animal communication


animal communication
Transmission of information from one animal to another by means of sound, visible sign or behaviour, taste or odour, electrical impulse, touch, or a combination of these.

Most animal communication uses sound (e.g., birds calling, crickets chirping). Visual communication usually indicates an animal's identity (species, sex, age, etc.) or other information through specific characteristics (e.g., horns, patches of colour) or behaviour (e.g., the bee's "dance" describing a source of food). Chemical communication involves pheromones (chemical signals) produced by the animal's endocrine system. Eels and some other fishes use electrical impulses to communicate.

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Introduction

      the transmission of information from one animal to another by means of sound, visible sign or behaviour, taste or odour, electrical impulse, touch, or a combination of these mediums. 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 (display behaviour), 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.

The functions of communication
      Because the complexity of social interactions makes experimental manipulation difficult, human understanding of the role of 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 functions. 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 this 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 accipitrine 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 (reproductive behaviour) 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 aids 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 (i.e., members of the order Galliformes (galliform)) 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 (migration) 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 (Zonotrichia capensis) 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 functional explanation may be involved in the evolution of human dialects.

      Signals have evolved, the primary function of which lies in communication between, rather than within, species—particularly in cases in which identifying markings or displays of dangerous or distasteful animals provide information to potential predators or in those in which innocuous species mimic the signals. Other species, such as some forest falcons (Micrastur) of the New World tropics, apparently employ vocalizations as a kind of lure to attract prey species for capture; in this case the information is of use only to the individual providing it. An American ethologist, Martin Moynihan, has shown that elaborately specialized means of communication have evolved in bird species that join to form large mixed foraging flocks. These signals attract individuals to the flocks and help to maintain the cohesiveness of the assemblages as they move through the trees.

Modes of information transfer
      The evolution of animal behaviour and structures toward a communication function has yielded a mode of communication adapted for each externally oriented sensory receptor system—e.g., organs of vision, hearing, and taste. Each mode, although specialized, has limitations with respect to such properties as energy utilization; the ability to surmount environmental obstacles; the ease with which the source of the communication can be located; the persistence or transitoriness of the signal; and the available range of physical complexity. These differences have been exploited during evolution.

      Because sound disseminates and fades rapidly, a given unit of information does not remain to interfere with, or garble, succeeding units. In addition, sound can be varied with regard to pitch, clarity or harshness, duration, loudness, and rate of repetition, with each variable providing greater range of ability to encode. One advantage of sound as a medium of communication is that vocal displays can be uttered by animals who need to keep their appendages free for other activities and can be received by individuals who need not face the communicator in order to receive the signals.

      It is usually a simple matter for an animal with two ears to locate the source of a sound, although some modifications (described below) can help to conceal the location of the transmitter from potential dangers. Virtually all of the animals for which sound is important are bilaterally symmetrical and hence have paired hearing organs. Sound is a superb means of encoding information that must pass around environmental obstacles, such as trees or other vegetation. Apparently, some animals utilize frequencies that are particularly good at bypassing obstacles; this appears to be the case at least in the vocalizations of forest birds. Because the highest frequencies are obstructed in the forest and attenuated relatively rapidly by wind and air in open habitats, they apparently are not selected for use in the communication of at least the majority of vertebrate species.

      The most obvious examples of the use of sound in displays are the vocalizations (vocalization) characteristic of most of the better known air-breathing vertebrates (i.e., reptiles, birds, and mammals). Many nonvocal means of producing audible displays exist, although none match the potential for elaboration found in vocalizations. Many invertebrates produce sounds by rubbing one body part against another (stridulation); this technique is also used by fishes and is, in some ways, comparable to the chest beating done by the male gorilla (Gorilla gorilla). Gorillas also beat upon the ground and upon other objects in their environments; alarmed beavers slap the surface of water with their broad tails. Some vertebrates have elaborated on this sort of behaviour. Many woodpecker species, for instance, seek out certain dead limbs or even the tin roofs of buildings on which to produce their drumming displays. The North American ruffed grouse (Bonasa umbellus) produces a sound like the beating of a low drum by beating air toward its chest with its broad wings. Many other birds use specialized wing or tail feathers to produce sounds during display flights—such as the “winnowing” flight of snipe.

      Some forms of vibrational signalling are not perceived as sound, at least not by the relevant participants. Thus, although the sounds employed during the social interactions of honeybees are audible to man, it is likely that the bees perceive them primarily as vibrations through receptors in their feet. Some other displays of this type are not audible to man. Males of some web-building spiders, for example, approach females for mating very cautiously, signalling their presence and identity by strumming on the females' webs.

