Optical communication is found in many species with well-developed vision and sociality, especially insects, crustaceans, cephalapods, fishes, amphibians, reptiles, mammals, and birds. Although a few animals generate their own light (see chapter 8 of this volume), most signal by modulating reflected sunlight. This short review concentrates on traditional concerns of ethology: the origins and structure of signals and the dynamics of the signaling process. A broader and more speculative discussion of optical signals is available as a monograph (Hailman, in press). To keep documentation short, examples are drawn chiefly from studies subsequent to Sebeok (1968), which may be used as an entry into older literature.
There is no unambiguous way for the casual observer to know that two animals are communicating. I believe that certainty requires comparison of two situations: one in which a reputed signal is given and another one in which it is not, other things being equal. Then if the behavior of the reputed receiver differs in the two situations, one can state operationally that communication occurred. In other words, communication is shown by the correlation between a difference in the behavior of the reputed sender and the reputed receiver. This chapter primarily considers conspecific animals sending signals to one another: reciprocal social communication.
It is difficult to specify what constitutes a visual signal. Virtually anything about an animal that can be perceived by another can be a signal: slight tensing of muscles, an action performed out of its usual sequence, or just normal, ongoing behavior. Even when a behavioral pattern obviously generates stimuli that affect another animal's behavior, one cannot be certain what aspect of the stimulus is effective. Many visual stimuli accompany the production of sound, marking by scent, or generation of tactile stimuli. We humans are so visually oriented that the tail slap of a fish may seem to be a visual signal until we realize that the displacement wave it causes is readily detected by another fish's lateral line system. I refer for convenience to behavioral patterns and morphological structures as signals, although it is actually the stimuli they generate that constitute the signal.
It is useful to discuss determinants of visual communication under the four classes of causes and origins that apply generally to behavior (Tinbergen, 1963; Hailman, 1967; Klopfer and Hailman, 1967). One may ask how a behavioral system works in the immediate sense of the relations among external inputs, internal mechanisms, and behavioral outputs—the determinants of dynamic control. The control system of a particular individual at a particular time of its life is structured by the past interactions of the organism with its environment, keyed by the organism's genetic endowment from its parents. These ontogenetic origins may involve particular experiences during development, or the ultimate control system may develop relatively similarly regardless of experience and environment. In turn, the genetic endowment of individuals in a population is structured by natural selection acting on phenotypic variation. Genes leading to beneficial phenotypes are preserved and those leading to other phenotypes are trimmed from the population: the adaptive function of a behavioral pattern depends on such natural selection. Finally, the evolutionary history of selection acting on the population also determines behavior, since the phylogenetic origins impart a directional impetus to evolutionary processes. One must ultimately understand all these behavioral determinants—control, ontogeny, function, and phylogeny—to understand communicative behavior.
Control of Visual Communication
A simple sort of reciprocal visual communication takes place as follows: Animal A sends a signal to animal B, and then animal B performs some act that constitutes a signal back to animal A. Animal A then responds with a second signal, and so on, with each receiving and replying in turn. It is the vogue to analyze animal communication as if that were the communicative interaction, but there seems to be no truly convincing case of animal communication's working in this way.
There are relatively few signals that yield a discrete and constant reply in the recipient. Indeed, the general lack of strict correlation between stimulus and "response" in animal behavior constantly challenges the ethologist. The lack implies either that the receiver is somehow different at different times of receipt or that the signal's effect depends on external factors apart from the signal itself. Both complications usually apply, the first being subsumed under "motivational" factors and the second under the "context" of communication (Smith, 1965, 1968). Therefore, recording the mere exchange of signals between two animals yields an incomplete picture of communication.
Another criticism of the simplistic model of communication was recently articulated by Schleidt (1973), who points out that many signals have long-lasting, not merely immediate, effects. One animal may incessantly signal to another to maintain some state of readiness in the recipient. There may be no obvious exchange of signals even though important communication transpires. When the recipient does deliver an identifiable signal it may be produced by the totality of ongoing external and internal processes—not merely in response to the other animal's immediately previous signal.
I skirt the difficulties in attempting to provide a complete framework for the communication control and concentrate instead on the visual signal itself: what kind of signals are utilized by animals and how are they to be descriptively classified?
EXTRINSIC AND INTRINSIC VISUAL SIGNALS
One may divide visual signals into those that have a physical existence apart from their creator, the sender (extrinsic signals), and those that are part of the animal itself (intrinsic signals). Bowerbirds decorate display structures called bowers with brightly colored objects such as flower petals (Marshall, 1954) (see Fig. 1). Many physical objects created or rearranged by one animal may serve as visual signals to another. Tracks in snow or mud, nests or burrows, browse marks or other evidence of feeding all could act as visual signals. Olfactory marking by dung or by rubbing on the bark of trees is common in mammals, but without experimentation one cannot know if the visual component of such signals is important. The blackbuck brushes its antlers through grass as a form of visual place-marking (Schmied, 1973) (Fig. 1), and red squirrels stack spruce cones as visual marks of their territorial boundaries (Kilham, 1954). Printed words such as these constitute a complex form of extrinsic visual signaling.
More extensively studied are the intrinsic or behavioral visual signals used by animals: postures, gestures, and other aspects of behavior that generate visual stimuli.
DIMENSIONS OF INTRINSIC VISUAL SIGNALS
Three primary dimensions describe an intrinsic visual signal: the orientation of the signaling animal or some part of it with respect to the intended receiver; the shape, or configuration, of the animal, which is the relative orientation of its parts; and the movement patterns of the animal or its parts. Some signals depend primarily on one dimension, whereas others utilize various combinations.
Mere orientation of one animal relative to its conspecifics constitutes an important visual signal in many animals (e.g., Scruton and Herbert, 1972; Figler, 1972; Stanley, 1971; Golani and Mendelssohn, 1971; Dunham, 1966), as illustrated in Fig. 2. Baylis (1974) notes that orientation of a cichlid fish with respect to the environment also carries information, an example of context, mentioned previously.
Specific shapes or postures also act as visual signals, even when there is no special orientation of the signaling animal toward its conspecifics. There are at least three mechanisms employed to create bodily shapes: motor adjustments of bodily parts such as appendages in different relations with one another (e.g., Tinbergen, 1959; Barash, 1973; Hall and Miller, 1968; R. R. Phillips, 1971; Spinage, 1969; Fox, 1969); pilomotor responses of fur and feathers (e.g., Fox, 1969 McBride et al., 1969; Rood, 1972; Krämer, 1969 Schweinsburg and Sowls, 1972; Ewer, 1971 Schmidt and van de Flierdt, 1973); and inflation of structures with air (e.g., Evans, 1961; Carpenter, 1963; Kahl, 1966); examples appear in Fig. 3. The wildebeest may hold the record static posture: it stands in the "static-optic advertising" shape for up to an hour (Estes, 1969).
Movements that occur without specific orientation or body shape may be difficult to recognize as visual signals. There are two classes: movements of the entire animal, such as incipient locomotion (Daanje, 1950; Andrew, 1956) and other movements (e.g., Walter and Hamilton, 1970; Figler, 1972; Saayman et al., 1973; Rood, 1972); and movements of part of an animal, such as its tail or an appendage (e.g., Smythe, 1970; Saayman et al., 1973; Spinage, 1969; Barash, 1973; Quanstrom, 1971; Cole and Ward, 1969; LaFollette, 1971). Some examples are shown in Fig. 4.
Two of the three dimensions—orientation, shape, and movement—are often used together as elements of a unitary signal. Oriented static postures are common (e.g., Andrew, 1957; Estes, 1969; Steiner, 1971; Packard and Sanders, 1971; Wells and Wells, 1972; Frey and Miller, 1972; Dubost, 1971). Movement and shape are less-commonly combined (e.g., Walther, 1964, 1969; Estes, 1969). The combination of orientation and movement without a body shape different from normal is extremely common (e.g., Reese, 1962; Hazlett and Bossert, 1965; Hazlett and Estabrook, 1974a, 1974b; Hunsaker, 1962; Carpenter, 1963; Kühme, 1961; Otte, 1972; Markl, 1972; Rovner, 1968). Movement of part of the body does, of course, alter the animal's shape so that it becomes an empirical question as to whether shape is an important part of the signal. In other cases, the simple approach of one animal toward another involves no special shapes (e.g., Frey and Miller, 1972; Figler, 1972; Saayman et al., 1973; Sturm, 1973; Shank, 1972). Examples of combinational signals of two dimensions are given in Fig. 5.
The majority of intrinsic visual signals probably combine all three dimensions of orientation, shape, and movement. Many human facial signals are so composed, having orientation toward the intended receiver, changes in the shape of the mouth, eyes, or eyebrows, and dynamic movements such as eyelash fluttering or laughing. Examples of "three-dimensional" visual signals among animals are extremely common (e.g., van Lawick-Goodall, 1968; Andrew, 1963; Bovet, 1972; Stamps and Barlow, 1973; Zumpe and Michael, 1970; Lorenz, 1958); see Fig. 6.
