Signalling and Reception Secondary article Leena Lindstro¨m, University of Jyva¨skyla¨, Jyva¨skyla¨, Finland Article Contents Janne S Kotiaho, University of Jyva¨skyla¨, Jyva¨skyla¨, Finland . Introduction . What is Communication? Communication, a widespread natural phenomenon, occurs in both animals and plants. Honesty of Signals Signals are evolved traits that transfer information from one individual (the signaller) to . Signal Design and Receiver Psychology another (the receiver); they can occur in any sensory modality. Signalling in Plants Introduction transmitted through a surrounding environment, and Communication is a widespread phenomenon in the received by a receiver. natural world, occurring not only in animals, but also in Signals can be either directed at conspecifics or at plants. Signals are traits that have evolved specifically to members of other species. Intraspecific signals (i.e. those transfer information from one individual (the signaller) to occurring between individuals in the same species) can be another (the signal receiver). They can occur in any sensory used to attract a mate, to deter rivals, to maintain social modality, and some signals are even sent in several sensory grouping, or to warn kin of approaching danger. Signals channels simultaneously. Signals have evolved to provide aimed at individuals of another species are often anti- useful information for receivers; a signaller provides this predator signals, for example, alarm calls, warning signals information in an attempt to manipulate a receiver’s and mimicry. Signals produced by plants are often aimed at behaviour to its own advantage. This conflict of interest animal receivers; for example, flowers attract insects or between signaller and receiver means that to be mutually birds to pollinate them, although, as we shall see, there is beneficial signals need to be honest, although under certain some evidence of communication between plants. How- circumstances, deceptive signals can evolve. The sensory ever, first we will consider animal communication in more and psychological capabilities of signal receivers have also detail. influenced signal design, and the signals that we see today are the product of selective pressures on both signal How do animals communicate? content and how efficiently that information can be transferred. Animals have evolved an incredibly diverse array of traits that they use for communication. Based on the sensory capabilities of the signal receivers, signals can be con- sidered to fall into three main categories: acoustic or What is Communication? vibrational signals; olfactory or chemical signals; and visual signals. In addition, electric signals are found in Communication is the process whereby individuals send some fish species that emit pulses from discrete electric and receive information about each other and their organs derived from modified muscle (Andersson, 1994). surroundings. Communication is achieved through the Acoustic signals, as well as all other vibrational signals, use of signals, traits that have specially evolved to transfer are mechanical stimuli that are transmitted through a information between one individual (the signaller) to medium such as air, water or a variety of solid materials. another (the signal receiver). Because signals have evolved Depending on the medium in which the signal is sent, these to transmit information, this distinguishes communication signals may be used for very short range communication from simple information acquisition. Psychologists often (e.g. plant stem vibrations produced by tropical wandering use the term ‘signal’ to describe any stimulus in an animal’s spiders), medium range communication (e.g. airborne bird environment that might alter its behaviour. For example, a song) and very long range communication (e.g. underwater tone might be a cue from which an animal learns to predict sounds and songs of whales). the arrival of food and move towards a food dispenser, but Olfactory and other chemical signals are transmitted the sound is not a signal in this evolutionary sense as it is an through a medium by diffusion, which can be a slow arbitrary cue designed by the experimenter. In addition to process if the viscosity of the medium is very high. communication between individuals, communication can Olfactory signalling is commonly used by mammals, where occur at other levels of cellular organization within it plays a potent role in their social organization. Olfactory individuals, for example in cell to cell signalling. However, communication is also widespread among insects: many here we focus only on those signals that are transmitted insects produce pheromones, a class of species-specific between individual animals and plants. These can be chemical compounds or molecules that are produced to characterized by the signals being produced by a signaller, communicate between members of the same species. In ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net 1 Signalling and Reception addition, the use of scents or pheromones is not warn relatives of the presence of a predator. Interspecific uncommon even among birds, fish, amphibians and signals are perhaps less common, with signals that are reptiles. As with vibrational signals, olfactory signals can aimed at predators being the most well-studied. be very short range, localized signals (e.g. urine marking of many mammals) or can travel considerable distances (e.g. Sexual signalling moth pheromones). The purpose of sexual signals is either to attract mates of In addition to sounds and odours, animals produce a the opposite sex or to deter rivals of the same sex. variety of visual signals. These traits are usually extremely Ornaments are morphological traits that are used for mate striking (e.g. the bright plumage coloration of many birds), attraction and include the plumage and feather ornaments and often take conspicuous shapes and forms (e.g. the of many birds, the colour markings of some lizards and eyespots and enlarged fins of some fish). In addition to fish, and the leg tufts of certain species of spiders these morphological traits, some visual displays make use (Andersson, 1994). Traits produced in other sensory of foreign artefacts and construction. For example, modalities can also attract mates: bird song and the calls tropical bower birds, such as the Satin bower bird of crickets and frogs are examples of acoustic signals that (Ptinolorhynchus violaceus), adorn their bowers with animals use, and olfactory signals are also common, colourful objects that attract females. Male satin bower especially in insects. Other morphological traits and signals birds collect blue objects, ranging from flowers and bottle evolve in response to competition between members of one caps to colourful glass, with which to decorate their sex for access to mates in the other. These are used in bowers. Movement is often used in visual displays, perhaps contests between individuals where weaponry and threat to increase the detectability of morphological traits, as signals are produced to deter rivals. Perhaps the most these can only be received over relatively short distances. striking examples of these are traits that are used as This combination of movement with visual signals may be weapons, such as the antlers of many ungulates and the regarded as a behavioural display, which often incorpo- horns of some beetle species. rates signals from other sensory modalities. Social signalling Multicomponent signalling Social signals are used by conspecifics living in tight Many animals use complex signals that combine multiple groups. They can reveal the dominance and hierarchical components. Multicomponent signals may have evolved status of a particular individual within the group. Social because they allow more effective information transfer, or signals can also be cooperative when animals transfer convey more information than single component signals. information that benefits both signaller and receiver; for These signals may be potentially more reliable, or more example, small passerine birds use alarm calls to inform difficult to ‘eavesdrop’ (see below). Multicomponent individuals within the same flock of an approaching signals can be either unimodal, i.e. they are perceived in predator. Alarm calls benefit receivers as it provides them only a single sensory modality by the receiver, or they can with an opportunity to flee, but also protects the signaller be multimodal and produced in more than one sensory as it is less likely to be attacked in the flock than if it were to modality. For example, ladybirds (Adelia bipunctata) are flee on its own. aposematic, in that they taste unpleasant to birds and they signal this unpleasantness to birds using conspicuous Warning signalling colouring but also by releasing a strong pyrazine odour Not all communication occurs between conspecifics; when attacked. Birds learn to associate these multiple signals can also be directed at receivers in other species, signals with unpalatability, and association is most such as predators. Usually the function of signals that are effective when both the colour and the odour are present directed towards predators is to escape predation. Warn- (Marples et al., 1994). Many sexual signals also have ing signals, or aposematic signals, inform the predator that multiple components, and signallers often incorporate there is a cost of attacking the signaller. Usually this cost is movement in a vivid behavioural display. that the prey is