      Visibly encoded information provides for much easier pinpointing of source than either sound or chemical signals, although visible displays are also much more easily obscured by structures in the environment. Ease of locating the source of a signal is often extremely important as, in a gull colony, for example, in which a large number of individuals are present in small space and it is important for the recipient to identify the individual that is displaying. The sight of the communicator also provides information about his orientation and so functions (like the human signal of pointing) in selection of a relevant recipient. To avoid the problem of being obscured by the environment the communicator is often able to select a display position that makes him more easily seen by the most relevant recipients. When the latter are at a distance, for example, the communicator is likely to display from a highly placed station, or, in the case of many grassland bird species that have no high perches available, to perform a flight display above the vegetation. Visible displays in many species of social birds and in at least some primates (such as baboons) are often combined into relatively complex assortments that are thought to convey unusually precise information.

      Unlike sounds, which are usually very transient and can be difficult to maintain, visible posturings can sometimes be maintained with relative ease, although they usually interfere with the communicator's ability to engage in other forms of behaviour. Many animals have surmounted this problem by the evolution of badges—morphological specializations, such as bright patches of skin, fur, or feathers; horns; casques; and crests. Badges convey information about the general identity of the communicator (i.e., species, sex, age) and some information about his physiological state. Animals have also evolved ways to utilize sources of information that supplement displays and badges. Some species provide information of some highly relevant samples of the environment; the honeybee, for example, uses a drop of nectar in the dance at the hive to indicate the identity of the food source. Courting (courtship) males of many bird species feed their mates or provide them with bits of nest material. Unmated male weaver birds make nests at which they display and which are subsequently used for breeding; male bower birds build “bowers” (a variety of display structures), and male manakins clear leks (special display arenas) that serve only a communicative function. The use of constructions in some cases extends to supplying information in the absence of the communicator, as in territorial marking. Rabbits and other mammals use dung heaps (both visible and scented) for this purpose, and bears scratch marking posts.

      Despite their flexibility, visible means of signalling have disadvantages in addition to being easily obscured. They may be too easily located, drawing undesirable attention of predators and other inappropriate recipients to the communicator. Moreover, the signals are available only if the recipient looks at the source directly; this hinders his freedom to do other things simultaneously.

Chemicals
      Many species have evolved special chemical products that are released under particular circumstances. Some of these substances are used as defense mechanisms against predators; they apparently function primarily by being distasteful or even injurious, but some may serve to warn the predator that its intended prey can harm it, thus eliminating the need for actual contact. A variety of chemicals called pheromones (pheromone) are used for communication within a species. Fishes and other aquatic animals secrete certain pheromones into the water; moths release pheromones as sexual attractants into the air to be wafted downwind; and various highly social insects mark surfaces with pheromones or spray them into the air, sometimes secreting them onto their own body surfaces so that chemicals can be directly tasted by other individuals. Scents (odour) are used by many species to mark territories, as in the well-known urination patterns of domestic dogs.

      Much research has been done on the use of pheromones by species of ants. It has been found that, depending on circumstances, different pheromones are secreted from different glands; for example, one type is secreted when the ants are laying trails, another when they are indicating alarm. The rate of release and the response thresholds of the different pheromones have been adjusted during the course of evolution so that the fading time of the odour correlates with the functions performed. Alarm pheromones, for example, which cause the ants to congregate, diffuse very rapidly and fade quickly, unless continuously renewed by additional secretion from newly attracted individuals who also take alarm. Pheromones used for group identification and for simple assembly (in the absence of alarm), fade much more slowly. Even though pheromones are less persistent than certain structures used for marking, they are used primarily for their persistence—at least in the case of species that have other means of communication available. Extremely rapid fading of the odour is a problem, however, as is the recovery rate of the recipient's chemoreceptors. Pheromones are probably an inadequate means of communication in social events that change rapidly.

      Specialized patterns of touching—i.e., tactile displays—are often overlooked in studies of animal communication unless they involve distinctive movements. Although a recipient animal may be aware of a distinctive touch, an observer may not; nonetheless, many forms of touch are evident. The remarkable dances of honeybees are customarily performed on a vertical comb in the darkness of the hive. Recipients of these displays follow the communicators closely, maintaining contact with their antennae; thus, much of the information is probably received as changing tactile patterns.

      In many mammalian species, the members of social groups engage in bouts of grooming (called allogrooming when performed on another individual). Although visible to group members other than those in the interaction, allogrooming probably functions largely as a tactile display. Specialized touches with the hands are now suspected to be precopulatory signals in female rhesus monkeys (rhesus monkey) (Macaca mulatta). Individuals of the South American monkey Callicebus (titi) moloch rest together in trees with their tails intertwined, a tactile display that probably serves a function similar to that served by allogrooming in social groups of baboons and macaques.