In most studies of visual signals all the above types are described or implied. Examples come from all kinds of animals from cephalopods and arthropods to fish, birds, and mammals (e.g., van Rhijn, 1973; McBride et al., 1969; Krämer, 1969; Myrberg, 1972; Packard and Sanders, 1971; Ewer, 1971; Hall and Miller, 1968; Dingle and Caldwell, 1969; Gibson, 1968; Ewing and Evans, 1973; Spivak, 1971; Potts, 1973; R. E. Phillips, 1972; Wells and Wells, 1972; Kleiman and Eisenberg, 1973; Albrecht, 1969; Kahl, 1972; Winkler, 1972; Franck, 1968; Simpson, 1968; Tinbergen, 1953, 1959; Hinde, 1955/6; Andrew, 1957, 1963).
MORPHOLOGICAL ELEMENTS OF INTRINSIC VISUAL SIGNALS
Darwin (1871) observed that many animals possess elaborate morphological elements that are prominently displayed before conspecifics. Indeed, the ethological concept of a "display" stems directly from such phenomena. The elements may be structural shapes or specializations of the light-reflecting surface. These elements are so well known that only a few remarks are necessary.
Structural elements, which alter the animal's shape, include those used for other functions as well as those that appear specialized for signaling. The former include horns and antlers of many ungulates, structures that intrigued Darwin and have recently been reviewed by Geist (1966). Used physically in fighting, especially among males during the breeding season, these weapons (Fig. 7) are also used as visual signals. Some elaborate plumages of birds, on the other hand, seem not to be used except for visual display. Wattles, crests, and other elements, although also possessing certain special surface structures for reflection, impart quite different shapes to various birds. Other such signals include extendible throat pouches in lizards (e.g., Crews, 1975), swordtails on fish (e.g., Hemens, 1966; Franck and Hendricks, 1973) and ear tufts on certain cats (e.g., Kleiman and Eisenberg, 1973); examples are shown in Fig. 8.
Surface elements that reflect light in particular ways are divisible along two dimensions: the kind of stimulus they create and the relative permanence of the particular reflection. These elements may create specific brightness contrasts, colors, shapes, or orientations of shapes (Fig. 9).
Surface elements may also be classified according to their relative permanence along a continuum: permanent coloration, such as that of the zebra's stripes or the cardinal's red feathers (also see Noble, 1936); labile coloration of seasonal or relatively long duration, such as the starling's yellow beak during the breeding season (see also Marler, 1955; N. G. Smith, 1966); and modulated coloration that can be changed within the course of a single day, sometimes within seconds, such as blushing in humans and color changes in the octopus (Packard and Sanders, 1971). Cephalopods are particularly adept at rapid color changes (see also Wells and Wells, 1972; Warren et al., 1974), but bony fish may hold the record for both rapidity and diversity of modulated colors (e.g., Frey and Miller, 1972; Figler, 1972; Wickler, 1969; Myrberg, 1972; Markl, 1972; Sale, 1971; Gibson, 1968; Ewing and Evans, 1973; Machemer, 1970; Hamilton and Peterman, 1971; Keenleyside, 1972; Albrecht, 1969; Apfelbach and Leong, 1970; Noakes and Barlow, 1973). Some examples of colorations of varying permanence are illustrated in Fig. 10. Baylis (1974) provides an extensive discussion of the rapidity of color changes in a cichlid fish having elements of permanent coloration, a yellow ground color that requires several days to attain, coloration that is gained or lost in ten seconds, and an overall blanching that requires but two seconds.
OTHER ASPECTS OF INTRINSIC VISUAL SIGNALS
Structural and surface elements of visual signals are usually combined with behavioral elements of orientation, shape, and movement to produce unitary signals. The whippoorwill flashes its usually hidden white tail feathers (Bruce, 1973), and the orientation of attack and threat in canids is correlated with species-specific body markings (Fox, 1969). Morphological elements, such as a rack of antlers or the male cardinal's red plumage, are virtually always visible. It is not always evident whether the coloration is emphasizing behavior, or a movement is displaying a particular color. In some cases the former situation appears to hold (e.g., R. G. B. Brown et al., 1967; Gutherie, 1971a) and in other cases the latter (Otte, 1972; Kahl, 1966; Dingle and Caldwell, 1969; Dunham, 1966). Some examples are shown in Fig. 11.
The intensity of a signal usually refers to various levels or shapes along a single dimension, such as the angle to which a crest is raised in a jay (J. L. Brown, 1964). Such signals show "duality" of patterning (Hockett and Altmann, 1968) because they have qualitative (e.g., crest raised) and quantitative (angle to which raised) aspects. Morris (1957) noted that animals often show modal points of usage along such a continuum—a concept he calls the "typical intensities" of the signal. Varying two display elements along different continua creates a whole range of different visual signals, as in eye-color variation and dorsal-fin-raising in a damselfish (Rasa, 1969) (Fig. 12).
Jenssen (1970) rearranged the sequential and temporal patterns of head movements and dewlap extensions in lizards by means of film loops and showed that atypical patterns reduced the effectiveness of the visual signal in inducing approach by females. The temporal and sequential aspects of signaling are therefore also important and deserve more attention.
PERCEPTION AND PHYSIOLOGICAL MECHANISMS
Unless the effectiveness of a reputed signal is investigated, the assertion that some orientation, posture, movement, structure, or coloration is actually a signal remains hypothetical. The "flehmen" posture of many male mammals is concerned with olfactory communication (Estes, 1972), but in the chamois it may actually be a visual signal as well (Krämer, 1969). On the other hand, many behavioral patterns that look like visual signals (Fig. 13) probably are not (examples in Bovet, 1972; Parker, 1972; Pilleri and Knuckey, 1969; Estes, 1969; Barth, 1970).
G. K. Noble pioneered the use of models to test which elements of a reputed signal affect recipients (e.g., Noble, 1934a, 1934b, 1936; Noble and Vogt, 1935; Noble et al., 1938), and the tradition is still laudably active (e.g., Franck and Hendricks, 1973; Markl, 1972; Hailman, 1967, 1971; Ducker, 1970; Lill, 1968a, 1968b; Peeke, 1969; Stout and Brass, 1969; Crews, 1975; Payne and Swanson, 1972; D. G. Smith, 1972; Keenleyside, 1971; Fox, 1971; Potts, 1973; Youdeowei, 1969; Peeke et al., 1969; Peek, 1972; Deiker and Hoffeld, 1973;Jenssen, 1970; Grantetal., 1970); see Fig. 14.
The experimental analysis of reputed signals should lead to hypotheses about the underlying sensory mechanisms, but there is little progress to report (Hailman, 1970). Tinbergen (e.g., 1951) drew attention to "supernormal" experimental stimuli—those more effective than naturally occurring ones—and other workers continue to find new examples(e.g., Grant et al., 1970; Payne and Swanson, 1972). Such stimuli imply organizational principles about perception, but few studies have pursued the quest to actual mechanisms. Hazlett (1972) tried to relate hermit crab displays to the compound eye, and Fig. 15 summa rizes some of my attempts to uncover visual mechanisms of gull chicks responding to parental signals of shape, orientation, and coloration (Hailman, 1967, 1970, 1971).
Ever since the classic studies of von Hoist and Saint Paul (1963), attempts have been made to find brain areas that when stimulated elicit some signaling behavior. Âkerman's (1965a, 1965b) results may be the most convincing: he elicited normally appearing displays through stimulation of the preoptic nuclear complex and related brain areas of the pigeon, the behavior including bowing, nest demonstration, threat postures, and various pilomotor responses of agonistic behavior (Fig. 16).
Although there is an active literature on hormonal bases of agonistic and reproductive behavior in general, there is little study of visual communication. Orcutt (1971) switched male pigeons from primarily bow-coo to the bow display more prevalently given by females through longterm injections of estrogen. Ducker (1970) injected estradiol into male birds that usually react to the red coloration of other males. The injected males behaved like females in not responding to red, but no treatment of females caused them to respond to red.
CONCLUSIONS CONCERNING CONTROL
In the widest sense there has been little study of the dynamic control of visual communication, either of behavior or underlying physiological mechanisms. Greatest emphasis has been accorded elements of behavior and morphology that act as individual visual signals. The diversity created by various extrinsic signals as well as by orientations, shapes, movements, structures, and colorations of intrinsic signals imparts to communication by reflected light a large informational capacity, which is further increased by "intensities" of signals and their sequential and temporal relations.
