Carl W Hoagstrom. Salem Press Encyclopedia of Science, 2022.
Pheromones are chemical signals. The term pheromone designates any chemical or chemical mixture that, when released by one member of any species, affects the physiology and/or behavior of another member of the same species.
Pheromones are semiochemicals that carry information between members of a single species. To do so, the pheromone must be released into the atmosphere or placed on some structure in the organism’s environment. It is thus made available to other members of the species for interpretation and response. Semiochemicals that affect the physiology or behavior of members of a different species are called allelochemicals and include allomones (if they benefit the emitting animal), kairomones (if they benefit the receiving animal), or synomones (if they benefit both the emitting and receiving animal).
For example, klipspringer antelope mark vegetation in their environment with a chemical secreted from a special gland . Other klipspringer investigate the marks to gather information on the marking individual. Ticks that parasitize the klipspringer, however, are also attracted to the chemical marks and thus increase their chance of attaching to a host when the mark is renewed or when another klipspringer investigates it. The tick is using the semiochemical as a kairomone.
To appreciate fully the complexity of the interactions under consideration, it is important to remember that a pheromones only carry information between members of the same species. There are two general types of pheromone: those that elicit an immediate and predictable behavioral response, called releaser or signal pheromones, and those that bring about a less obvious physiological response, called primer pheromones (because they prime the system for a possible behavioral response). Pheromones are also categorized according to the messages they carry. There are trail, marker, aggregation, attractant, repellant, arrestant (deterrent), stimulant, alarm, and sex pheromones. Their functions are suggested by the terms used to name them.
Pheromone Composition and Function
The chemical compounds that act as pheromones are numerous and diverse. Most are lipids or chemical relatives of the lipids, including many steroids . Even a single pheromonal message may require a number of different compounds, each present in the proper proportion, so that the active pheromone is actually a mixture of chemical compounds.
Different physical and chemical characteristics are required for pheromones with different functions. Attractant pheromones must generally be volatile to permit atmospheric dispersal to their targets. Many female insects emit sex-attractant pheromones to advertise their readiness to mate. The more widely these can be dispersed, the more males the advertisement will reach. On the other hand, many marking pheromones need not be especially volatile because they are placed at stations that are checked periodically by the target individuals. The klipspringer marking pheromone is an example. Some pheromones are exchanged by direct contact, and these need not have any appreciable volatile component. Many mammals rub, lick, and otherwise contact one another in social contexts and exchange pheromones at these times.
Specificity also varies for pheromones with different functions. Sex attractants usually need to be very specific, directed only to members of the opposite sex. Alarm pheromones, on the other hand, need not be so specific. These pheromones simply alert other members of the same species to a disturbance. It is usually harmless, and sometimes even helpful, to alert members of other species as well by emitting an allelochemical alert. In keeping with this argument, related groups of ant species produce species-specific sex-attractant pheromones: Each female attracts only males of its own species. In contrast, alarm allelochemical of any species in the group will stimulate defensive reactions in individuals in many of the species.
Pheromone Sources and Receptors
The sources of pheromones are also diverse. Some pheromones are produced by specialized glands; many insect species have glands specialized for the production of pheromones. One example is the harvester ant’s alarm pheromone, which is produced in the mandibular gland at the base of the jaws. Other pheromones seem to be by-products of other bodily functions. The lipids of mammalian skin are probably primarily important in waterproofing and in maintaining the outer layer of the skin, but many also function as pheromones.
The reproductive tract is an important source of pheromones in many species. These usually act as sex-attractant or sex-stimulant pheromones or as signal pheromones that give information on the sexual state of the emitter. The urine and feces of many species also contain pheromones used to mark territory boundaries and to transmit other information about the marking individual. Many pheromones seem to be produced not by the sending organism alone but by microorganisms living on the skin or in the glands or cavities of the sender’s body. These microbes convert products of their host into the actual signal molecules, or pheromones, used by the host.
The receptors for pheromones are also of many different types, and the chemical receptors for taste and smell are often involved. In vertebrates, the vomeronasal organ (Jacobson’s organ) seems to be an important receptor for many pheromones. It is a pouch off the mouth or nasal passages, and it contains receptors similar to those for smell. It is nonfunctional in humans, but it functions in more primitive mammals and seems to be of great importance to snakes and other reptiles. Insects and other invertebrates have many specialized structures for receiving pheromonal messages. Perhaps the best-known example is the feathery antennae of many male moths, which are receptors for the female moth’s sex-attractant pheromone. Some pheromones seem to be absorbed through the skin or internal body linings and to bring about their effects by attaching to some unknown internal receptor.
Pheromones are widespread in nature. Most are poorly understood. The best-known are those found in insects, partly because of their potential use in the control of pest populations and partly because the relative simplicity of insect behavior allowed for rapid progress in the identification of pheromones and their actions. Despite these advantages, much remains to be learned even about insect pheromones. Mammalian pheromones are not as well known, although they may also be of economic importance. The more complex behavior of mammals makes the study of their responses to pheromones much more difficult.
