The capabilities of a chemical communication system are dictated by the physical and chemical properties of the compounds used. Signals, which delineate boundaries of territories, may be stable and nonvolatile whereas compounds employed for a short-term warning of danger may be chemically unstable and extremely volatile. Furthermore, communication under water requires water-soluble signals, whereas terrestrial organisms rather use volatile compounds or compounds which can be attached to small particles that travel in agitated air and transport the actual signaling component. In this context, the term volatility is frequently used to describe a substance's propensity to evaporate. The volatility depends strongly on the molecular weight, polarity, and general structure of the compound, which can be summarized as its vapor pressure (Figure 2 a). Volatile compounds (i.e., with a high vapor pressure) have low molecular weights, a low polarity, and certain functional groups such as esters or lactones.
On the other hand, smaller molecules allow only a decreased structural diversity to be realized. Increasing the number of C atoms leads to an approximately exponential increase in the number of possible hydrocarbons, which can be generated by permuting site chains of variable lengths within the carbon skeleton or introducing functional groups (e.g., alcohol, aldehyde, keto groups). Even stereoisomers, optical isomers, and geometric isomers can occasionally be distinguished by the receiver, thus further extending the list of potential signaling compounds. Although general rules determine the physical and chemical properties of a compound, different organisms may use completely different molecules to convey qualitatively similar messages. There is no way to predict a priori which kind of compound will be used by a particular species to carry a message.
Organisms that use the chemical channel to communicate face a signal-to-noise problem. Chemical noise is always present in the environment as originating from other signal emitters, decaying material, geochemical processes, and so on. A signal that is involved in chemical communication has to emerge out of this chemical background to be detectable for the receiver. Sufficient contrast to the environmental background can be generated by producing unique compounds (Figures 2 c and 2d) or a mixture of compounds (Figure 2b). Blends of molecules can also contain ordinary compounds, yet are often emitted in a specific relative composition with slight changes, sometimes completely abolishing the biological activity.
The ability of a species to employ a certain chemical channel successfully depends strongly on the biosynthetic equipment of the sender as well as the sensory armamentarium of the receiver. Hence, especially closely related species compete for the limited number of compounds they may produce and/or recognize. This is reflected, for example, in the taxonomic distribution of lepidopteran sex pheromones. Here it could be shown that on average, taxa with larger numbers of species use more components per blend than smaller taxa and have relatively fewer different kinds of components per species compared to smaller taxa. Competition for communication channels is avoided by adding additional compounds to a blend or slightly changing different ratios of these chemicals (Figure 2b).
Another solution to overcome the signal-to-noise problem is a temporal change in the emission pattern of the signal by modulating its sequence and duration. Cellular slime molds (Dyctiostelium discoideum), for example, use temporal gradients to organize their aggregation behavior. By emitting the chemoattractant cAMP (cyclic adenosine 3',5'-monophosphate) in pulse waves with a 7 min period, translocation of cells is mediated even in the absence of a spatial gradient. Temporal changes in the emission pattern of a chemical signal can also help partition the chemical channel among closely related species. Nonoverlapping daily mating rhythms appear to be the primary barriers to cross-attraction in three saturniid moths in South Carolina: Callosamia promethea is active from 10:00 to 16:00 h, C. securifera from the late afternoon until dusk, and C. angulifera from dusk till about midnight.
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