      In some cases the communicator leaves information available for individuals it will never meet. Female wasps of at least one species are able to indicate to their offspring the direction in which to seek egress from the cells in which the eggs are laid. The information is stored in the geometry and texture of the walls constructed by the females to seal the nest chambers.

Electrical energy
      A number of fishes that live in muddy waters produce regular patterns of electrical discharges as an active sensory system (active in the same sense as a bat's sonar scanning of the environment). There is good evidence that some species respond to electrical discharges of individuals of the same species, and that some aspects of the discharges, including their cessation, may function as displays.

The role of displays

The display repertoire
      The individual animal may have a repertoire of up to 40 displays. For most relatively social adult fishes, birds, and mammals, the range of repertoire size for different species varies from 15 to 35 displays. Further, most species have evolved displays adapted for the different sensory modality of their recipients; thus, their repertoires comprise sets of displays (e.g., both visual and audible) that overlap in the information they convey. A bird can convey information about the probability of flying by fluttering its wings and by uttering a particular call and may often do both simultaneously. Recipients sometimes are able both to see and to hear the communicator; at other times, however, he may be obscured behind foliage, or the sounds may be masked or distorted by wind noise or by the calls of other birds. The redundancy typical of communication systems probably exists primarily to facilitate information transfer in such noisy environments that may interfere with any one form of transmission.

      In view of the immense value of language to man, it is remarkable that evolutionary pressures have not produced a greater elaboration of communication in animals other than man. This implies that further elaboration would impair rather than facilitate communication. Although the nature of this impairment is still a matter of theoretical conjecture, a promising explanation is that of Moynihan, who argues, in part, that it is difficult for a species to evolve the ability to make rapid and finely tuned responses to rare events. Rapid escape from a surprising stimulus (a relatively rare event) has frequently evolved but it is not a finely tuned response. As the size of a display repertoire increases, appropriate circumstances for the use of certain displays would become increasingly rare; Moynihan proposes that the number of displays in an animal's repertoire should not exceed that at which its rarer displays have little usefulness. This number of displays cannot yet be predicted. Other factors also act to limit the number of displays. Among them is the need for each display to be sufficiently distinctive to make it unlikely that a recipient will mistake it for another. Beyond some point, as repertoire size increases distinctiveness requires increasingly elaborate displays that may be time-consuming to produce and require too much attention by the recipient.

      The restriction in the size of the display repertoire undoubtedly has had considerable influence on the evolutionary selection of the kinds of information that are encoded. An American zoologist, W.J. Smith, has suggested that this restriction explains two striking characteristics of displays: the paucity of narrowly precise messages and the relative abundance and widespread occurrence of information about such general behavioral patterns as locomotion or social association. Each species has so few displays available to it that only a minimal number can be used in restricted situations. When practical, a display has selective advantages if it conveys information that can be of use in mediating a variety of social interactions. Some types of activities, such as attack and escape, are too critically important in most species to be indicated by ambiguous signals. Others, including many of the acts that bonded (e.g., paired) individuals can perform in each other's company without endangering the fabric of social organization, may be safely left for a recipient to predict based on its experience in similar contexts, provided it has the information that the only likely activities are bond-limited ones. There is strong selection pressure toward both the use of general messages (in situations in which immediacy of correct response is not of overriding importance) and similar messages (for species with a wide overlap of social habits).

The information content of displays
      The study of animal displays was once primarily concerned with the motivational states of the animals using the displays. It was recognized that, if an animal uses a display only when in a certain motivational state, then the display informs a recipient that the communicator is in that state; the recipient thus should be able to predict, at least in part, the subsequent activities of the communicator. Because only certain aspects of the communicator's motivational state are common to all uses of a given display, however, these aspects are the only ones that can be said to be encoded in the display. Thus, a recipient must obtain other information in order to establish the presence of other motivational aspects. It is more useful, therefore, to study the behaviour (rather than the motivation) of communicating animals in order to correlate specific behaviour patterns with specific displays. Because communication is a social process, the most productive course of study concerns the nature of the information that can be inserted into social interactions by displays. This information helps to determine the course of the interaction and is concerned primarily with making the behaviour of the interactants more predictable.