Ontogeny of Visual Communication
Ontogenetic determinants of behavior are often divided into genetic and experiential influences, although their interaction provides the most coherent understanding (e.g., Hailman, 1967). Since there is no overall understanding of dynamic control of visual communication, it is difficult to analyze how ontogenetic factors lead to developmental end points. This fact probably accounts for the relative paucity of studies on the ontogeny of optical communication.
GENETICS OF VISUAL COMMUNICATION
The fragmentary evidence about the genetics of specific displays comes primarily from studies of interspecies hybrids and cross-fostering. Gorman (1969) found that displays of a hybrid lizard filmed in the field resembled one parental species in total duration, the other parental species in number of head bobs, and both in aspects of the tail-flick components. Davies (1970) found in hybrid doves that bowing displays resembled one parent or the other, or were intermediate between the parental species, or showed a range of variation that exceeded that bounded by the two parental types (Fig. 17). Analyses of displays of hybrid ducks (Sharpe and Johnsgard, 1966; Kaltenhäuser, 1971) produced similar results. The results suggest that visual displays are polygenically controlled.
By rearing the young of one genotype with parents of another and then testing their behavioral choices in adulthood one can see whether genetic endowment or individual experience plays the major role in recognition of visual displays. Immelmann (1969) found that male estrildid finches courted females of their foster-parent species in preference to their genetic parents. Walter (1973) and Immelmann (1969) reared zebra finches with albino and normally pigmented parents and found that males reared by albinos chose albino mates. Walter further showed that males reared by normally pigmented parents preferred these, whereas those reared by mixed pairs showed no choice of mate color. However, females always preferred pigmented males. It appears in this case as if the male's preference is determined experientially and the female's genetically.
EXPERIENCE AND VISUAL COMMUNICATION
The foregoing results on cross-fostering demonstrate that early experience can affect mate preferences presumed to be primarily visually mediated. There is relatively little evidence, however, concerning experiential effects on the recognition of specific visual signals, on the production of signals, or how the signals are used in communicative behavior. We know from several studies (e.g., Peeke et al., 1969; Clayton and Hinde, 1968) that repeated presentation of a visual signal leads to decremental responses in the recipient, so that at least simple kinds of experience do affect responses to signals.
It is possible that an animal can respond at different ages to the same visual signal, but its perception of that signal changes as a function of experience. For instance, a gull chick will peck at red areas on the parent's beak. Perception in newly hatched chicks is simply coded (Fig. 15), but as chicks accumulate experience with the parents and are fed for responding to the stimulus, they develop a more highly structured, Gestalt A ike preference (Fig. 18). At all ages chicks confine responses primarily to the same physical object (the parent's beak), but the ideal signal changes as a result of experience (Hailman, 1967)—a process I call "perceptual sharpening." Such perceptual sharpening will not be evident from mere field observation.
The extent to which animals learn to produce visual signals is virtually unknown. Tayler and Saayman (1973) showed that a captive dolphin imitatively produced all kinds of behavioral patterns of another species, including visual displays. More complicated is the question of whether specific use of signals requires previous experience. Stephenson (1973) has given examples from Japanese macaques in which the same physical signal is used differently in different troops. Feekes (1972) reported experientially dependent development of ground pecking as a visual display in domestic fowl. Apparently a bird learns that feeding during tension-producing agonistic encounters lowers the psychological tension. Ground pecking then becomes incorporated into the ongoing behavior in this situation and thereby comes to act as a visual signal to the opponent. Furthermore, ground scratching also becomes incorporated by generalization from the related pecking behavior.
Many studies show that the social environment of rearing affects social relations. Most studies are quite general, but Fox (1971) found in canids that social experience facilitated inguinal presentations toward a visual model of a dog, and Fox and Clark (1971) separate general stages of development in which action patterns of display and other behavior become incorporated into increasingly complex sequences. Anthoney (1968) reports that lip smacking in baboons develops first from intention movements of sucking the mother's pink nipples. Young then respond to the mother's own lip smacking, which involves her visually similar pink tongue, so that the learning is facilitated by perceptual transfer. Furthermore, the young baboon's face has pink areas that help to elicit lip smacking from the mother. Social bonds built by the exchange of this signal later generalize to sexual visual communication, where the female's pink sexual skin and the male's pink penis further aid in the transfer of responses (see Fig. 19).
CONCLUSIONS CONCERNING ONTOGENY
There exists only fragmentary evidence for both genetic and experiential effects on visual communication. Genetically determined propensities guide development by experience, perhaps by assuring certain initial behavior that changes through learning processes. Experience can help determine the form, use, or recognition of a visual signal. Probably both the taxonomic group and learning capacities of an animal, as well as the social structure and type of environment, influence the degree to which the development of various aspects of visual communication are experientially dependent.
Function of Visual Communication
The word "function" is used variously in biology (Hailman, 1975); I use it here as shorthand for "adaptive function" or "selective advantage." Were a detailed understanding of the dynamic control of visual communication possible, then one could ask after the pressures of natural selection that shape the total ontogenetic sequences leading to such control. Instead, goals must be limited to exploring the adaptiveness of two aspects of visual communication: situations that favor vision over other modalities, and selective pressures that act to structure the types of visual signals.
SELECTION PROMOTING VISUAL COMMUNICATION
At least three factors promote specific modalities: the environment in which signaling takes place, the kind of animals communicating, and the functional use of the communication. Communication via reflected light obviously requires light for reflection. An environmental factor given less attention is the transparency of the medium, since the turbidity of water or vegetation of a habitat may discourage communication by light. Wootton (1971) notes that visual signals are less well developed in a species of stickleback living in thick vegetation and tea-colored water than in its congeneric relatives in more transparent media. Similarly, Catchpole (1973) shows that the open-habitat sedge warbler has more visual displays than the congeneric reed warbler, which depends instead on vocal displays in its more vegetated habitat. Busnel (1968) points out that high ambient noise may discourage acoustic communication and thereby favor vision or other modalities.
Not all species in groups with well-developed vision use extensive visual communication; among insects, particularly, visual communication is relatively unusual. Perhaps more motile than sessile animals have developed visual signals because sessile animals cannot effectively modulate ambient light even if they have good photoreceptors. Even motile species that possess good vision may conduct important communication at night (e.g., orthopteran insects and anuran amphibians)—perhaps to escape predation and utilize humid conditions—so that their mating signals and other exchanges are largely acoustic. Some, diurnal insects do have visual signals (e.g., Waage, 1973; Otte, 1972), but others enigmatically lack them. Otte (1972) reports Syrbula grasshoppers have visual signals, whereas related genera living in the same habitat lack them. Similarly, Schremmer (1972) notes that male Bombus confusus bees perform looping flights before the female, whereas their congeners rely strictly on pheromonal courtship signals.
The visual mode may have certain advantages over communication by other means. Extrinsic visual signals, for instance, may have a persistence that is difficult to match in other modalities; olfactory signals may persist for hours or days but extrinsic visual signals may persist for years. The great diversity of visual signal elements impart a huge informational capacity to optical communication. Visual signals may be directed toward specific receivers, whereas many other signals (pheromones, sounds, electrical fields) tend to radiate indiscriminately. Visual signals also have high potentialities for indexical and representational qualities: that is, visual signals can point out a specific spatial locus or can mimic in form some physical object. Furthermore, since many visual signals are evolved from intention movements of nonsignal activities (see major section on phylogeny, below) such signals carry predictive information about the subsequent behavior of the sender, information that may be more difficult to encode in other modalities.
SELECTION FOR SIGNAL QUALITIES
It seems unlikely that visual signals are arbitrarily established in evolution. Natural selection is constrained to work on the available variation in a population, so that the physical attributes of a signal may be partly constrained by the phylogenetic origins of the signal (see major section on phylogeny, below). However, many qualities of a signal may be directly adaptive to signaling of certain kinds in certain environments.
The most emphasized signal attribute is species-specificity. It may be of advantage for each similar species of a monophyletic assemblage to have distinctive signals, either to insure monospecific flocking and aggregation or to prevent hybridization or gamete wastage in reproductive behavior. Tinbergen (1951) emphasized Lorenz's suggestion of species-specificity of speculum patterns in the wings of ducks (Fig. 20) as an example of the first case, and much literature emphasizes the species-specificity of courtship displays (e.g., Salmon, 1967; Purdue and Carpenter, 1972; Kroodsma, 1974); see Fig. 21. I doubt if selection for species-specificity is as important as is often believed. Convergence of signals among various species is known (e.g., Moynihan, 1968; Cody, 1969), and the attributes of the convergent signals are still to be explained. Furthermore, many closely related species have quite similar signals, suggesting that slight but consistent differences are sufficient for species recognition. A new search should be instituted for other factors that promote specific attributes of signals.
