Social behaviour commonly includes communication, which may be defined as the sending of a message through a medium to a receiver so as to change the status and perhaps the behaviour of the receiver. In general, communication is employed by animals to attract or repel other individuals of particular groups and to establish and maintain distinct forms of social organization that range from relatively simple pair and family bonds to the highly structured troops of some primates and the complex colonies of social insects.
The information involved in animal communication can come from many sources; any facet of the environment perceived is considered information. In linguistic communication the primary function of words is to convey information. Similarly, animals including man have modes of behaviour that, in the course of evolution, were selected for their value in providing vehicles for conveying information. During the evolutionary process some of these vehicles also retained more direct functions, but many became specialized for a communicative function alone. These communicative acts, known as displays, include various posturings and movements; sounds; particular ways of making contact among individuals; the release of specialized chemicals called pheromones; and even electrical discharges. Displays have been studied as important means for transmitting information in animal communication. There are, of course, other information sources in animals, some of which have also undergone evolutionary specialization toward a communication function. Among them are what may be called badges — i.e., attributes that are merely structural and nonbehavioral in nature: the red breast of the robin, the red underside of the breeding male stickleback fish, and the mane of the male lion. Many other sources of information can be found in the repeated forms of interaction that develop during prolonged relationships between two individuals and in individual expectations about the nature of the roles in which they encounter others, both familiar associates and strangers. The activities of individuals who interact socially provide a constant and usually rich information source, but, in the study of nonhuman communication, the bulk of systematic research thus far has been directed toward displays and badges; it is, therefore, these highly specialized categories that are of the greatest concern here.
Because the complexity of social interactions makes experimental manipulation difficult, human understanding of the signalling in the social life of animals remains largely based upon inference. It is difficult to repeat an example many times with rigid control of all variables except the one being investigated, and attempts to structure the testing situation to simplify the form of interaction often obviate the interaction. Displays are universal among animals of any degree of structural complexity, however, so that they would not have been evolved and retained if they lacked important function. But the function of a display is likely to differ, depending upon the individuals involved. A small bird seeing an approaching hawk, for example, may utter a vocal display indicating the high probability that it (the communicator) is, or soon will be, engaged in an attempt to escape. Other small birds, upon hearing the vocalization, may seek cover immediately. Hence, the function of the vocalization is to give them a better opportunity to remain alive and not to increase the immediate chances of survival of the communicator — indeed, its chances for survival may slightly decrease. The display functions for the communicator in that it protects individuals whose continued existence provides a benefit to him greater than the cost of using the display. These individuals may be his offspring or associates whose similar responses to the environment will provide him future protection and, through their alertness in the future, make it possible for him to spend less time scanning his surroundings for predators.
From the ways and circumstances in which displays are used and from the apparent responses of recipients, it is possible to enumerate the general functions of animal communication. First, displays guide animals to one another, thereby enabling one to advertise its presence and behavioral predispositions to potential recipients. Displays enable individuals in a group to respond selectively to particular associates at appropriate times.
Second, communication permits animals to identify one another. Individuals can thus select information of importance to them — usually from members of their own species and often particular individuals. Special cases exist, however; members of different species that normally coexist in the same environment may attend each other’s signal. Thus, the maximum alarm communicated by one songbird when it discovers a falcon or hawk in its environment is attended by all other songbirds species in the area. In addition, by facilitating identification, communication acts at a premating level to help maintain reproductive isolation among species.
Third, communication reduces the amount of actual fighting and fleeing among animals, an excess of which could disrupt social encounters. In functionally aggressive encounters, such as territorial or dominance disputes, this reduction is achieved by threat displays that often lead to some form of capitulation by one opponent before fighting occurs. In less aggressive circumstances, communication enables animals to appease and reassure one another that each is not likely to be initially aggressive in his present state. Fourth, communication aids in synchronizing the behaviour of individuals who must come into appropriate physiological states in order to breed. This is necessary within pairs and, in some species, among whole colonies of pairs.
Fifth, displays enable individuals to use each other to monitor the environment, not only on a relatively long-term basis but also on a very immediate basis. Thus, in species that spend much of their time living in compact social groups, such as flocks, coteries, or troops, an indication by any one individual that it is fleeing precipitously — often a vocal display in addition to the flight itself — usually correlates with the presence of a dangerous predator and leads to evasion, hiding, or alertness on the part of the other members of the group.
Finally, communication facilitates the maintenance of special relationships between individuals by making available information about the readiness of each to engage in certain activities. The maintenance of individual relationships in cohesive groups is furthered by communication, which keeps members aware both of the behaviour of associates whom they may not be able to see and of the readiness of associates to change their activities. For example, vocal displays usually precede flight by a member of a resting family of geese, and the family then tends to depart as a unit. Within some types of relationship, display behaviour also aides in eliciting general classes of responses; for example, offspring usually signal to arouse various forms of care-giving behaviour from their parents.
The functions in which communication appears to be used vary considerably among different species; each has specialized features, some quite remarkable. It has been demonstrated, for example, that vocalizations and other sounds made during hatching by chicken-like birds influence the rate of hatching of sibling chicks, so that all members of the brood can leave the nest simultaneously. It has been suggested that birds migrating in flocks may use signals in order to inform each other of their position in the night sky, so that the individuals in the flock can perhaps compensate for small individual navigational errors.
One interesting aspect of birdsong is the occurrence of dialectal differences regional variations among populations of a single species living in different areas. Several such changes that are known to occur between adjacent populations of the South American rufous-collared sparrow correlate with relatively major habitat changes. Very few dialectal changes occur over an enormous range on the Argentine pampas, but in this case the habitat of the species also changes little. The habitat changes markedly in the Andes mountains over short distances, however, as elevation rapidly increases, and, concurrently, many more dialectal changes occur there in birds’ songs. The suggested function of the correlations between display and features of the habitat is that they provide markers that identify populations adapted to different local conditions; such markers would permit more appropriate selection of mates than would otherwise occur, at least in the marginal areas between populations. It has been suggested that a similar structural explanation may by involved in the evolution of human dialects.
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