Chapter 3

Innate Recognition
 

Every organism is bombarded by a vast number and variety of stimuli which come flooding in from all sides, and the more acute its sensory organs are the more numerous will be the stimuli which it picks up. How, then, can an animal select from this profusion of sensory messages those which are essential to its existence and relevant to the survival of itself and its species? This is the gargantuan problem with which every animal "construction" has had to contend in the course of evolution.
The animal must, logically, perform the motions of feeding only when it has found suitable nourishment, otherwise they would be pointless and even distracting. Flight motions are expedient only when danger threatens, and danger must be recognized. The complicated series of movements associated with courtship and mating are also pointless unless the animal has found a suitable mate. The central nervous system must exercise an investigatory and selective function here. It must somehow recognize specific combinations of stimuli.
As numerous experiments have shown, IRM's (innate releasing mechanisms) are finely attuned to particular combinations of stimuli, much as a key is tailored to a lock. Just as the key opens the lock, so a particular stimulus acts on the IRM in such a way that it gives the go-ahead to the nerve impulses, and the appropriate hereditary coordination begins to operate. The inhibitory block is thus removed or unlocked. In this context,

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the ethologist refers to the combination of stimuli which reacts on an IRM as a key stimulus. The term is misleading in so far as it suggests that only one stimulus is ever involved. This obtains only in the rarest instances. Almost without exception the key stimulus consists of a relatively complex plurality of stimuli.
To the ant lion and web-spinning spider, a more or less characteristic form of vibration represents a key stimulus. The ant lion lurks at the bottom of its sand crater. If an earthbound insect crawls into the trap, its efforts to escape cause grains of sand to trickle to the bottom of the crater. The ant lion promptly shoots sand upward, creating a sort of avalanche which hastens the insect's progress to the bottom of the crater. If a few grains of sand are dislodged with a blade of grass, the ant lion responds with the same reaction. The spider, in its turn, responds to the vibration of its web and hurries to overpower the captive insect which has caused it. Touch the web with a vibrating tuning fork and the spider will respond in the same way.
In the Indian Ocean, Eibl and I were able to observe how the scent of blood induces deliberate searching movements in sharks. Certain sea snails are roused by the perception of substances excreted by predatory starfish. Male silk moths find a key stimulus in a sexually attractive substance secreted by the female, to which they are extraordinarily sensitive and respond at a great distance. To certain moths the ultrasonic echosounding cries of the bat are a key stimulus which prompts them to perform special evasive maneuvers such as banking or diving. A simple optical key stimulus is the speckled pattern on herring-gulls' eggs. As Kruijt ascertained, these gulls will not roll eggs back into their nests unless this marking is present. Far more complicated and difficult to define, by contrast, is the stimulus pattern which prompts kittens, chicks, kids, and lambs to halt on the edge of a sudden drop. That this also depends on optical perception is demonstrated by the fact that they still come to a halt when the drop is covered with a sheet of glass.
Some reflexes can be activated by a wide variety of stimuli and are therefore termed unselective. Other reflexes respond to a very specific stimulus and are called selective. The