      It is important for the recipient to be able to identify the communicator with some degree of precision; if he identifies himself, through displays or some other means, as belonging to a category of individuals important to the recipient, the latter will pay attention to him, insofar as is practical or necessary. Of prime importance is specific identity; the most important communication occurs among members of the same species. Thus, most of the flow of communication among members of other species is irrelevant. Within a species it may be useful for the recipient to know the sex and perhaps the age of the communicator; even the general physiological state (e.g., breeding or nonbreeding condition) may be significant. All of this information, successfully communicated, narrows the range of behaviour that can be expected by the recipient of a display. For bird and mammal species with complex social behaviour, some features of the form of a display may be peculiar to the individual. A female passerine bird (a perching bird) newly arrived from spring migration to the region where she will breed encounters many individuals of several different species. By using certain vocalizations (usually called song), some of the birds identify themselves as males of her species and also communicate to her that they are in breeding condition and probably the information that they are unmated. This information enables the female to predict their responses to any approaches she might make to them. Also, after she begins to form a pair bond with a male, she must be able to identify him.

Information displayed by mammals
      All displays studied thus far identify the communicator to some degree. The amount or kind of identifying information differs according to the social requirements of the species.

      As already noted, identification is not only encoded by animals in displays but also in badges. The strikingly distinctive breeding season plumages of drake waterfowl and of males of many other bird species attest to this. Female birds of many species are less distinctively coloured and it is usually they, not the males, who select a mate; male birds of such species apparently accept any female who attempts to form a bond, but females respond only to males with the correct plumage. The evolution of unusual specializations apparently has been necessary in only a few cases. Gulls, for instance, are able to distinguish species by a combination of eye colour and its degree of contrast with the white head.

      Closely related in many ways to information concerning identification is information specifying location. An animal may need to transmit its message without fully revealing its location. It may, for example, use a visible display to a nearby recipient while otherwise concealed by vegetation from potential and undesirable recipients that would perceive vocal displays. In some cases, however, animals provide information about their location in proportion to the need of the recipients. A small bird on fleeing from a hawk, for example, may utter a high-pitched warning vocalization that carries little information about its own location. Natural selection has favoured characteristics of this vocalization that make its source difficult for a two-eared predator to locate. Many small birds hear the sound and are warned that one of their associates has found desperate reason to flee; they need not know his exact location or even why he is fleeing. The hawk, who needs primarily the location information, is denied it, largely since he is unable to locate the source of the vocalization and often has not yet seen the fleeing communicator. It is significant that the begging vocalizations of many nestling birds are similar to the warning call; the reason also is similar—to prevent predators from discovering the location of the nest, which the nestlings' parents, of course, already know.

      In certain cases a vocalization can vary in ways that increase or decrease its locatability. One example, which has been studied in detail, is the call given by males of the Panamanian frog Engystomops pustulosus as they await females at spawning ponds. The call of a lone male contains fewer clues to his location than does the call of a male in a chorus of other males. The former probably can rely on the persistence of any passing female to find him, but, because he is alone he is more vulnerable to predators and so must give as little information as possible about his location. On the other hand, the male within a group gets some protection from predators by virtue of being among other frogs; not only is the approaching predator likely to encounter one of the other frogs first but, collectively, they are more likely to detect the predator. After the chorus has attracted a female to the immediate region, however, any male that is less easily located than his fellows is at a disadvantage; under these circumstances each male maximizes the ease with which he can be located.

      Much of the remainder of the information encoded by animals in their stylized displays is behavioral—it indicates the likelihood with which a recipient can expect a communicator to engage in different types of activities.

      The bared teeth, growls, curled lip, and bristled hair of a watchdog at the approach of a stranger indicate that the animal may attack. The same display components may also indicate that it may not attack, that something, perhaps fear, is holding its aggression in check, however tenuously. Much of the displaying done by animals seems to indicate the probability of attack or escape. Such agonistic behaviour patterns must be controlled in the establishment and maintenance of organized social units such as pairs, families, and troops. A species may encode information about antagonistic behaviour in more than one display. The green-backed sparrow (Arremonops conirostris), for example, uses a vocalization that sounds like “chuck” when it is less likely to attack than to escape, but, when both attack and escape are equally probable, it utters either of two calls, which Moynihan has described as “medium hoarse notes” or “hoarse screams.” Although the last two vocal displays indicate equal probabilities of the two acts, they do not carry exactly the same information. The “hoarse screams” are used when attack or escape is very likely to happen, and the “medium hoarse notes” are used when the bird's indecision between the two alternatives makes it less likely that either will occur. The two displays thus carry information not only about attack and escape behaviour but also about the probability that some act will occur. In fact, information about the probability of a specific act is apparently encoded in all displays, but not always in the way described above. The eastern kingbird (Tyrannus tyrannus) encodes the information that it may attack in a vocalization with a sound similar to “zeer.” The likelihood that it may attack when using this call varies, increasing as the abruptness and harshness of the vocalization increases.