Experiments are necessary to see what aspect of a visual signal is effective. Crews (1975) found that movements and dewlap extension of the male Carolina anole were important in stimulating the female, but the color of the dewlap was not. The color might (for instance) be adaptive in promoting conspicuousness of the display in a particular kind of visual environment. Rand and Williams (1970) point out considerable redundancy among the visual signals of anole species on Hispaniola; each species differs from the others in many ways. They suggest that some signal features get the message through in one sort of environment, whereas other features are more effective in other habitats.
The effect of the medium on visual signaling in aquatic organisms has not been thoroughly explored. Luria and Kinney (1970) report that as turbidity increases, absorption of light becomes greater at short wavelengths (blue end of the spectrum). Baylis (1974) interprets the yellow (long wavelength) signal coloration of a cichlid fish as adaptive in combating its turbid medium.
Other aspects of signals appear explicable by factors having to do with neither species-specificity nor the signaling environment. The advantage of visual signals in having representational qualities is illustrated by Fig. 19. Gutherie and Petocz (1970) have generalized this notion of "automimicry," in which the visual signal mimics the appearance of some other feature of the animal. Wickler (1967) suggested that the face of the mandrill mimicks its genitals, although this suggestion is controversial (Anthoney, 1968; Dunbar and Dunbar, 1974). Gutherie and Petocz (1970) review various structures and color patterns that resemble canine teeth or antlers and horns in various mammals, and also note submissive signals of males that mimic postures of juveniles or estrous females (see Fig. 22). Gutherie (1971b) interprets rump-patch signals of mammals as a sort of elaborate automimicry of more restrictive ano-genital display, which serves to remotivate the observer from aggression to sex.
Another possibility is that the form of the visual signal is directly related to the function served by the signal. The most frequently repeated example (e.g., Marier, 1968; Wilson, 1972) is that threat displays are selected to make the threatening animal appear larger (or perhaps closer) than it really is. Larger animals do more readily attack smaller individuals, but this fact does not bear on the structure of the signal. Hazlett (1970) provides the only direct experimental evidence: he increased the visual size of a hermit crab's shell and found that this change increased the probability of the crab's winning an encounter. Enigmatically, the same result obtained when the shell's weight (but not its size) was increased!
No discussion of signal qualities is complete without Darwin's (1873) principle of antithesis. Darwin believed that emotions were expressed outwardly as some inevitable result of neural activity, but did not quite realize that such expressions are often visual signals enhanced by natural selection for communication. He discovered several sets of opposite-appearing expressions and hypothesized that "opposite emotions" give rise to opposite expressions; he cited expressions of domestic animals, such as a dog with "hostile intentions" versus one "in a humble and affectionate frame of mind." Fig. 23 shows an example of two postures that appear quite different, if not opposite, and it is parsimonious to think in terms of distinctiveness of signals rather than oppositeness of postulated emotions. New examples of antithesis in visual signals continue to be reported (e.g., Krämer, 1969; Stanley, 1971; Ewer; 1971). Klopman (1968) reports that the Canada goose signals "high attack probabilities" with mouth open, neck coiled, and head aimed at the opponent, and "low attack probabilities" with mouth closed, neck extended horizontally, and head directed away from the opponent.
CONCLUSIONS CONCERNING FUNCTION
The selection pressures promoting visual communication over other types and the pressures that shape attributes of visual signals have not been studied extensively. Certain environmental variables encourage visual signaling, only certain animals are equipped to send and receive optically, and certain aspects of communication favor the use of vision. Species-specificity, although real, is probably overworked as an explanation for signal diversity, yet conditions of the environment that promote specific kinds of signals remain largely unexplored. Visual signals may sometimes have a direct relation with their apparent informational content, but this topic also presents mainly unsolved problems.
Phylogeny of Visual Communication
Natural selection shapes phenotypes of a population but is constrained by the starting materials. It may not be possible to understand the qualities of a visual signal unless one knows its phylogenetic origins.
The postures of gulls in Fig. 24 illustrate Darwin's antithesis principle discussed above. A comparison of Figs. 23 and 24, however, reveals a curious aspect of signals not explicable by antithesis: the threat posture of the sparrow is a head-forward, horizontal stance, whereas the threat posture of the gull is an upright, headretracted stance. Similarly, the fear or anxiety posture of the sparrow is vertical, whereas that of the gull is horizontal. Although both sparrow and gull obey the antithesis principle, they do so with opposite polarities, so that further explanation is required. Since the sparrow attacks by pecking directly at the opponent and the gull attacks by pecking down on top of the opponent, it seems reasonable to assume that the threat postures of these two birds were evolved from their movements of attack. In other words, the probable phylogenetic origins of visual signals help to explain their qualities.
Moynihan (1955) has suggested that the term "display" be restricted to those signals whose evolution has been influenced by their function as signals, even if they serve other functions as well. The evolutionary process by which non communicative behavior becomes a display has been called "ritualization" (Tinbergen, 1951, 1952). In this section I deal with noncommunicative behavioral patterns from which visual displays appear to have evolved: that is, with the phylogenetic origins of visual signals.
ORIGINS IN INTENTION MOVEMENTS
Animals may show incipient or incomplete performances of a motor act, often just prior to or just after the act itself. These incomplete acts are called "intention movements" (Daanje, 1950), without connotation of conscious intention. They often occur in behavioral situations that favor their elaboration and standardization as signals. A bird about to attack an opponent may open its beak in preparation for biting, and the movement, posture, and orientation of such a beak-open act can signal probable attack to an opponent; see Fig. 23.
There appear to be two major sources of visual displays among intention movements—agonistic and reproductive activities—although locomotion, feeding, and other activities have also been reported as origins. Many cases were reviewed by previous authors (e.g., Tinbergen, 1951), so I concentrate on illustrating the diversity of origins with recent examples.
"Agonistic" behavior includes all those activities associated with situations in which fighting, fleeing, or appeasement occur. Many visual displays that have an intimidating effect on the opponent ("threat" displays) bear a striking resemblance to the fighting methods of the species. Examples of sparrows (Fig. 23) and gulls (Fig. 24) were given above, and Fig. 25 illustrates other examples (see also Krämer, 1969; Tyler, 1972; Schweinsburg and Sowls, 1972; van Lawick-Goodall, 1968; Kahl, 1966; Allin and Banks, 1968). Protective responses or flight from the opponent can also lead to visual signals, as in the neck withdrawal of the Canada goose (Raveling, 1970) or leaning away from the opponent in the lemming (Allin and Banks, 1968). Eibl-Eibesfeldt (1970) asserts that ducking, combined with breaking eye contact, has become a submissive signal in human communication, its exact form (nodding, sweeping bows, etc.) being culturally determined.
Many examples of reproductive signals seem evolutionarily derived from noncommunicative reproductive activities (Fig. 26). Intention movements of copulation appear as signals in courtship of many animals, these having been reviewed for birds by Andrew (1957). Lordosislike postures appear to be used not only as a signal of readiness by female mammals (e.g., Tyler, 1972), but also as general submissive signals by both sexes (e.g., van Lawick-Goodall, 1968). And penile erection is an evident display in many primates (e.g., van Lawick-Goodall, 1968). Intention movements of nest building have become visual signals in the courtship of many birds (e.g., Kahl, 1966; Kunkel, 1969; Güttinger, 1970; R. E. Phillips, 1972; Baltin, 1969). Güttinger (1970) reports courtship displays of finches evolved from behavior associated with parental care of the young, and submissive displays appear often to be derived from, or else mimic, the behavior of juvenile animals (e.g., Tinbergen, 1959; Güttinger, 1970; Anthoney, 1968).
A rich source of signals is locomotory intention movements (Daanje, 1950). Since so many behavioral situations involve locomotion, this general activity is contextually placed so as to be easily ritualized (Fig. 27). Earlier works review many examples (e.g., Daanje, 1950; Andrew, 1956; Tinbergen, 1951, 1959) and new examples continue to appear (e.g., Myrberg, 1972; Fishelson, 1970; Sale, 1971; Ewer, 1971; Kunkel, 1967; Kahl, 1966).
Foraging, feeding, and associated activities have also evolved into visual signals (Fig. 28): for instance, the pecking and ground scratching of fowl (Feekes, 1972) and the nursing movements that are the phylogenetic origins and also the ontogenetic origins of lip smacking in baboons (Anthoney, 1968). Smythe (1970) argues that white flash patterns of some mammals, used as warning signals to conspecifics, were originally evolved to entice a predator into betraying its hiding place or making a premature charge, so that antipredator behavior may also give rise to visual signals.
Responses involving orientation of the sense organs are situationally placed so as to be available for evolution into signals themselves (Fig. 29). Looking at an opponent is a visual signal in primates, carnivores, and probably many other animals (e.g., van Lawick-Goodall, 1968; Kleiman and Eisenberg, 1973). The origin need not be visual itself, though, since the ears-up signal of ponies is an acoustical orientation that leads to a visual signal (Tyler, 1972).