Behavioral and Chemical Research
Both behavioral and chemical techniques are required in order to study pheromones and other semiochemicals. The observation of behavior, either in nature or in captivity, often suggests pheromonal functions. These hypothesized functions are then tested by presenting the pheromone to a potentially responsive organism and observing the response. Situations may be arranged that demand the subject’s response to a particular pheromone under otherwise natural conditions. Alternatively, the organisms may be observed in enclosures to help control the experimental context. The presentation of the hypothetical pheromone may be in the form of another organism of the same species or some structure to which the presumed pheromone has been applied. The observed response (or lack of response) gives information on the status of the presented chemical as a pheromone in that behavioral context.
While the pheromonal function of secretions from a gland or other source can be determined from these behavioral tests, the tests can give information on specific chemical compounds only if the compounds can be isolated and identified. The isolation and identification of pheromonal compounds are challenging because of the great complexity of the secretions in which they are found and the exceptionally small amounts that are required to elicit a response. Many separation and identification techniques are used. One of the most powerful is a combination of gas chromatography and mass spectrometry.
Gas chromatography is used to separate and sometimes to identify chemicals that are volatile or can be made volatile. The unknown chemical is mixed with an inert gas, called the mobile phase of the gas chromatography system. This mixture is passed through a tube containing a solid, called the stationary phase. The inert gas does not interact with the solid; however, many of the compounds mixed with it do, each to an extent determined by the characteristics of the compound and the characteristics of the stationary phase. Some members of the mixture will interact very strongly with the solid and so move slowly through the tube, whereas others may not interact with the solid at all and so pass through rapidly. Other members of the mixture interact at intermediate strengths and so spend intermediate amounts of time in the tube. The different compounds are recorded and collected separately as they exit from the tube.
For identification, the compounds are often passed on to a mass spectrometer. In mass spectrometry the compound is broken up into electrically charged particles. The particles are then separated according to their mass-to-charge ratio, and the relative number of particles of each mass-to-charge ratio is recorded and plotted. The original compound can usually be identified by the pattern produced under the specific conditions used. After separation and identification, the individual chemicals may be subjected to behavioral studies.
Uses for Pheromones
Pheromones and other semiochemicals are of interest simply from the standpoint of understanding communication between living things. In addition, they have the potential to provide effective, safe agents for pest control. The possibilities include sex-attractant pheromones to draw pest insects of a particular species to a trap (or to confuse the males and keep them from finding females) and repellant pheromones to drive a species of insect away from a valuable crop species. One reason for the enthusiasm generated by pheromones in this role is their specificity. Whereas insecticides generally kill valuable insects as well as pests, pheromones will be specific.
These chemicals were presented as a panacea for insect and other pest problems in the 1970s, but most actual attempts to control pest populations failed. Many people in the field have suggested that lack of understanding of a particular pest and its ecological context was the most common cause of failure. They maintain that pest-control applications must be made with extensive knowledge and careful consideration of pest characteristics and the ecological system. In this context, pheromones have become a part of integrated pest management (IPM) strategies, in which they are used along with the pest’s parasites and predators, resistant crop varieties, insecticides, and other weapons to control pests. In this role, pheromones have shown great promise.
Some consideration has been given to the control of mammalian pests with pheromones, though this field is not as well developed as that of insect control. Pheromonal control of mammalian reproduction has received considerable attention for other reasons: Domestic mammals are of great economic importance, and many wild mammalian species are endangered to the point that captive breeding has been attempted. The manipulation of reproductive pheromones may be used to enhance reproductive potentials in both cases. The complexity of mammalian behavioral and reproductive systems, however, and the subtle changes brought about by mammalian pheromones present a particular challenge. As with insect pest control, the key to progress is a complete understanding of the entire system being manipulated.
Pheromones and other semiochemicals are of great potential economic importance as substitutes for or adjuncts to toxic pesticides in pest management. Mammalian reproductive pheromones are being explored as tools to enhance reproductive efficiency in domestic and endangered mammals. A complete understanding of the complex roles of pheromones in each of the systems being managed is necessary for success in all these endeavors.
Principal Terms
Allelochemic: a general term for a chemical used as a messenger between members of different species; allomones and kairomones are allelochemics, but hormones and pheromones are not.
Allomone: a chemical messenger that passes information between members of different species, resulting in an advantage to the sender.
Kairomone: a chemical messenger that passes information between members of different species, resulting in an advantage to the receiver.
Primer pheromone: a chemical that generates an often subtle physiological response in another member of the same species; a behavioral response may follow.
Semiochemical: a chemical messenger that carries information between individual organisms of the same species or of different species; pheromones and allelochemics are semiochemicals, but hormones are not.
Volatiles: chemical compounds that are vapor or gas at environmental temperatures or that readily release many of their molecules to the vapor phase.