      Of the information widely encoded among diverse animals in displays, little is apparently as narrowly defined as attack and escape signals. Although the encoding of information regarding attempts by a communicator to engage in social play would seem useful in view of the fact that many mammals (and even some birds such as the hobby falcon, Falco subbuteo) play socially, information that an approaching communicator is not likely to be vicious but intends only to play is apparently encoded only by a few primates.

      Adults of many vertebrate species appear to lack common reasons for coming into contact with their fellows other than aggressively. Many animals thus have difficulty in approaching one another closely enough to copulate. The displays of only a few species provide information that an individual is approaching specifically to copulate; in fact, the general information provided by the individual in other displays apparently is often sufficient, given the right contextual circumstances, to indicate the intention to copulate. So far as is known, the information provided by most birds and mammals through their displays is in large part of a general nature.

      In many birds and mammals, approach in order to copulate is restricted to individuals who have established a pair bond. In this relationship each partner adopts, and recognizes as appropriate in the other, behaviour patterns that characterize certain roles, special restrictions, and special privileges. Such behaviour patterns are regular in occurrence and apparently expected to some degree by each individual. The complexity of this behaviour varies considerably among species with different degrees of social complexity—i.e., the individuals may be merely temporarily mated or permanent members of a troop of numerous individuals—but the appropriate behaviour will, for some relationships, include the possibility of copulation. Many species employ at such times displays that are used in a variety of behaviour characteristic of closely interacting, bonded individuals. These displays indicate that the communicator will probably select behaviour patterns appropriate to the bonded relationship, without specifying what the behaviour will be.

      The somewhat more precise information that may be available among bonded individuals usually appears to specify that appropriate behaviour will not involve physical contact. In these circumstances, the communicator simply associates with the other individual, remaining in his presence but doing nothing that requires bodily contact. Such information about association is apparently basic and is encoded by many species, at least of birds and mammals. A superficially similar sort of information that may also be widespread indicates that the communicator will engage in a variety of activities, from social encounters to nonsocial acts of individual maintenance (e.g., foraging), but will neither attack nor attempt to escape from the individuals to whom the display is directed. He will thus behave nonagonistically.

      General information that has been identified in displays of three mammalian species (gorilla, man, and the black-tailed prairie dog) appears to be so basic that it, too, may be widespread among social vertebrates. The communicator provides information that he will probably hesitate to engage in a certain social interaction. This information is usually provided when the communicator is engaged in an activity that would be hampered by interruption. How widespread this sort of information is among animals is not known.

      Other kinds of general information known to be widely encoded are less directly social in nature. The best understood information involves the probability that the communicator will engage in some form of locomotion. One kind of display may be used when an unmated male bird moves from point to point along the boundaries of his territory, occasionally stopping to advertise. The same display may be used by a mated male as he approaches his rather aggressive and perhaps sexually unreceptive mate. It may also be used by either mate on approaching a predator near its nest, or later by the nearly independent fledglings when importuning increasingly unwilling parents. Nor does this exhaust the range of situations in which this one display is used. In all cases the display fails to indicate with any precision just what the communicator's probable activities will be, but it is well correlated with a behavioral indication of uncertainty over whether the communicator will initiate or cease flying, or change direction if in flight. In any case the activities of the communicator indicate what it may attempt to do, and the display appears to provide information as to whether or not this apparently probable behaviour will be performed. The information the communicator thus provides indirectly implies that there are factors acting to counter his obvious motivational tendencies. These implications have different social relevance in different circumstances.

      Displays are also known that encode even more general information—exactly how general is not yet clear, but the following example is illustrative. The blacktailed prairie dog (Cynomys ludovicianus) has a combined visual and vocal display in which the communicator throws its foreparts vigorously into the air, directs its nose straight up, and utters an abrupt, two-part vocalization; the performance has been named the Jump-Yip. Jump-Yips are employed on many occasions, all of which have in common a certain probability (usually less than 50 percent) that the communicator will begin or continue to flee. Other forms of behaviour associated with the Jump-Yip differ greatly, depending on the circumstances; for instance, the communicator may greet or associate with its mate or another individual (such as a member of its family group); approach its mate sexually; challenge a neighbour aggressively; or do one of various nonsocial acts such as foraging or dust bathing. The display seems to function either as an indication that some form of behaviour may be interrupted or prevented by fleeing but that this other behaviour may be socially more important than the fleeing; or that the other behaviour may be negligible and the possibility of flight itself the critical feature of the situation for the particular recipient. The display is not used unless there is an alternative to flight, but it does not provide information about what that alternative is. Such information is usually clear enough contextually, however, The unspecified alternative might be anything in a prairie dog's behavioral repertoire.

