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.

Fig. 1. Examples of extrinsic visual signals. Left: Bower of a regent bowerbird (Sericulus chrysocephalus) decorated internally with palm seeds; like other avenue builders, this species paints the inside walls with juice of plants and its saliva, but other species have far more elaborate display structures (after Marshall, 1954:plate 17). Right: A blackbuck (Antilope cervicapra) creating a visual mark of its territory by sweeping its horns through the grass. (After Schmied, 1973:165.)

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.

Fig. 2. Examples of orientational visual signals. Left: A male jackal (Canis aureus) stays behind and slightly to the side of a female during precopulatory behavior (after Golani and Mendelssohn, 1971:Fig. 35). Right: Rose-breasted grosbeak (Pheucticus ludovicianus) facing its opponent by turning the head to the side in a resting posture. (After Dunham, 1966:164.)

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).

Fig. 3. Examples of signal shapes. Lett: A wildebeest (Connochaetes taurinus) assumes the "static-optic advertising display" by postural adjustments of body parts. (After Estes, 1969:313.) Right: A collared peccary (Dicotyles tajacu) creates the "intense curiosity" shape primarily through piloerection along its back. (After Schweinsburg and Sowls, 1972:142.) A third method of creating a specific shape is inflation of body parts with air.

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.

Fig. 4. Examples of signal movements. Left: "Rolling" by an Argentinian cavy (Galea musteloides) creates a visual signal by gross body movements. (After Rood, 1972). Right: "Tail flicking" of the Richardson's ground squirrel (Spermophilus richardsonii) creates a visual signal by movement of a body part, in which the tail is raised, lowered part way, moved in a circle, and then lowered to the ground. (After Quanstrom, 1971:647.)

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.

Fig. 5. Examples of combinational pairs of signal elements. Top left: A male Mediterranean octopus (Octopus vulgaris) assumes the "sucker display" involving a special posture and orientation toward the female, but no movement. (After Wells and Wells, 1972:300.) Top right: "Stotting" by a Thomson's gazelle (Gazella thomsoni) involves a special posture and movement, but no apparently specific orientation toward the intended receiver. (After Walther, 1964:872.) Bottom: "Transverse approach" by the piranha (Serrasalmus nattereri) involves oriented movement toward the opponent without a change from normal body shape. (After Markl, 1972:192.)

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.

Fig. 6. Examples of visual signals combining shape, orientation, and movement simultaneously. Left: Male chimpanzee (Pan troglodytes) "brandishing a stick prior to throwing it towards his mirror image." (After van Lawick-Goodall, 1968:240.) Right: The "fan" display of an anole lizard (Anolis aeneus), in which the legs are extended such that the head is raised, the dewlap is extended down, and the posture is displayed laterally to a conspecific. (After Stamps and Barlow, 1973:69.)

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.

Fig. 7. Examples of structural elements secondarily elaborated for visual signaling. Top: Teeth and facial warts of a suid pig (Phacechoerus), showing structures greatly elaborated for visual display. (After Geist, 1966:194.) Bottom: Horns in the bovid sheep (Ovis dalli) on left and antelope (Antelope) on right, elaborated in different ways for visual display. (After Geist, 1966:203.)

Fig. 8. Examples of structural elements evolved primarily for visual signaling. Top: The swordlike tail of the swordtail (Xiphophorus hellerii), used in various displays. (After Hemens, 1966:293.) Bottom: Ear tufts of the caracal (Felis caracal), cited by Kleiman and Eisenberg (1973:646) as enhancing the "ear-flipping" signal. (Drawn from a photograph.)

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.

Fig. 9. Examples of how coloration encodes visual information. Top left: Contrast polarity is opposite in the male hooded warbler ( Wilsonia citrina), which has a yellow mask on a black head, and the male yellowthroat (Geothlypis trichas), which is a warbler with a black mask on the yellow head. Top right: Differences in shape are evident in the white facial spot of the common (Bucephala clangula) and Barrow's goldeneye (B. islandica) male ducks. Bottom left: Differences in orientation of similar color markings are evident in the American green-winged teal (Anas crecca carolinensis), which has a vertical white stripe, and the European common teal (A.c. crecca), in which the white stripe is horizontal. Bottom right: Even when patterns of coloration are similar the color itself may differ, as in these three species of orioles (Icterus), from top: the yellow Scott's (I. parisorum), orange Baltimore (I. g. galbula), and russet orchard oriole (I. spurius). The communicative functions of coloration patterns in this figure have not been studied experimentally.

