Communication is only one behavior in an animal's repertoire and whether or not to communicate at any instant is influenced by many factors outside the scope of this article. However, once the decision to communicate has been made, we would expect communication behavior to be as effective as possible. Both the physical environment and the social environment can have major influences on the effectiveness of communication; these are manifested as decisions concerning when and where a signal is broadcast and in the structure of the signal. A complication is that both types of environment change over a range of timescales and often vary cyclically. For example, vegetation changes over an annual cycle in temperate terrestrial habitats whereas the activities of other signalers can change from signal to signal. A limitation on our understanding of such matters is that signaling is a much more conspicuous behavior than receiving, and therefore most of the information on when and where to communicate relates to signaling behavior.
In this section, we will consider the physical environment first because generally its effects are less complex and change less frequently than the effects of the social environment. However, it is important to remember that both physical and social environment contribute to the variation in communication behavior and signal structure that we observe.
All habitats modify signals during transmission. Signals are both attenuated and degraded, that is, signal energy is absorbed and signal details are distorted by the environment. Often degradation will limit the effective range of a signal before attenuation - it is a common personal experience to know that someone at a distance has said something, but to be unable to understand the words; the signal has been detected but the detailed information it contains cannot be discerned. Habitats can be thought of as imposing selection pressures on the form of signals and as a result most signals are adapted to some degree to their transmission environment. For example, birds found in open habitats (e.g., grassland) tend to sing songs with rapid temporal patterning (the songs contain more trills than whistles) and with a wide frequency range. By contrast, woodland bird songs tend to have more low frequency whistles than high frequency trills. This difference contributes to minimizing the effect of the habitat on signals during transmission; trills suffer less degradation from the irregular changes in air temperature and wind speed characteristic of open habitat, while low frequency whistles suffer less degradation induced by reflections from leaves and trunks in woodland.
Transmission conditions also change within a habitat on daily and seasonal timescales; for example, the effects on visual and acoustic signals of a deciduous forest in winter are very different from those in the same forest in summer. Such changes often affect to a greater extent when and where signals are produced than they affect signal structure.
Effects of the Physical Environment on When and Where to Communicate
The most effective time to signal is often related to cyclical changes in the physical environment. An example on a seasonal timescale is communication underlying breeding behavior; breeding is seasonal (e.g., spring in temperate zones and the rainy season in equatorial regions) and so are the long-range mate attraction and territory defense signals such as birdsong and the choruses of frogs and insects. Cycles occurring over shorter time-scales are also accompanied by appropriate signaling behavior. Circalunar and tidal cycles are common in aquatic or intertidal environments and animals such as fiddler crabs produce their claw-waving visual signal at low tide. Daily (circadian) cycles give rise to choruses of bird song at dawn and of frogs and fireflies at dusk. It seems reasonable to expect cycles of receiver activity to coincide with those of signalers. However, it is possible that the amount of information available during periods of very high signaling activity (e.g., intense chorusing) could exceed receivers' processing abilities, with the result that receivers may avoid such periods.
The most effective place to signal from is affected by several factors including transmission conditions and the area to be encompassed by the signal. Environmental factors affecting signal transmission often vary from location to location within habitats. For example, studies that broadcast bird song and record it at different locations to investigate location effects on transmission have shown that the best perch height for maximizing song transmission (i.e., where signalers should be found) is lower than that which favors sound reception (where receivers should be found). As many visual signals rely on reflected light (see above), it is not surprising that signaler location is strongly affected by the incident light regimes in terrestrial and aquatic environments. Some species take things further and alter the properties of the signaling location to enhance signal transmission. For example, mole crickets excavate a calling burrow in an exponential horn shape to amplify their call and some tree crickets create acoustic baffles from leaves to modify the spreading pattern of their call. Signaling from a single location can be less effective than signaling from several locations, often because the signal does not encompass the required area. When several signaling locations are used, the signaler faces a series of decisions including how to apportion effort between locations and the interval before returning to each location. Often solutions to these issues based on a priori considerations will be overridden by the activity of adjacent signalers and receivers, that is, the social environment can modify and often override effects of the physical environment.
The most noticeable effects of the social environment on signal form occur when other signalers and receivers have adverse influences that cannot be avoided by ceasing signaling or signaling from another location. One example involves the species specificity or species distinctiveness of signals and is a form of character displacement. If the signal characteristics ofsympatric species overlap then we would expect selection for the signals to diverge because communicating with the appropriate species is important in many contexts. Character displacement is commonly assumed to be the cause of a lack of overlap of signal characteristics between species in sympatry when compared with the same species in allopatry.
A common influence of the social environment is an aspect ofcommunication networks, that is, the presence of receivers in addition to the intended receivers (Figure 1). These additional receivers can intercept signals intended for others, usually at a cost to the signaler. Often such interceptive eavesdroppers are predators or parasites that use broadcast signals to locate prey and hosts. Examples are predatory bats that feed on calling male frogs and flies that use calls to locate frog hosts for their parasitic larvae. Signalers rarely call in the presence of such eavesdroppers (i.e., eavesdroppers affect decisions on when and where to communicate; see below), but there are examples in which the form of the signal reduces the opportunity for interceptive eavesdropping. The clearest examples are signals that warn of predators because such warning calls must be given in a predator's presence and could be intercepted by the predator. For example, the warning call (seeet) given by some northern temperate species to an aerial predator has a frequency of about 8 kHz, to which the calling species is more sensitive than the eavesdropping predator.
Interceptive eavesdroppers may also be the same species as the signaler and therefore potential competitors and rivals of the signaler. For example, a male fiddler crab reacts by producing courtship waving signals when he intercepts the courtship signals produced by a male neighbor in response to the female, even though he cannot see an approaching female, and in doing so he becomes a competing suitor. During close-range, high-intensity courtship or aggressive interactions, some song birds change from a far-carrying advertising song to a form of song that carries less far as a consequence of its lower amplitude and greater high-frequency content. Such a change in signal form should restrict the potential for eavesdropping at times when the signaler can suffer high costs.
Effects of the Social Environment on When and Where to Communicate
The most readily observed effect of the social environment on communication is the avoidance of adverse influences (e.g., risk of predation, interference from other signalers) by ceasing signaling or signaling from another location. Interference from other signalers of the same species is particularly severe in high-density communication networks (e.g., breeding colonies, chorus aggregations) and can result in changes in communication behavior. For example, chorusing male frogs have less success attracting a female when another calling male is close by. In response to such interference males may vary the timing of their calls both in relation to close neighbors and the overall synchrony of the chorus. As males may fight to displace others calling from close by and as the larger male usually wins, small males often move away when a large male begins calling near them. It is likely that similar considerations apply to receivers.
Signaling interactions between others are one aspect of the social environment in which eavesdropping would be favored because information on relative aspects of the signalers is readily available from such interactions. In such social eavesdropping (cf. interceptive eavesdropping, above), the eavesdropper does not take part in the signaling interaction (Figure 1). Social eavesdropping by both sexes has been demonstrated in laboratory experiments on fish using visual signals and in field experiments with territorial songbirds: males and females eavesdropped on aggressive signaling interactions between other males, and females eavesdropped on malefemale courtship interactions. There are indications that receivers position themselves in space and time to facilitate social eavesdropping. The specific circumstances of an interaction and the individuals involved will determine whether the signalers involved will gain or lose from the presence of social eavesdroppers (e.g., the winner of an aggressive interaction may gain while the loser may not) and therefore how the eavesdroppers will affect when and where the interaction occurs.
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