      Of widespread occurrence in the displays of birds and mammals is information that the communicator, however strongly motivated to perform a particular act, lacks the opportunity to do so. Thus, when, in a social encounter, a juvenile gorilla tries to flee but finds itself cornered, it adopts a special posture indicative of its frustrated escape. An eastern kingbird or a black-headed gull (Larus ridibundus) performs a very stylized and spectacular aerial display when it is strongly motivated to attack but lacks a suitable opponent—such as an intruder into its territory. A Carolina chickadee (Parus carolinensis) that appears to be seeking a border encounter with a territorial neighbour employs a special form of song if the neighbour fails to appear in response to its challenges. Various songbirds of the New World tropics have vocal displays that are used only when one bird becomes separated from the companions with which it associates while foraging; the calls cease after reunion occurs. In short, it appears that if an animal is frustrated from engaging in activities of sufficient social importance to merit communication, it may be able to encode its frustration with respect to that activity.

Information displayed by nonmammals
      The foregoing describes the types of basic information presently known to be widespread among adult birds and non-human mammals. Other types of information are known to be encoded by other animals: ants provide directional information in odour trails, honeybees in “dances.” The fire ant (Solenopsis) odour trail appears to indicate specifically the route to a food source, but the honeybee dance is less specific—it can be used to provide information about food sources, water sources, and potential sites for new hives. Both cases reflect the evolution of communication within complex social organizations of invertebrates, based on evolutionary properties quite different from those of vertebrates.

      Even among vertebrates, some systems of social organization differ markedly from those of most of the birds and mammals whose communication patterns have been studied. Among nocturnal frogs, for instance, breeding males congregate loosely and call to attract females. They attempt to mate with any individual that comes near them—whether it be a male or a female. A male or a female who has already laid her eggs usually utters a distinctive call and is released; the encoded information appears to be that the individual that has been clasped does not belong to the class that will shortly lay eggs (i.e., gravid females), a rather precise but entirely suitable sort of information. Precision in such narrowly specific messages may well be the rule in groups in which complexity of social behaviour is minimal.

      Immature animals dependent on adults often have social requirements quite different from those of adults and provide, in part, different information, although they too may have displays encoding attack, escape, and frustration information. At least among infant mammals, much displaying is done to indicate a need state—such as hunger, pain, need for social interaction—for which the infant must depend on others. Some displays do not specify the needs that are relevant—the most common forms of human crying, for instance, are rather imprecise. Others, such as the widely gaping mouth of a juvenile passerine bird, appear to correlate primarily with readiness to eat. Infants, because of their often limited behavioral repertoires, are somewhat difficult subjects for behavioral studies of the kind normally employed by students of animal communication. They provide an important challenge, however, as attempts are made to learn both what they encode in their displays and how this information is altered during development into adult forms of encoding.

Interpretation of information
      The information exchanged during social interactions among animals allows them to select their subsequent activities with greater certainty that their actions will be more appropriate than would be possible without the new information. When an animal is offered information through a display, it can select a response that is based, in part, upon this information. Depending upon the abundance and the type of information, the response must be such as to permit flexibility—i.e., a generalized response may be practical. On the other hand, receipt of abundant and varied information permits the individual to choose among less generalized, and probably less flexible, responses with a greater certainty of making the appropriate selection. Complete understanding of animal communication, therefore, requires knowledge of the responses that individuals make to the information communicated. This is the study of what Colin Cherry has referred to as the “meanings” that recipients draw from the circumstances in which communication occurs.

      The communication process between two animals is summarized in the following paragraphs. Through displays, badges, and other aspects of his appearance and activities, one individual adds information to another individual's environment, in which much information is already available. The human observer is certain that communication has occurred only when the second individual selects a response, demonstrating the meaning the event has had for him. Unfortunately, the study of responses in social situations is technically very difficult.

      The recipient animal certainly ignores most of the many sources of information available and weighs others as he selects his response. In most instances, his response is not directly available to the human observer, for the recipient's mental state or disposition may change without an overt indication. For most displays the response observed most commonly is indistinguishable from that of inattention. When the recipient does alter his activity immediately after exposure to a signal, his first acts may be common to many different kinds of events—he may simply face the source of information, move a little closer to it, or a little farther away.

      The first response of a recipient to a variety of signals conveying many possible types of information about the communicator (including high probability of attack, intent to escape or to engage in association, social play, or copulation) may be to approach the communicator. The predisposition of the recipient to react further is likely to depend on the signal, and further activity may not occur until the recipient is supplied with further information.