Maintenance activities—those action patterns involved with preventive maintenance of the interior and exterior of the body, such as stretching, preening, scratching, grooming, shaking, yawning, etc.—appear in many communicational situations, although not always in an obviously ritualized form (Fig. 29). Sometimes actions of the entire animal are involved, as in rolling, which is elicited by wet fur but is also used as a signal to inhibit flight in conspecifics (Castell et al., 1969). Other times only a limb movement is involved, as in scratching by chimpanzees (van Lawick-Goodall, 1968). The visual signal may in some cases be highly ritualized, as in the bowing and curtseying of waxbills, which evolved from bill wiping (Kunkel, 1967). McKinney (1965) has made an unusually complete analysis of comfort movements in waterfowl and their uses as unritualized and ritualized visual signals.
ORIGINS IN AUTONOMIC RESPONSES
Autonomic responses—those mediated by the sympathetic and parasympathetic nervous systems of vertebrates and usually involving smooth rather than striated muscle—often accompany more dramatic actions of animals and hence are in excellent behavioral situations to become used as signals.
Pilomotor actions—changing of the feather postures of birds or the fur of mammals—are parts of many visual displays (Fig. 30). Morris (1956) proposed that these display components originate in thermoregulatory responses: sleeking feathers decreases the thickness of the insulating layer, whereas fluffing increases it. In extreme cases, Morris (1956) says that feathers may be ruffled so that the air pockets are opened and the insulation made less effective. McFarland and Baher (1968) could not confirm the last point experimentally with ring doves, but did show temperature control of the other feather postures and also their use in presumptive communicative situations.
Tyler (1972) reports yawning (respiratory responses) in ponies as a visual signal, as does van Lawick-Goodall (1968) in chimpanzees. Vasoresponses—shunting the blood differentially to various parts of the body by control of restriction and dilation of blood vessels—has been known for a long time to carry visual consequences (e.g., Cannon, 1915) such as human blushing in embarrassment, flushing in anger, paling in fear, and so on. Longer-term vasoresponses are involved in sex skin coloration of some primates (e.g., Dunbar and Dunbar, 1974).
Stanley (1971) notes the importance of eye closure in hopping mouse behavior, and it is difficult to resist the speculation that the widespread colored eye-rings in birds accentuate the degree of eye closure. Pupillary responses are also used as visual signals, both in cats (Kleiman and Eisenberg, 1973) and in humans (Hess, 1965). Dr. R. Jaeger has pointed out to me that European women formerly used the drug from nightshade (Atropa belladonna) to dilate their pupils to make themselves more attractive to men.
OTHER ORIGINS OF VISUAL SIGNALS
A few phylogenetic origins of visual signals are not readily classified. Many animals possess the ability to match their background coloration (e.g., Gibson, 1968), even though the mechanisms for such color change may be quite different in different species. Perhaps this camouflage-related ability is the basis of many visual signals employing color changes.
There also exists "secondary ritualization," in which a signal evolved for communication in another sensory modality takes on visual properties and may be further changed to enhance the visual component. Several authors report visual signals that have evolved from scent-marking activities of mammals (Rood, 1972; Estes, 1969; Schmied, 1973), as noted in Fig. 31. The production of sound often involves assuming a specific posture, which may then become a visual signal, as in the head-tipping display of the chimpanzee ritualized from "soft-bark" calling (van LawickGoodall, 1968).
CONCLUSIONS CONCERNING PHYLOGENY
The evolutionary origin of a particular signal may never be identified with the same degree of certainty that other conclusions in ethology can be secured. Yet comparative study among species and the comparison of visual signals with noncommunicative behavior within a species offer convincing probabilities of many origins. Virtually any behavior that occurs in a situation of interaction between animals may be enhanced in some way to increase its value as a visual signal.
Clearly there remains much to be learned about the control, ontogeny, function, and phylogeny of optical communication. To judge from recent studies, the most immediately promising areas for new results may be motivational and contextual factors in communication, temporal and sequential patterns of signals, ontogenic and traditional determinants of all aspects, mechanisms of perception, and environmental structuring of signals. We are a very long way from a theoretical framework that will encompass the claw waving of a fiddler crab and the bower of the bowerbird under the same roof as the writing, painting, sculpture, and dance of man.
Åkerman, B., 1965a. Behavioural effects of electrical stimulation in the forebrain of the pigeon. I.Reproductive behaviour. Behaviour, 26:323-38.
Åkerman, B., 1965b. Behavioural effects of electrical stimulation in the forebrain of the pigeon. II. Protective behaviour. Behaviour, 26:339-50.
Albrecht, H., 1969. Behaviour of four species of Atlantic damselfishes from Colombia, South America (Abundefduf saxatilis, A. taurus, Chromis multilineata, C. eyanea; Pisces, Pomacentridae). Z. Tierpsychol., 26:662-76.
Allin, J. T., and Banks, E. M. 1968. Behavioural biology of the collared lemming Dicrostonyx groenlandicus (Traill): I. Agonistic behaviour. Anim. Behav., 16:245-62.
Andrew, R.J., 1956. Intention movements of flight in certain passerines, and their use in systematics. Behaviour, 10:179-204.
Andrew, R. J., 1957. The aggressive and courtship behaviour of certain emberizines. Behaviour, 10:255-308.
Andrew, R. J., 1963. The origin and evolution of the calls and facial expressions of the primates. Behaviour, 20:1-109.
Anthoney, T. R., 1968. The ontogeny of greeting, grooming, and sexual motor patterns in captive baboons (superspecies Papio cynocephalus). Behaviour, 31:358-72.
Apfelbach, R., and Leon, D., 1970. Zum Kampfverhalten in der Gattung Tilapia (Pisces, Cichlidae). Z. Tierpsychol., 27:98-107.
Baltin, S., 1969. Zur Biologie und Ethologie des Talegalla-Huhns (Alectura lathami Gray) unter besonderer Berücksichtigung des Verhaltens während der Brutperiode. Z. Tierpsychol., 26:524-72.
Barash, D. P., 1973. The social biology of the Olympic marmot. Anim. Behav. Monogr., 6:171-245.
Barth, R. H., 1970. The mating behavior of Periplaneta americana (Linnaeus) and Blatta orientalis Linnaeus (Blattaria, Blattinae), with notes on 3 additional species of Periplaneta and interspecific action of female sex pheromones. Z. Tierpsychol., 27:722-48.
Baylis, J. R., 1974. The behaviour and ecology of Herotilapia multispinosa (Teleostei, Cichlidae). Z. Tierpsychol., 34:115-46.
Bovet, j., 1972. On the social behavior in a stable group of long-tailed field mice (Apodemus sylvaticus). I. An interpretation of defensive postures. Behaviour, 41:43-54.
Brown, J. L., 1964. The integration of agonistic behaviour in the Steller's jay Cyanocitta stellen (Gmclin). Univ. Calif. Publ. Zool., 60:223-328.
Brown, R. G. B.; Blurton Jones, N. G.; and Russell, D. J. T.; 1967. The breeding behaviour of Sabine's gull, Xema sabini. Behaviour, 28:110-40.
Bruce, J. A., 1973. Tail flashing display in the Whippoor-will. Auk, 90(3):682.
Busnel, R. G., 1968. Acoustic communication. In Animal Communication, T. A. Sebeok, ed. Bloomington: Indiana University Press, pp. 127-53.
Cannon, W. B., 1915. Bodily Changes in Pain, Hunger, Fear and Rage. New York: W. W. Norton.
Carpenter, C. C., 1963. Patterns of behavior in three forms of the fringe-toed lizards (Uma Iguanidae). Copeia, 1963:406-12.
Castell, R.; Krohn, H.; and Ploog, D.; 1969. Rückenwälzen bei Totenkopfaffen (Saimiri sciureus): Körperpflege und soziale Funktion. Z. Tierpsychol., 26:488-97.
Catchpole, C. K., 1973. The functions of advertising song in the sedge warbler (Acrocephalusschoenobaenus) and the reed warbler (A. scirpaceus). Behaviour, 46:300-20.
Clayton, F. L., and Hinde, R. A., 1968. The habituation and recovery of aggressive display in Betta splendens. Behaviour, 30:96-106.
Cody, M. L., 1969. Convergent characteristics in sympatric populations: a possible relation to the interspecific territoriality. Condor, 71:222-39.
Cole, J. E., and Ward, J. A., 1969. The communicative function of pelvic fin-flickering in Etroplus maculatus (Pisces, Cichlidae). Behaviour, 35:179-99.
Crews, D., 1975. Effects of different components of male courtship behaviour on environmentallyinduced ovarian recrudescence and mating preferences in the lizard, Anolis carolinensis. Anim. Behav., 23:349-56.