Fig. 10. Examples of relative permanence of signal coloration. Top left: The common flicker (Colaptes auratus) has many permanent plumage markings that may be visual signals. The female (left) lacks the male's moustache mark, which is black in some parts of the species' range and red in others. Noble (1936) showed the moustache mark to be a critical visual signal in sex recognition. Top right: The Kumlien's gull(Larus glaucoides kumlieni) possesses a red eye-ring and red beak-spot, both of which are dull during the nonbreeding season but intensify in color during breeding. N. G. Smith (1966) showed that eye-ring color is a visual signal in courtship and species recognition, and several studies have shown the red spot in related species to be a visual signal eliciting begging by the chicks. (After Smith, 1966:frontispiece.) Bottom: Two labile color patterns of the octopus (Octopus vulgaris) used in visual display. In the "fighting display" (left) the animal becomes entirely red, and in the "zebra crouch" (right) it assumes dark bars on a light background. (After Packard and Sanders, 1971:784.)

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.

Fig. 11. Examples of the use of color in visual display. Left: A mantis shrimp (Gonodactylus bredini) in the "meral spread display," in which the small, dark meral spots emphasize the posture. (After Dingle and Caldwell, 1969:120.) Right: Summary scheme of relation of body markings to social behavior in a stylized canid. Arrows point out color markings correlated with specific movements, postures, and orientations. (After Fox, 1969: plate XVII.)

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).

Fig. 12. Example of intensity of a visual signal. The crest of the Steller's jay (Cyanocitta stellen) varies in angle from 90° (left) to 0° (right) along a continuum. (After J. L. Brown, 1964:296.)

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).

Fig. 13. Examples of action patterns that look like visual signals. Left: The "wing-raising display" of the male cockroach (Periplaneta americana) is believed to provide chemical-release and tactile signals to the female. (After Barth, 1970:725.) Right: Tail slapping by two fish provides tactile stimuli by means of displaced water waves. (After Tinbergen, 1951:25.)

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).

Fig. 14. Examples of models used to assess visual elements of a reputed signal. Top: Four of several models used to test for general shape and textural details of visual stimuli from glaucous-winged gulls (Lams glaucescens). Bottom: Four of several models used to test for effects of body postures. (After Stout and Brass, 1969:44-45.)

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).

Fig. 15. Example relating perception of signals to vision physiology. The newly hatched chick of the laughing gull (Larus atricilla) begs by pecking at the red beak of its parent (a), which is pointed downward at feeding. The chick's perceptual ideal (b) is dark red or blue vertically elongated object on a light yellow or green background, of a certain width, moved horizontally and across its long axis. The receptive field of a cat's visual neuron (c) is optimally stimulated by dark bar on light background, and thus would encode chick's preference (d) with respect to contrast polarity (e), width (f), and vertical orientation (g). (Drawing a after Hailman, 1967: Plate I; c and d after Hailman, 1970:143; remaining parts after Hailman, 1971:330.)

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.

Fig. 16. Examples of visual displays elicited by brain stimulation of the pigeon (Columba livia). Top row: Stimulation of the preoptic area elicits erection of head and body, ruffling of feathers, and movements of the crop, as well as walking, then walking in circles with bowing and lowering of fanned tail, and finally looking around. (After Åk erman, 1965a:326.) Second row: Nodding, chest lowering, and wing vibration of the "nest-demonstration" ritual elicited by stimulation of the preoptic area. (After Åkerman, 1965a:333.) Third row: Stimulation of the ventral diencephalis paraventricular gray elicits ruffling of feathers, depression of tail, crouching, and wing waving. (After Åkerman, 1965b:341.) Bottom row: Stimulation of the lateral hypothalamus elicits crouching, head turning, deep crouching ("cringing"), tail lowering, and even flight. (After Åkerman, 1965b:344.)

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.

Fig. 17. Example of display of an interspecies hybrid. Left: The bowing of a barbary dove (Streptopelia roseogrisea-risoria) male. Right: The bowing of turtle dove (S. turtur). Center: The bow of an interspecies hybrid, which resembles the barbary dove in lack of neck plumage fluffing and the turtle dove in the posture at the beginning of the bow. However, the bottom of the bow goes below the horizontal, as in the barbary dove, so the hybrid shows elements of both parental displays. (After Davies, 1970.)

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.

Fig. 18. Example of ontogenetic change in perception of visual signal. Pecking of laughing gull chicks (Larus atricilla) toward models of the adult parent shows little discrimination in newly hatched chicks (white bars), in which only the parent's beak is important in the signal (see Fig. 15). After a few days' experience in the nest, the presence and shape of the adult's head becomes very important (black bars). (After Hailman, 1967:89.)

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.

Fig. 19. Examples of similar visual features involved in ontogenetic transfer of communicative responses. The pink objects associated with elicitation of lip smacking in the baboon (Papio cynocephalus) include the nipple of the female (upper left), penis of the black infant male (upper middle), tongue of the lipsmacking adult female (upper right), sexual skin of the estrous female (lower left), and face of the black infant (lower right). (After Anthoney, 1968:363.)