Modified displays
      Although the nature of displays and badges and of the types of information that animals encode in them is important in the process of animal communication, it provides only part of the story; for example, a display is a vehicle for a particular type of information transfer, but the communicator, in the manner of its displaying, can modify some aspects of this information. The probability of attack, as encoded in the vocal display of a bird species, can be ascertained by the abruptness or harshness of a vocalization. The intensity of the communicator's involvement in the situation can also be indicated by variations in the rate at which the vocalization is uttered, or in its loudness. Such modifications provide information about the nature of the expected attack—i.e., whether it will be vigorous or perfunctory—and about the communicator's state with the passage of time.

      The position of the communicator as it vocalizes may indicate a particular recipient as the most relevant in a given instance. There is little reason to conclude that the orientation of the communicator has become ritualized as a part of the display. Sometimes an intermediate orientation becomes part of a ritualized display, and orientation must very often provide important information.

      It appears that the information obtained by most animals about situations in the environment is either associated with stylized displaying or is implied by it with fair probability. For instance, a bird that utters a call when it sights and then flees from a dangerous aerial predator is not specifying the presence of a hawk, even though it is likely that it has been frightened by a hawk; under some circumstances (e.g., if the bird (birdsong) has nestlings) a predator normally less dangerous than a hawk, such as a cat, is responded to with the call (vocalization) usually given for hawks. The bird advertises its fear in the presence of different stimuli at different times. It does not specify what the stimuli are, but instead, what its probable response is or will be. Although a bird does not specify directly that it has a territory, it regularly indicates different degrees of readiness to behave aggressively to inappropriate individuals of (usually) its own species—and so defines a region, which can be labelled its territory, that it will protect from intrusions. A forager bee does not state that it has found food at a certain place; rather, she describes a direction of flight (perhaps a flight she is likely to make again shortly) and, on request, provides a sample of the food. On the other hand, an ant that lays an odour trail only between a food source and its home colony is providing information that food is present; such narrow specific use for a display, however, is the exception, at least among birds and mammals.

Evolution of communication
      In the evolution of communication within a species, one normal constraint usually is that the exchange of information must be useful to both the communicator and the recipient. The behavioral patterns evolved by the communicator enable him to transmit information that increases the probability of a social response suited to his needs. A recipient evolves the tendency to respond to this information only when the response suits his needs, which often differ, at least superficially, from those of the communicator. When their needs are not compatible, lack of selection pressure for the recipient to respond appropriately usually removes any advantages for the communicator, or at best yields an evolutionarily unstable situation of misinforming, to which recipients are always counter-adapting. Yet, in certain relationships between individuals belonging to different species, particularly in predator–prey relations, selection pressures for providing misinformation are such that it is a widespread phenomenon. In Batesian mimicry, potential prey species advertise that they are actually unpalatable or dangerous to predators, which either learn to leave most of them unharmed or evolve avoidance responses. Other potential prey species, lacking defense mechanisms or unpalatability, may mimic the behavioral and morphological specializations of the unpalatable ones, thus surviving by providing predators with false information. The predators, of course, are under evolutionary pressure to develop means of distinguishing the true from the false information.

      The evolutionary process by which the transmission of information between members of the same species can become specialized has been studied by the pioneer European ethologists Konrad Lorenz (Lorenz, Konrad) and Nikolaas Tinbergen (Tinbergen, Nikolaas). They have established that displays are specialized activities that have evolved from precursors, or predecessors, of several types. Important among these are intention movements—i.e., incomplete performances of acts, such as taking flight or turning away, which are usually performed by an animal not quite committed to a given course of action. Through the process of ritualization, components of some intention movements become exaggerated and divorced from their directly functional roles. The exaggeration is often evident in an increased conspicuousness (perhaps with the concurrent evolution of morphological badges, to which the display draws attention) or an increase in the conspicuousness of only some components, thus creating a difference between the display and its evolutionary precursor.

      A second source of precursors in the evolution of displays lies in inconspicuous but complete movements, such as eyebrow-lowering movements that help protect the eyes; such movements have been incorporated into the facial expressions (frowning) of numerous primate species. Other movements, such as jabbing motions of attack, are ritualized by being aimed in a stiff and often repetitive fashion away from their customary targets; these are called redirected activities. Still other movements appear to be occurring outside their customary functional contexts, as if displaced. Called displacement activities (displacement activity), they remain perhaps the least understood, particularly since it is not always clear that they are as functionally irrelevant as they sometimes appear. Displays apparently derived from displacement activities often resemble the activities devoted to individual maintenance—self-grooming, feeding, and drinking. Such behaviour often occurs in close conjunction with other kinds of socially relevant activities and so is perhaps easily available for evolutionary specialization. Further, some maintenance activities, which are directly related to the physiological results of exertion (as when a bird ruffles its feathers to cool its body) and thus to active social encounters like chasing and fighting, are commonly exaggerated as components of display behaviour. Other autonomic (involuntary) responses, such as urination and defecation by thwarted or frightened mammals, may have been sources for the evolution of some marking displays.