Daanje, A., 1950. On locomotory movements in birds and the intention movements derived from them. Behaviour, 3:48-98.
Darwin, C., 1871. The Descent of Man. New York: D. Appleton.
Darwin, C., 1873. Expression of the Emotions in Man and Animals. New York: D. Appleton.
Davies, S.J.J. F., 1970. Patterns of inheritance in the bowing display and associated behaviour of some hybrid Streptopelia doves. Behaviour, 36:187-214.
Deiker, T. E., and Hoffeld, D. R., 1973. Interference with ritualized threat behaviour in Cichlasoma nigrofasciatum. Anim. Behav., 21:607-12.
Dingle, H., and Caldwell, R. L., 1969. The aggressive and territorial behaviour of the mantis shrimp Gonodactylus bredini Manning (Crustacea:Stomatopoda). Behaviour, 33:115-36.
Dubost, G., 1971. Observations ethologiques sur le Muntjak (Muntiacus muntjak) Zimmermann 1780 et M. reevesi (Ogilby 1839) en captivité et semi-liberte. Z. Tierpsychoi, 28:387-427.
Ducker, G., 1970. Untersuchungen über die hormonale Beeinflubbarkeit der Farbbevorzugung von Feuerwebern (Euplectes orixfranciscanus). f. Ornithoi, 111(1): 19—29.
Dunbar, R. I. M., and Dunbar, P., 1974. The reproductive cycle of the gelada baboon. Anim. Behav., 22:203-10.
Dunham, D. W., 1966. Agonistic behavior in captive rose-breasted grosbeaks, Pheuticus ludovicianus (L.). Behaviour, 27:160-73.
Eibl-Eibesfeldt, I., 1970. Ethology: The Biology of Behavior. Holt, Rinehart and Winston. 530pp.
Estes, R. D., 1969. Territorial behavior of the wildebeest (Connochaetes taurinus Burchell, 1823). Z. Tierpsychoi, 26:284-370.
Estes, R. D., 1972. The role of the vomeronasal organ in mammalian reproduction. Mammalia, 36:315-41.
Evans, L. T., 1961. Structure as related to behavior in the organization of populations in reptiles. In: Vertebrate Speciation, W. F. Blair, ed. Austin: University of Texas Press.
Ewer, R. F., 1971. The biology and behaviour of a free-living population of black rats (Rattus rattus). Anim. Behav. Monogr., 4:127-74.
Ewing, A. W., and Evans, V., 1973. Studies on the behaviour of cyprinodent fish. I. The agonistic and sexual behaviour of Aphyosemion bivittatum (Lönnberg 1895). Behaviour, 46:264-78.
Feekes, F., 1972. "Irrelevant" ground pecking in agonistic situations in Burmese red junglefowl (Gallus gallus spadiceus). Behaviour, 43:186-326.
Figler, M. H., 1972. The relation between eliciting stimulus strength and habituation of the threat display in male Siamese fighting fish, Betta splendens. Behaviour, 42:63-96.
Fishelson, L., 1970. Behaviour and ecology of a population of Abudefduf saxatilis (Pomancentridae, Teleostei) at Eilat (Red Sea). Anim. Behav., 18:225-37.
Fox, M. W., 1969. The anatomy of aggression and its ritualization in Canidae: a developmental and comparative study. Behaviour, 35:242-58.
Fox, M. W., 1971. Socioinfantile and socio-sexual signals in canids: a comparative and ontogenetic study. Z. Tierpsychoi, 28:185-210.
Fox, M. W., and Clark, A. L. 1971. The development and temporal sequencing of agonistic behavior in the coyote (Canis latrans). Z. Tierpsychoi, 28:262-78.
Franck, D., 1968. Weitere Untersuchungen zur vergleichenden Ethologie der Gattung Xiphophorus (Pisces). Behaviour, 30:76-95.
Franck, D., and Hendricks, R., 1973. Zur Frage der biologischen Bedeutung des Schwertfortsatzes von Xiphophorus hellen. Behaviour, 44:167-85.
Frey, D. F., and Miller, R. J., 1972. The establishment of dominance relationships in the blue gourami, Trichogaster trichopterus (Pallas). Behaviour, 42:8-62.
Geist, V., 1966. The evolution of horn-like organs. Behaviour, 27:175-214.
Gibson, R. N., 1968. The agonistic behaviour of juvenile Blenniuspholis L. (Teleostei). Behaviour, 30:192217.
Golani, I., and Mendelssohn, H., 1971. Sequences of precopulatory behavior of the jackal (Canis aureus L.). Behaviour, 38:169-92.
Gorman, G. C., 1969. Intermediate territorial display of a hybrid Anolis lizard (Sauria: Iguanidae). Z. Tierpsychoi, 26:390-93.
Grant, E. C.; Mackintosh, J. H.; and Lerwill, C. J.; 1970. The effect of a visual stimulus on the agonistic behaviour of the golden hamster. Z. Tierpsychoi, 27:73-77.
Gutherie, R. D., 1971a. The evolutionary significance of the cervid labial spot. J. Mammal., 52:209-12.
Gutherie, R. D., 1971b. A new theory of mammalian rump patch evolution. Behaviour, 38:132-45.
Gutherie, R. D., and Petocz, R. G., 1970. Weapon automimicry among mammals. Amer. Nat., 104:58588.
Güttinger, H. R., 1970. Zur Evolution von Verhaltensweisen und Lautäusserungen bei Prachtfinken (Estrilidae). Z. Tierpsychoi, 27:1011-75.
Hailman, J. P., 1967. The ontogeny of an instinct. Behaviour Suppl., 15:1-159.
Hailman, J. P., 1970. Comments on the coding of releasing stimuli. In: Development and Evolution of Behavior, L. R. Aronson, E. Tobach, D. S. Lehrman, and J. S. Rosenblatt, eds. San Francisco: W. H. Freeman, pp. 138-57.
Hailman, J. P., 1971. The role of stimulus-orientation in eliciting the begging response from newlyhatched chicks of the laughing gull (Larus atricilla). Anim. Behav., 19:328-35.
Hailman, J. P., 1976. Uses of the comparative study of behavior. In: Evolution, Brain and Behavior: Persistent Problems, R. B. Masterson, W. Hodos, and H. J. Jerison, eds. Hillsdale, N.J.: Erlbaum, pp. 13-22.
Hailman, J. P. In press. Optical Signals: Animal Communication and Light. Bloomington: Indiana University Press.
Hall, D. D., and Miller, R. J., 1968. A qualitative study of courtship and reproductive behavior in the pearl gourami, Trichogaster leeri (Bleeker). Behaviour, 32:70-84.
Hamilton, W.J. Ill, and Peterman, R. M., 1971. Countershading in the colourful reef fish Chaetodon lunula: concealment, communication or both? Anim. Behav., 19:357-64.
Hazlett, B. A., 1970. The effect of shell size and weight on the agonistic behavior of a hermit crab. Z. Tierpsychol., 27:369-74.
Hazlett, B. A., 1972. Stimulus characteristics of an agonistic display of the hermit crab (Calcinus tibicen). Anim. Behav., 20:101-107.
Hazlett, B. A., and Bossert, W. H., 1965. A statistical analysis of the aggressive communications systems of some hermit crabs. Anim. Behav., 13:347-73.
Hazlett, B. A., and Estabrook, G. F., 1974a. Examination of agonistic behavior by character analysis. I. The spider crab Microphrys bicornutus. Behaviour, 48:131-43.
Hazlett, B. A., and Estabrook, G. F., 1974b. Examination of agonistic behavior by character analysis. II. Hermit crabs. Behaviour. 49:88-110.
Hemens, J., 1966. The ethological significance of the sword-tail in Xiphophorus hellerii (Haekel). Behaviour, 27:290-315.
Hess, E. H., 1965. Attitude and pupil size. Sei. Amer., 212(4):46-54.
Hinde, R. A., 1955-56. A comparative study of the courtship of certain finches. Ibis, 97:706-45; 98:128.
Hockett, C. F., and Altmann, S. A., 1968. A note on design features. In: Animal Communication, T. A. Sebeok, ed. Bloomington: Indiana University Press, pp.61-72.
Holst, E. von, and St. Paul, U., 1963. On the functional organization of drives Brit. J. Anim. Behav., 11:1-20.
Hunsaker, D., 1962. Ethological isolating mechanisms in the Sceloporus torquatus group of lizards. Evolution, 14:62-74.
Immelmann, K., 1969. Uber den Einfluss frühkindlicher Erfahrungen auf die geschlechtliche Objektfixierung bei Estrildiden. Z Tierpsychol., 26:675-91.