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.

Fig. 20. Example of species-specificity in visual signals. Wing speculum patterns in some North American ducks of the genus Anas.

Fig. 21. Example of species-specificity in a visual display. The courtship claw-waving display of the fiddler crabs (genus Uca) involves different movement geometries and tempos in the four species pictured here; from left: U. mordax, U. rapax, U. pugnax and U. speciosa. Diagrams at bottom show spatial geometry of the movement with cross-marks indicating jerks (two smooth movement patterns in U. speciosa at right). (After Salmon, 1967:452.)

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.

Fig. 22. Examples of automimicry in visual signals. Five artiodactyls in which ears, facial markings, or other signals mimic the horns, from left: Sylvicapra grimmia, Acelaphus buselaphus, Artilocapra americana, Oreamnos americanus, and Oreotragus orcotragus. (After Guthrie and Petocz, 1970:587.)

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.

Fig. 23. Example of Darwin's principle of antithesis in visual displays. Left: The fox sparrow (Passerella iliaca) threatens a conspecific by crouching in a horizontal posture with dorsal feathers flattened and lateral feathers fluffed. Right: A "fearful" bird assumes a nearly opposite configuration, with vertical posture, dorsal raising, and lateral compression of feathers. (Drawn from field notes of the author.)

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.

Fig. 24. Example of phyogenetic influence of visual displays. The upright threat posture (left) of the laughing gull (Lams atricilla) is antithetical to the submissive, hunched posture (right) of the young lava gull (Larus fuliginosus), so that gulls also illustrate Darwin's antithesis principle. However, comparison with Fig. 23 shows that the polarity of the antithesis is reversed in sparrows and gulls, a fact that is explained by considering phylogenetic origins of the displays. (Drawn from photographs by the author.)

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.

Fig. 25. Examples of visual displays ritualized from agonistic behavior. Left: The "lateral display threat" of the chamois (Rupicapra rupicapra) ritualized from fighting movements. (After Krämer, 1969:918.) Right: The "erect gape" display of the Marabou stork (Leptoptilos crumeniferus) ritualized from escape behavior. (After Kahl, 1966: Plate V.)

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).

Fig. 26. Examples of visual displays ritualized from reproductive behavior. Left: The "scraping display" of the killdeer (Charadrius vociferus) ritualized from nest building. (After R. E. Phillips, 1972:3.) Right: Display element of the estrildid finch Spermestes bicolor ritualized from the movement of feeding begging young. (After Güttinger, 1970:1054.)

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).

Fig. 27. Examples of visual displays ritualized from locomotion. Above: Male bicolor damselfish (Eupomacentrus partitas) on left leads female by using swimming pattern with exaggerated tail movements. (After Myrberg, 1972:216.) Below: Display postures of the male house sparrow (Passer domesticus) (left) and European tree sparrow (P. montanus) (right) evolved from different phases of flight-intention movements. (After Daanje, 1950: Figs. 18 and 19.)

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).

Fig. 28. Examples of visual displays ritualized from foraging and antipredator behavior. Left: A Burmese red junglefowl (Gallus gallus spadiceus), ancestral species to domestic fowl, pecks at nonedible particles as part of agonistic display interactions. (After Feekes, 1972:258.) Riglit: Tail flashing of the whitetail deer (Odocoileus virginianus), believed to be a social alarm signal; Smythe (1970) has proposed that many such signals may have evolved originally as antipredator behavior and then later as social signals.

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.

Fig. 29. Examples of displays ritualized from maintenance activities. Left: Squirrel monkey (Saimiri sciureus) rolls on back, displaying genitals with the same motor pattern as used when rolling out wet fur. (After Castell et al., 1969:490.) Right: Andean goose (Chloëphaga melanoptera) throws its head back in a display that closely resembles oiling movements. (After McKinney, 1965: Plate V.)

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.

Fig. 30. Examples of visual signals evolved from autonomic responses. Above: Barbary dove (Streptopelia risoria), showing areas of body in which pilomotor responses were given in temperature and social experiments. (After McFarland and Baher, 1968:172.) Below: Female gelada baboon (Theropithecus gelada), showing beaded red chest that signals estrus. (After Dunbar and Dunbar, 1974: Plate VII.)

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.

Fig. 31. Examples of visual signals secondarily evolved from signals in other modalities. Left: A wildebeest (Connochaetes taurinus) defecates during a social encounter, one form of scent marking that also serves as visual communication. (After Estes, 1969:322.) Right: Chimpanzee (Pan troglodytes) assumes particular facial expression while hooting. (After van Lawick-Goodall, 1968: Plate 9.)

Future Prospects

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.

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