      The precursors of ritualization are more easily imagined for visible displays and perhaps tactile displays than they are for other forms, although there is no reason to believe that the evolutionary process is fundamentally different in any case. Chemical displaying seems highly specialized: the chemicals per se have in most cases probably originated from metabolic waste products, and the acts of releasing different chemicals (the displaying) may have evolved from precursors classifiable as individual maintenance activities or “autonomic responses.” The evolutionary precursors of vocal displays are also conjectural, and many extant vocalizations undoubtedly arose from preexisting ones. Vocalizations must have arisen originally from some form of noisy, controlled breathing; Darwin's (Darwin, Charles) suggestion that the breathing patterns of terrified or sexually aroused animals would provide a source for specialization has not been bettered.

      The evolutionary process of ritualization yields two somewhat distinctive classes of displays. As described above, much ritualization functions to yield a display distinctly different from the act from which it was derived. The act, perhaps a movement preparing a bird to take flight, remains in the behavioral repertoire of the species, serving its original functions, and, to be fully effective, the display must be distinguishable from it. There are, however, cases in which the form of the act is not altered, but its frequency of usage is in one of several ways. This is possible primarily in cases in which the evolutionary precursor is not maladaptive when done to excess (with respect to its original function). Cases in which increased frequency of performance occur are known primarily from social acts, whereas acts that are transformed in the process of ritualization may have social or nonsocial precursors. The best known example of ritualization through increased usage is what is known in mammals as allogrooming and in birds as allopreening—care given by one individual to the condition of the body surface of another individual. In highly social birds and mammals this occurs much more frequently than is necessary for cleansing of the fur or feathers, is done among animals that have bonded relationships, and is often expressed asymmetrically with respect to some feature of the social organization of a species—that is, in one species, subordinate individuals may groom dominant ones more than the former are groomed by the dominants, but in another species the reverse may be true. In addition to being a sanitary procedure, allogrooming apparently expresses the acceptance of bond-limited relationships by both the groomer and the groomed.

      The evolutionary process of ritualization operates within a number of limits in addition to those imposed by the process of communication. Each species has a history, in which the origins of its attributes are, ultimately, products of genetic chance.

      Closely related species evolve (evolution) different solutions to the problems and opportunities of communication from those of more distantly related species. Each lineage has developed a working system based on the opportunities it has received, or, having failed to develop such a system, has become extinct. The products of ritualization are not ideal solutions to communication tasks; rather, they are practical ones. The similarities and differences of displays among species contain clues to phylogenetic relationships, although, like all other such clues, they are most safely used when their full functional significances are understood.

      By no means do all of the differences among the communication patterns of different species result from the different genetic peculiarities of the different lineages. Species differ in the nature of their social behaviour. Birds, for instance, may be paired through the nesting season and flock in the remainder of the year; be paired only briefly, followed by dispersal on individual territories; be paired for life (year round); or utilize various other types of social structure. The great range of patterns of social organization determines many things about the sorts of behavioral interactions occurring among individuals and, hence, the functions required of communication patterns. The form of social organization is likely to be set, in part, by characteristics of the species' habitat, and the habitat thus must indirectly influence the directions of ritualization. Habitats also have a direct influence on ritualization of form, because they differ very much in the degree to which they obstruct information or mask it by noise. The environment of most species also contains other species, some of which communicate with similar displays, creating a need for specific distinctiveness in form.

      A basic limitation exists in the nature of the sensory receptor organs available to different kinds of animals. Social insects make much use of chemical and tactile signalling, but visual signals are relatively more important to fishes and visual and auditory signals to birds. Among mammal groups there is considerable specialization, but on the whole mammals make considerable use of all sensory modalities in display behaviour.

W. John Smith Ed.

Additional Reading
Animal communication is the focus of Charles Darwin, The Expression of the Emotions in Man and Animals, 2nd ed. (1890, reissued 1989), the first major attempt to trace the evolution of facial signals; Wesley E. Lanyon and William N. Tavolga (eds.), Animal Sounds and Communication (1960), accompanied by a phonograph record; Martin Lindauer, Communication Among Social Bees (1961), a fascinating account of evolution in and experiments on social bees; Rene Guy Busnel (ed.), Acoustic Behaviour of Animals (1963), a general survey; and Thomas A. Sebeok (ed.), Animal Communication (1968), with good coverage of various aspects of the subject.

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Universalium. 2010.

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