck, 2006; Pearson, 2006).

References

ISBN links support NWE through referral fees

• Barnard, C. 2004. Animal Behaviour: Mechanism, Development, Function and Evolution. Harlow, England: Pearson/Prentice Hall. ISBN.
• Gangrade, B.K and C.J. Dominic. 1984. Studies of the male-originating pheromones involved in the Whitten effect and Bruce effect in mice. Biol Reprod 31(1):89-96.[1]
• Karlson, P. and M. Lüscher. 1959. Pheromones: a new term for a class of biologically active substances. Nature 183:55-6.
• Pearson, H. 2006. Mouse data hint at human pheromones. Nature. 442(7102):495. [2]
• Wade, N. "Gay Men are found to have Different Scent of Attraction." New York Times, Retrieved on date.
• Whitten, M.K. 1957. Effect of exteroceptive factors on the oestrous cycle of mice. Nature 180(4599):1436. [3]
• Wyart, C., Webster, W.W., Chen, J.H, Wilson, S.R, McClary, A., Khan, R.M., and N. Sobel. 2007. Smelling a single component of male sweat alters levels of cortisol in women. J Neurosci 27(6):1261-5. [4]
• Wyatt, T.D. 2003. Pheromones and Animal Behavior. Cambridge, England: Cambridge University Press. ISBN 0-521-48526-6.
• {{cite web |url=http://www.news.cornell.edu/releases/Feb98/antpheromone.hrs.html | title=Excited ants follow pheromone trail of same chemical they will use to paralyze their prey Retrieved on March 14, 2006.
• {{cite web | url=http://animal.discovery.com/news/afp/20051128/ants.html Ants Use Scents Like Road Signs. Retrieved on March 14, 2006.

Further reading

• Kohl, J.V., Atzmueller, M., Fink, B. and K. Grammer. 2001. Human Pheromones: Integrating Neuroendocrinology and Ethology. Neuroendocrinology Letters 22(5), 319-31. Full text
• Liberles, S.D. and L.B. Buck. 2006. A second class of chemosensory receptors in the olfactory epithelium. Nature 442:645-50.
• McClintock, M.K. 1984. Estrous synchrony: modulation of ovarian cycle length by female pheromones. Physiological Behavior 32: 701-5.
• Wilson, E. O. and W.H. Bossert. 1963. Chemical communication among animals. Recent Progress in Hormone Research 19: 673-716.

External links

• Pherobase, the database of insect pheromones
• SFSU study shows that synthetic pheromones in women's perfume increase intimate contact with men
• Male Axillary Extracts Contain Pheromones that Affect Pulsatile Secretion of Luteinizing Hormone and Mood in Women Recipients
• Sexual Orientation, in the Brain
• Review of published debate about HOW insects detect pheromones over large distances, even when the wind seems unfavourable
• Male sweat boosts women's hormone levels — from UC Berkeley, February 2007
• The Effect of Male Sweat on Women's Hormone Levels — from Science Daily, February 2007
• For women, nothing's like the smell of men's sweat — from Reuters (February 2007)
• Pheromones In Male Perspiration Reduce Women's Tension, Alter Hormone Response — from Science Daily (March 2003)

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