Jenssen, T. A., 1970. Female response to filmed displays of Anolis nebulosus (Sauria, Iguanidae). Anim. Behav., 18:640-47.
Kahl, M. P., 1966. Comparative ethology of the Ciconiidae. Part 1. The Marabou stork, Leptoptilos crumeniferus (Lesson). Behaviour, 27:76-106.
Kahl, M. P., 1972. Comparative ethology of the Ciconiidae. Part 4. The "typical" storks (Genera Ciconia, Sphenorhynchus, Dissoura, and Euxenura). Z. Tierpsychol., 30:225-52.
Kaltenhausen D., 1971. Über Evolutionsvorgänge in der Schwimmentenbalz. Z Tierpsychol., 29:481-540.
Keenleyside, M. H. A., 1971. Aggressive behavior of male longear sunfish (Lepomis megalotis). Z. Tierpsychol., 28:227-40.
Keenleyside, M. H., 1972. The behaviour of Abudefduf zonatus (Pisces, Pomacentridae) at Heron Island, Great Barrier Reef. Anim. Behav., 20:763-74.
Kilham, L., 1954. Territorial behavior of the red squirrel.J. Mammal., 35:252-53.
Kleiman, D. G., and Eisenberg, J. F., 1973. Comparisons of canid and felid social systems from an evolutionary perspective. Anim. Behav., 21:637-59.
Klopfer, P. H., and Hailman, J. P., 1967. An Introduction to Animal Behavior: Ethology's First Century.Englewood Cliffs, N.J.: Prentice-Hall. 297pp.
Klopman, R. B., 1968. The agonistic behavior of the Canada goose (Branta canadensis canadensis). I. Attack behavior. Behaviour, 30:287-319.
Krämer, A., 1969. Soziale Organisation und Sozialverhalten einer Gemspopulation (Rupicapra rupicapra L.) der Alpen. Z Tierpsychol., 26:889-964.
Kroodsma, R. L., 1974. Species-recognition behavior of territorial male rose-breasted and black-headed grosbeaks (Pheucticus). Auk, 91 ( 1 ):54—64.
Kühme, W., 1961. Beobachtungen am afrikanischen Elefanten (Loxodonta africana Blumenbach 1797) in Gefangenschaft. Z Tierpsychol., 18:285-96.
Kunkel, P., 1967. Displays facilitating sociability in waxbills of the genera Estrilda and Lagonosticta (Fam. Estrildidae). Behaviour, 29:231-61.
Kunkel, P., 1969. Zur Rückwirkung der Nestform auf Verhalten und Auslöserausbildung bei den Prachtfinken (Estrilididae). Z Tierpsychol., 26:27783.
LaFollette, R. M., 1971. Agonistic behaviour and dominance in confined wallabies, Wallabia rufogrisea frutica. Anim. Behav., 19:93-101.
Lill, A., 1968a. An analysis of sexual isolation in the domestic fowl: I. The basis of homogamy in males. Behaviour, 30:107-26.
Lill, A. 1968b. An analysis of sexual isolation in the domestic fowl: II. The basis of homogamy in females. Behaviour, 30:127-45.
Lorenz, K., 1958. The evolution of behavior. Sei. Amer., 199(6):67-78.
Luria, S. M., and Kinney, J. A., 1970. Underwater Vision. Science, 167:1454-61.
McBride, G.; Parer, I. P.; and Foenander, F.; 1969. The social organization and behaviour of the feral domestic fowl. Anim. Behav. Monog., 2:127-81.
McFarland, D. J., and Baher, E., 1968. Factors affecting feather posture in the Barbary dove. Anim. Behav., 16:171-77.
Machemer, L., 1970. Qualitative und quantitative Verhaltensbeobachtungen an Paradiesfisch-♂♂, Macropodus opercularis L. (Anabantidae, Teleostei). Z. Tierpsychoi, 27:563-90.
McKinney, F., 1965. The comfort movements of Anatidae. Behaviour, 25:120-220.
Markl, H., 1972. Aggression und Beuteverhalten bei Piranhas (Serrasalminae, Characidae). Z. Tierpsychoi, 30:190-216.
Marler, P., 1955. Studies of fighting in chaffinches. 2. the effect on dominance relations of disguising females as males. Brit. J. Anim. Behav., 3:137-46.
Marler, P., 1968. Visual systems. In: Animal Communication, T. A. Sebeok, ed. Bloomington: Indiana University Press, pp. 103-26.
Marshall, A. J., 1954. Bowerbirds: Their Displays and Breeding Cycles. New York: Oxford University Press.
Morris, D., 1956. The feather postures of birds and the problem of the origin of social signals. Behaviour, 9:75-113.
Morris, D., 1957. "Typical intensity" and its relationship to the problem of ritualisation. Behaviour, 11:112.
Moynihan, M., 1955. Remarks on the original sources of displays. Auk, 72:240-46.
Moynihan, M., 1968. Social mimicry: character convergence versus character displacement. Evolution, 22:315-31.
Myrberg, A. A., Jr., 1972. Ethology of the bicolor damselfish, Eupomacentrus partitus (Pisces: Pomacentridae): a comparative analysis of laboratory and field behaviour. Anim. Behav. Monog., 5:197-283.
Noakes, D. L. G., and Barlow, G. W., 1973. Crossfostering and parent-offspring responses in Cichlasoma citrinellum (Pisces, Cichlidae). Z. Tierpsychoi, 33:147-52.
Noble, G. K., 1934a. Experimenting with the courtship of lizards. Nat. Hist., 34:3-15.
Noble, G. K., 1934b. Sex recognition in the sunfish, Eupomotis gibbosus (Linne). Copeia, 1934:151 -54.
Noble, G. K., 1936. Courtship and sexual selection of the flicker. Auk, 53:269-82.
Noble, G. K., and Vogt, W., 1935. An experimental study of sex recognition in birds. Auk, 52:278-86.
Noble, G. K.; Wurm, M.; and Schmidt, A.; 1938. Social behavior of the black-crowned night heron. Auk, 55:7-41.
Orcutt, F. S., Jr., 1971. Effects of estrogen on the differentiation of some reproductive behaviours in male pigeons (Columba livia). Anim. Behav., 19:27786.
Otte, D., 1972. Simple versus elaborate behavior in grasshoppers: an analysis of communication in the genus Syrbula. Behaviour, 42:291-322.
Packard, A., and Sanders, G. D., 1971. Body patterns of Octopus vulgaris and maturation of the response to disturbance. Anim. Behav., 19:780-90.
Parker, G. A., 1972. Reproductive behaviour of Sepsis cynipsea (L.) (Diptera:Sepsidae). I. A preliminary analysis of the reproductive strategy and its associate behaviour patterns. Behaviour, 41:172-206.
Payne, A. P., and Swanson, H. H., 1972. The effect of a supra normal threat stimulus on the growth rates and dominance relationships of pairs of male and female golden hamsters. Behaviour, 42:1-7.
Peek, F. W., 1972. An experimental study of the territorial function of vocal and visual display in the male red-winged blackbird (Agelaius phoeniceus). Anim. Behav., 20:112-18.
Peeke, H. V. S., 1969. Habituation of conspecific aggression in the three-spined stickleback (Gasterosteus aculeatus L.). Behaviour, 35:137-56.
Peeke, H. V. S.; Wyers, E. J.; and Herz, M. J.; 1969. Waning of the aggressive response to male models in the three-spined stickleback (Gasterosteus aculeatus L.). Anim. Behav., 17:224-28.
Phillips, R. E., 1972. Sexual and agonistic behaviour in the killdeer (Charadrius vociferus). Anim. Behav., 20:1-9.
Phillips, R. R., 1971. The relationship between social behavior and the use of space in the benthic fish Chasmodes bosquianus Lacepede (Teleostei, Blenniidae). I. Ethogram. Z. Tierpsychoi, 29:11-27.
Pilleri, G., and Knuckey, J., 1969. Behaviour patterns of some Delphinidae observed in the Western Mediterranean. Z. Tierpsychoi, 26:48-72.
Potts, G. W., 1973. The ethology of Labroides dimidiatus (Cuv. 8c Val.) (Labridae, Pisces) on Aldabra. Anim. Behav., 21:250-91.
Purdue, J. R., and Carpenter, C. C., 1972. A comparative study of the body movements of displaying males of the lizard genus Sceloporus(Iguanidae). Behaviour, 41:68-81.
Quanstrom, W. R., 1971. Behaviour of Richardson's ground squirrel Spermophilus richardsonii richardsonii. Anim. Behav., 19:646-52.
Rand, S. A., and Williams, E. E., 1970. An estimation of redundancy and information content of anole dewlaps. Amer. Nat., 104:99-103.
Rasa, O. A. E., 1969. Territoriality and the establishment of dominance by means of visual cues in Pomaeentrus jenkinsi (Pisces:Pomacentridae). Z. Tierpsychol., 26:825-45.
