Communication, broadly defined, is the transfer of information from one individual (signal emitter) to another (signal receiver). The signals used to transmit the information through the surrounding environment involve multiple sensory modalities. Based on the sensory capabilities of the receiver, the medium, which is used to convey information, and the distance, which has to be covered, signals can be considered to fall into three main categories.
The first group covers acoustic or vibrational signals which are transmitted through a medium such as air, water, or solid materials over short (e.g., vibration signals organize labor in honeybee colonies), medium (e.g., airborne songs of birds and frogs), and very long distances (e.g., infrasound communication of whales and elephants). The second type of signals involves a variety of visual signals such as the bright plumage coloration of birds, aposematic coloration of toxic insects and reptiles, and bioluminescence used by deep-sea fish to attract prey. Despite their striking appearance, these signals can only be received over relatively short distances. The third group of signals that may be used to transfer information are chemical or olfactory signals. Deploying chemical cues for intra- and interspecific communication is a very general principle that is used by nearly all taxonomic groups from microorganisms to primates in both terrestrial and aquatic habitats. A great structural diversity of chemical compounds is used for social organization, sexual attraction, or warning predators. Similarly to vibrational or acoustic signals, olfactory cues can act very locally (e.g., urine marking of mammals) or travel considerable distances (e.g., moth pheromones). Besides these three main groups of signals, some fish species use electric pulses for social communication.
Many animals use a combination of multiple signals, probably to increase efficiency and reliability of the signal, and to hamper unwanted eavesdropping by third parties (e.g., predators). Such multiple signals can either be perceived by a single sensory modality or be produced by multiple sensory modalities. Many aposematic species, for example, combine multiple signals such as odors, colors, and a certain behavior to warn predators that they contain toxic compounds, thereby increasing their chance of survival.
The focus of this article is on inter- and intraspecific communication through chemical signals. Besides presenting an overview over the structural diversity of the chemical compounds involved, the multifaceted interactions that are mediated by these compounds are elucidated, starting with an introduction into the relevant terminology.
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