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same applies to IRM's. One example of an IRM which responds very unselectively – or, to preserve our original metaphor, can be opened with a wide variety of keys – is the one which activates prey-catching behavior in the toad. As soon as the young toad has developed from the larva, it snaps-if hungry-at all small moving objects within reach. Since these are mostly insects, the young toad does not fare badly. It also snaps at moving leaves or stones, however, and does not learn until later how to distinguish inedible objects and insects which taste objectionable or sting. This is an additional faculty which it acquires only by experience. The innate element in its behavior is wholly unselective. It begins by snapping every little moving body in exactly the same way, whereas every large moving body activates another hereditary coordination, namely, that of flight.
The toad reacts just as unselectively at mating time when faced with the task of finding a mate. The male leaps indiscriminately at any moving body and embraces it. Should the object of its attentions be another male toad, the latter emits a rapid series of cries, whereupon the former releases its hold. The mating-minded toad sooner or later encounters a female, whose spawn it fertilizes, but it has no innate "image" of a prospective mate. Waggle your finger in front of a male toad and it will mount and embrace it in exactly the same manner.
In order to discover what characteristics go to form a key stimulus, the ethologist uses what he calls a dummy, or decoy. This consists of the simplest possible reconstruction of the appropriate stimulus situation. Judicious alteration of a dummy or the addition of further characteristics enables one to ascertain what the IRM under examination responds to. In young blackbirds, food begging can be stimulated by a dummy consisting of two circular disks of black cardboard, one large and one small. The young birds construe the larger disk as their parent's body and the smaller one – at which they point their gaping beaks – as the head. In the male fence lizard the blue stripe on the edge of its belly arouses fighting behavior in other males. Females have no such marking and are not attacked, but paint the blue pattern on a female and she will be attacked at once. Paint out the stripe on a male and it will be courted

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instead of attacked. A bunch of red feathers is enough to arouse fighting behavior in a male robin. Thus the word "mechanism" does possess justification here. The hereditarily fixed nerve structure responsible for recognition reacts like an automaton – in this sense, mechanically.
How little such reactions are associated with intelligence was shown by experiments with turkeys. To the turkey hen, the characteristic cheeping of turkey chicks is the key stimulus which arouses brood-tending behavior. Conceal a loudspeaker which emits this cheeping sound inside a stuffed polecat – one of the turkey's natural foes – and the turkey hen will take it protectively under her wing. Deprive the turkey hen of her

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hearing, on the other hand, and she will kill her own young because the appropriate key stimulus fails to reach her IRM. Of all the very numerous key stimuli, ethologists have taken a particular interest in one group which Lorenz, to whose pioneering investigations most of such research can be traced, has given the name "releasers." The peculiarity of key stimuli of this type consists in the fact that they not only release behavior patterns de facto but are bound to release them. They are signals conducive to understanding between members of the same species or animals of different species which are on "friendly" (symbiotic). terms with one another. Thus, they are releasers par excellence and are known as signal stimuli. In their case, unlike that of other key stimuli, there is a clear tendency for their emission to be as distinct and conspicuous as possible.
The following point should be borne in mind here: If predators can recognize animals of species A by means of a certain key stimulus, such animals are at an obvious disadvantage. It follows, in the course of evolution, that the members of the species which maintains – and, consequently, propagates – itself more efficiently are those in which the key stimulus dwindles most or is not emitted at all. Natural selection here demands an involution of the distinguishing features in question. Conversely, when a key stimulus aids mutual recognition – as, say, in the case of mating partners – it is beneficial to the species if this signal becomes as distinct as possible. This sets off a contrary selective process which gains strength in the course of evolution.
As in every transmitter-receiver relationship, releasers are dependent upon being as simple, unmistakable, and infrequent as possible. The simpler they are, the simpler need be the receiving apparatus and the less the effort involved. The more unmistakable they are, the fewer failures of communication; and the more infrequent they are, the smaller the risk that other animals will use the same signal and so give rise to dangerous misunderstandings. Quite simple spatial relationships between visual characteristics play a part here,. as Tinbergen was able to demonstrate in the case of the stickleback. The red belly of the male stickleback acts on other males as an aggression-