Raveling, D. G., 1970. Dominance relationships and agonistic behavior of Canada geese in winter. Behaviour, 37:291-319.
Reese, E. S., 1962. Submissive posture as an adaptation to aggressive behavior in hermit crabs. Z Tierpsychol., 19:645-51.
Rhijn, J. G. van, 1973. Behavioural dimorphism in male ruffs, Philomachus pugnax (L.). Behaviour, 47:153-229.
Rood, J. P., 1972. Ecological and behavioural comparisons of three genera of Argentine cavies. Anim. Behav. Monog., 5:1-83.
Rovner, J. S., 1968. An analysis of display in the lycosid spider Lycosa rabida Walckenaer. Anim. Behav., 16:358-69.
Saayman, G. S.; Tayler, C. K; and Brower, D.; 1973. Diurnal activity cycles in captive and free-ranging Indian Ocean bottlenose dolphins ( Tursiops aduncus Ehrenburg). Behaviour, 44:212-23.
Sale, P. F., 1971. The reproductive behaviour of the Pomacentrid fish, Chromis caeruleus. Z. Tierpsychol., 29(2): 156-64.
Salmon, M., 1967. Coastal distribution, display and sound production by Florida fiddler crabs (Genus Uca). Anim. Behav., 15:449-59.
Schleidt, W. M., 1973. Tonic communication: continual effects of discrete signs in animal communication systems .J. Theor. Biol., 42:359-86.
Schmidt, U., and van de Flierdt, K., 1973. Innerartliche Aggression bei Vampirfledermäusen (Desmodus rotundus) am Futterplatz. Z Tierpsychol., 32:139-46.
Schmied, A., 1973. Beiträge zu einem Aktionssystem der Hirschziegenantilope (Antilope cervicapra Linne 1758). Z. Tierpsychol., 32:153-98.
Schremmer, F., 1972. Beobachtungen zum Paarungsverhalten der Männchen von Bombus confusus Schenck. Z. Tierpsychol., 31:503-12.
Schweinsburg, R. E., and Sowls, L. K., 1972. Aggressive behavior and related phenomena in the collared peccary. Z Tierpsychol., 30(2): 132-45.
Scruton, D. M., and Herbert, J., 1972. The reaction of groups of captive talapoin monkeys to the introduction of male and female strangers of the same species. Anim. Behav., 20:463-73.
Sebeok,T. A., ed., 1968. Animal Communication. Bloomington: Indiana University Press.
Shank, C. C., 1972. Some aspects of social behaviour in a population of feral goats (Capra hircus L.). Z. Tierpsychol., 30:488-28.
Sharpe, R. S., and Johnsgard, P. A., 1966. Inheritance of behavioral characters in F2 mallard X pintail (Anas platyrhynchos L. X Anas acuta L.) hybrids. Behaviour, 27:259-72.
Simpson, M.J. A., 1968. The display of the Siamese fighting fish, Betta splendens. Anim. Behav. Monog., 1:1-73.
Smith, D. G., 1972. The role of the epaulets in the red-winged blackbird (Agelaiusphoeniceus) social system. Behaviour, 41:251-68.
Smith, N. G., 1966. Evolution of some arctic gulls (Larus)'. an experimental study of isolating mechanisms. Ornithol. Monogr., 4:1-99.
Smith, W. J., 1965. Message, meaning and context in ethology. Amer. Nat., 99:405-409.
Smith, W.J., 1968. Message-meaning analyses. In: Animal Communication, T. A. Sebeok, ed. Bloomington: Indiana University Press, pp.44-60.
Smythe, N., 1970. On the existence of "pursuit invitation" signals in mammals. Amer. Nat., 104:491-94.
Spinage, C. A., 1969. Naturalistic observations on the reproductive and maternal behaviour of the Uganda Defassa waterbuck Kobus defassa ugandae Neumann. Z Tierpsychol., 26:39-47.
Spivak, H., 1971. Ausdrucksformen und soziale Beziehungen in einer Dschelada-Gruppe (Theropithecus gelada) im Zoo. Z. Tierpsychol., 28:279-96.
Stamps, J. A., and Barlow, G. W., 1973. Variation and stereotypy in the displays of Anolis aeneus (Sauria: Iguanidae). Behaviour, 47:67-94.
Stanley, M., 1971. An ethogram of the hopping mouse, Notomys alexis. Z. Tierpsychol., 29:225-58.
Steiner, A. L., 1971. Play activity of Columbian ground squirrels. Z. Tierpsychol., 28:247-61.
Stephenson, G. R., 1973. Testing for group specific communication patterns in Japanese macaques. In: Symp. IVth Int. Congr. Primat., vol. 1 : PreculturalPrimate Behavior, W. Montagna and E. W. Menzel, Jr., eds. Basel: Karger, pp.51-75.
Stout, J. F., and Brass, M. E., 1969. Aggressive communication by Larus glaucescens. Part II. Visual communication. Behaviour, 34:42-54.
Sturm, H., 1973. Zur Ethologie von Trithyreus Sturmi Kraus (Arachnida, Pedipalpi, Schizopeltidia). Z. Tierpsychoi, 33:113-40.
Tayler, C. K., and Saayman, G. S., 1973. Imitative behaviour by Indian Ocean bottlenose dolphins (Tursiops aduncus) in captivity. Behaviour, 44:286-98.
Tinbergen, N., 1951. The Study of Instinct. London: Oxford Press. 228pp.
Tinbergen, N., 1952. "Derived" activities: their causation, biological significance, origin, and emancipation during evolution. Quart. Rev. Biol., 27:1-32.
Tinbergen, N., 1953. The Herring Gulls World. London: Collins.
Tinbergen, N., 1959. Comparative studies of the behaviour of gulls (Laridae): a progress report. Behaviour, 15:1-70.
Tinbergen, N., 1963. On aims and methods of ethology. Z. Tierpsychoi, 20:410.
Tyler, S. J., 1972. The behaviour and social organization of the New Forest ponies. Anim. Behav. Monog., 5:85-196.
van Lawick-Goodall, J., 1968. The behaviour of free living chimpanzees in the Gombe Stream Reserve. Anim. Behav. Monogr., 1 (3) : 161-311.
Waage, J. K., 1973. Reproductive behavior and its relation to territoriality in Calopteryx maculata (Beauvois) (Odonata: Calopterygidae). Behaviour, 46: 240-56.
Walter, M. J., 1973. Effects of parental colouration on the mate preference of off-spring in the zebra finch, Taeniopygia guttata castanotis Gould. Behaviour, 46:154-73.
Walter, R. O., and Hamilton, J. B., 1970. Head-up movements as an indicator of sexual unreceptivity in female medaka, Oryzias latipes. Anim. Behav., 18:125-27.
Walther, F., 1964. Einige Verhaltensbeobachtungen an Thomsongazellen (Gazella thomsoni Gunther, 1884) im Ngorongoro-Krater. Z. Tierpsychoi, 21:871-90.
Walther, F. R., 1969. Flight behaviour and avoidance of predators in Thomson's gazelle (Gazella thomsoni Guenther 1884). Behaviour, 34:184-221.
Warren, L. R.; Scheier, M. F.; and Riley, D. A.; 1974. Colour changes of Octopus rubescens during attacks on unconditioned and conditioned stimuli. Anim. Behav., 22:211-19.
Wells, M. J., and Wells, J., 1972. Sexual displays and mating of Octopus vulgaris Cuvier and O. cyanea Gray and attempts to alter performance by manipulating the glandular condition of the animals. Anim. Behav., 20:293-308.
Wickler, W., 1967. Socio-sexual signals and their intraspecific imitation among primates. In: Primate Ethology, D. Morris, ed. Chicago: Aldine, pp.69147.
Wickler, W., 1969. Zur Soziologie des Brabantbuntbarsches, Tropheus moorei (Pisces, Cichlidae). Z. Tierpsychoi, 26:967-87.
Wilson, E. O., 1972. Animal communication. Sei. Amer., 227(3):52-60.
Winkler, H., 1972. Beiträge zur Ethologie des Blutspechts (Dendrocopos syriacus). Das nicht-reproduktive Verhalten. Z. Tierpsychoi, 31:300-25.
Wooton, R. J., 1971. Measures of the aggression of parental male three-spined sticklebacks. Behaviour, 40:228-62.
Youdeowei, A., 1969. The behaviour of a cotton stainer, Dysdercus intermedium Distant (Heteroptera: Pyrrhocoridae) towards models and its significance for aggregation. Anim. Behav., 17:232-37.
Zumpe, D., and Michael, R. P., 1970. Redirected aggression and gonadal hormones in captive rhesus monkeys (Macaca mulatto). Anim. Behav., 18:11-19.