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releasing characteristic. Show the fish a wax sausage which has been painted red on the underside but bears no other resemblance to a fish (no fins or eyes) and it will be attacked no less fiercely. Turn the sausage over, however, so that the red paint is on top, and the dummy ceases to elicit aggressive behavior. In this instance the releaser is not simply "red" but "red underneath," as Tinbergen phrased it.
Different key stimuli can elicit the same form of behavior, and experiments with dummies enable one to present these to the subject separately. The males of one cichlid genus – they are blue with a black marking on their dorsal and ventral fins – confront their adversaries broadside on with fins spread, then aim tail strokes at their heads, and finally ram them with jaws open. Blue coloring and fin markings, transverse position, spread fins, tail strokes, and ramming technique-all these are key stimuli which, if presented separately, provoke a highly threatening reaction. The strongest reaction is observable in response to a combination of them all. Seitz, who was the first to recognize this, termed it the Reizsummenphänomen, or accumulated stimulus phenomenon. In many cases the ratio of effectiveness could even be accurately recorded in numerical terms.
Another discovery was that dummies can often be devised which surpass the efficacy of natural key stimuli. Koehler and Zagarus found that a ringed plover will abandon its own eggs in favor of one four times as large, even though it has no hope of hatching it. The cuckoo, as everyone knows, lays its eggs in other birds' nests, where its young are actually given preferential treatment by the unfortunate foster parents. This is attributable to the young cuckoo's wider throat, which acts as a stronger feeding release. Tinbergen and his associates established that the male brown butterfly prefers black female dummies to those of natural coloring. And for another species of butterfly, the silver washed fritillary, a rotating cylinder adorned with brown stripes running lengthwise, holds an even stronger sexual attraction than the sight of a female of its own kind. The ethologist refers in such cases to supernormal dummies.
Movements which initially served quite different purposes not infrequently developed into releasers. In gregarious ani-

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mals, for instance, skin-tending motions became signals of contact readiness for the eliciting of a friendly attitude from members of the same species. When a dog licks us in greeting, this is a friendliness-eliciting signal which developed from mutual fur cleaning among the dog's ancestors and has now been extended to human beings. Jane Goodall observed that chimpanzees living in the wild in Tanganyika greet each other by means of embraces and lip contact. These releasers of friendly behavior in others very probably stem from hereditary coordinations of the mother-child relationship (hugging the maternal neck and mouth-to-mouth feeding).
The same movement can possess quite different releaser functions in different species. Tail wagging, which signifies friendly excitation in the dog and hostile in the cat, has become a releaser of friendly behavior in the first instance and a means of intimidation, or releaser of fear in an adversary, in the second. Just as it is a matter of pure convention what significance we attach to this or that particular word in human speech, so, in hereditarily determined animal signals, no essential importance attaches to their individual nature or mode of origination. All that is important is that they are understood. There must thus develop, on the one hand, a hereditary coordination linked with signals of particular significance and, on the other, a re-

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ceptive mechanism capable of activating a particular reaction in response to such signals.
The courtship rituals performed by numerous creatures are designed not only to aid mutual recognition but to break down an equally innate fear of physical contact between mates. Courting is usually done by the male, though in many families, e.g., the Syngnathidae, the contrary is true. Prospective mates are prepared for copulation by a special form of "impressive" behavior and by numerous forms of display. Stimulative structures often develop within the context of this function. Male frigate birds, for example, impress their females by inflating a vivid red throat sac. Birds of paradise have developed peculiarly gorgeous plumage which they display in the most bizarre postures. Many fish don a wedding dress in that they become brilliantly discolored prior to mating. Among Australian bowerbirds, the male prefaces the act of copulation by constructing a special love bower which it decorates with flowers or brightly colored stones, then lures the female inside by means of courtship dances or other display behavior. The satin bowerbird goes to the lengths of painting its bower, applying a mixture of chewed berries and saliva with its beak or, sometimes, with a leaf or fragment of bark. The essential – and novel –
element in assessing all these procedures is that in every case intensified key stimuli are presented in order to elicit innate reactions from members of the same species (or, in the case of symbiosis, from those of another species). They are not, therefore, purely unilateral signals but a means of actively influencing the reactions of others.
While observing intraspecific battles between rival males, Lorenz discovered the so-called appeasement gestures – special postures which the defeated animal adopts in order to dissuade the victor from doing it further injury. These are releasers which activate an inhibitory mechanism and render the victor simply incapable of further aggression. For reasons which must be more fully discussed at a later stage, intraspecific fighting possesses a biological importance which accounts for its development in the course of evolution. On the other hand, natural selection encouraged forms of combat in which members of the same species did not injure themselves seriously. Ethologists

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speak of jousting in this context, and a certain resemblance to man's sporting contests really does exist. As in them, there is an established ceremonial with certain rules of war – though these are innate. An individual worsted in such a contest adopts an appeasement position (among iguanas from the Galapagos, for instance, the loser lies down flat in front of its adversary), "pacifies" the victor, and can then retire unscathed. Signal stimuli not only elicit movement, therefore; many of them activate an inhibitory mechanism which prevents a movement from occurring.
The development of releasers from other movements and their improvement in the sense of increased efficiency is known in the field of behavioral research as ritualization. One can trace the course of such a development in the black woodpecker. While preparing itself a hole in a tree, it produces a drumming noise. A quite similar drumming noise becomes a signal warning other males not to trespass on its territory. The noise meaning woodpecker at work thus developed into the threatening signal, "I'm at work here – keep out" In the case of cichlids, which invariably practice brood tending, one can trace the development of a similar signal. Among members of one branch of this family of fish, parents elicit a following reaction from their young by performing a peculiarly exaggerated swimming motion. The obvious significance of this is: "Watch out, children. I'm moving on now, so follow me!" Members of the genus Aequidens have developed the motion into an exaggerated wriggling movement performed for a short distance. This being even more striking and unusual, the likelihood that young fish may be misled by similar behavior in other fish becomes even smaller. Finally, among dwarf cichlids, we find an enticement motion in the form of exaggerated head shaking. Viewed in isolation, the origin o€ this signal would be hard to interpret. Comparison with related species of fish permits one to infer that this, too, is a last gestural relic of the swimming on-ahead movement – in other words, a piece of exceptionally advanced ritualization.
Knowledge of such factors has helped to explain many mysterious procedures. –  The male empid or dancing fly Hilara sartor, for instance, gives the female a "wedding present" in

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the shape of a balloonlike cocoon. The highly aggressive female busies herself with this gift, and the male seizes the opportunity to copulate with her. What is the significance of this cocoon? Comparison with other dancing flies teaches us that this, too, is the terminal phase of a ritualizing process. Originally – as is still true today of the Empis tessellata – a small insect was proffered as a wedding gift. The female takes the insect, sucks it dry – and is simultaneously mounted. In other species the males spin a veil around the gift before presenting it, thereby turning it into a sort of parcel. The females strive to open the parcel and the mating process takes places. In still other species only an empty cocoon is presented, and in the case of Hilara sartor this is a loose and quite transparent structure. Here, too, surviving species which have remained at this or that stage of development illustrate the course of evolution. The signals have become simplified but retain their activating effect.
Parallel with such ritualizations, parts of the body which participated in signaling movements became larger and more conspicuous. Since the signal stimulus gained in strength as a result, modifications of this kind – if they occurred in successive generations – also possessed selective value. Some snakes, for example; vibrate their tails as a threatening gesture. Although this was probably a quite general concomitant of excitation at first, it developed into a signal designed to intimidate adversaries. The rattlesnake, as a further improvement, developed horny rings which make a noise when its tail is vibrated and so emphasize the signal still more. In the porcupine, which erects its quills as a threatening gesture and also shakes its tail, an analogous development modified certain quills into noise-producing organs. The most surprising development of this type is to be found in the spotted hyena, which lives in packs and presents its erect penis as a gesture of appeasement and ceremonial greeting. Since the females did not possess such an organ, they could not perform this gesture. However, they carried their conformance with ritual so far that they, too, have developed a penislike organ which is erectile and enables them to greet their own kind in the male manner. From a purely external point of view, it is quite impossible to distinguish between male and female members of this species.

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We began with the question of how organisms are in a position to perform their innate movements at the proper place and time. It has been shown that special nerve structures, also innate, are responsible for accomplishing this. They are able to select very specific combinations of stimuli from a multitude of sensory messages, and whenever such combinations occur, they release the corresponding hereditary coordinations.
 

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