animal behaviour

animal behaviour


      any activity of an intact organism.

      A living animal behaves constantly in order to survive, and all animals must solve the same basic problems. They must, for instance, periodically replace their energy source (consume food), avoid dehydration (drink), avoid becoming another animal's energy source (avoid being eaten), maintain their body surfaces (clean and groom), and reproduce. This article discusses the basic behavioral activities of animals ranging from protozoans to higher vertebrates. Although references are made to human behaviour, the reader should consult human behaviour, for specific information. Likewise treated in passing are certain behavioral tendencies of plants that resemble or parallel simple unlearned behavioral responses and adaptive mechanisms of animals (see also plant development).

Diversity of behavioral activity
      Any animal may be regarded as an agglomeration of interacting and interdependent structures and behaviours that are responses to environmental (environment) conditions. The behavioral features of modern animals are the accumulated results of millennia of selective pressures acting on small variations inherent in individuals. This selection is relentless because environments are constantly changing.

      An understanding of comparative behaviour is helpful in understanding human behaviour, just as an understanding of comparative anatomy is helpful in understanding human anatomy. The reason for this is that both behaviour and anatomy have a genetic (heredity) basis. All vertebrates (vertebrate), for example, share certain anatomical features that distinguish them as vertebrates; and smaller groupings such as fish, amphibians, reptiles, birds, and mammals (mammal) may be distinguished from one another on the same basis. This type of distinction holds true between species and even between individuals within a species. Consequently, an understanding of the anatomy or behaviour of any species is helpful in understanding other species, including man himself. In general, the closer the relationship between any two species, the more similar are the structures and behaviours of the two species. The converse is also true. Exceptions, however, do exist. Human beings and chimpanzees (chimpanzee), for instance, are closely related genetically, but, because of historic differences in environment, the behaviour of humans is, in many ways, more like that of wolves, which experience many problems similar to those of ancient man. Such convergences and divergences are commonplace in biological evolution. Convergence occurs when unrelated animals independently evolve similar responses to similar environmental conditions—e.g., the similar body shapes of porpoises and sharks; the similar social behaviour of wolves and humans. Divergence occurs when closely related species are adapted to different conditions, with a resultant difference in behaviour and structure. This is the usual type of response; sometimes, however, divergence is extreme enough to obscure a close relationship. The males of many species of closely related hummingbirds, birds of paradise, pheasants, and ducks, for example, are superficially so different from one another that many of these species were formerly assigned to different genera.

      The study of behaviour has provided valuable information about relationships among animals. Aristotle was one of the first to use behaviour as a taxonomic aid, but only in recent times have behavioral features been important in animal taxonomy. Aristotle regarded pigeons and doves as closely related to the sand grouse, basing his view partly on their similar way of drinking. Pigeons, doves, and sand grouse, unlike most other birds, keep the bill in the water and drink with a pumping action.

      Behaviour may be quite simple, as in taxis (movement toward or away from a stimulus) and kinesis (undirected response proportional to the intensity of a stimulus). These two types of behaviour—most often descriptive of invertebrates (invertebrate)—may be further subdivided. Orthokinesis, for example, is a response that involves change in the speed of movement of the body as a whole. Klinokinesis involves changes in the rate of turning from side to side. Klinotaxis is a type of orientation to stimuli in which, in alternate body movements, external stimuli are received with equal intensity. In tropotaxis the orientation of the animal is similar to that in klinotaxis, but it depends upon stimuli acting simulaneously upon two receptors (receptor) or upon two parts of one receptor. These are stimulated unequally if the animal is not oriented directly toward or away from the source of stimulation. In telotaxis the animal orients to one or the other of conflicting stimuli affecting the same sensory mechanism. In menotaxis, or light compass response, animals (e.g., honeybees, ants) do not orient either directly away from or toward a source of stimulation but assume a constant angle to the direction of the stimulus. Complex behaviours such as nest building, courting, and fighting (aggressive behaviour) do not lend themselves to the simple labelling of taxes or kineses and are classified according to other systems, which are dealt with below.

Classification of behaviour

      An animal's behaviour may be described according to the nature of the muscular contractions involved or in terms of the consequences of the behaviour. Because the muscle contractions involved in specific behaviours are often complex, a kind of shorthand is commonly used to describe them. Terms such as tail flick, head bob, and threat posture are of this nature. In describing behaviour in terms of its consequences, terms such as avoidance, courtship, nest building, and burrowing are often employed. Sometimes a behaviour must be described both in terms of the type of muscle contraction and in terms of the consequences of the behaviour.

      Three types of behavioral classification are used most commonly: (1) on the basis of immediate causation, (2) on the similarity of evolutionary history, and (3) on the similarity of function.

Immediate causation
      Classification based on immediate causation requires that the causal factors first be identified. All behaviours triggered by similar causal factors are then grouped together. Whether or not behaviours share the same causal factor can be determined by either of two methods. One method is to administer the causal factor and see if all of the behaviours are elicited and affected similarly. The other method, used when the causal factor is not known, is to examine the chronological correlations between the activities in question. Two activities that consistently occur together are likely to be causally related. This method is often used in studies of agonistic (agonism) (attack–escape) courtship, feeding, egg laying, and similar complex behaviours.

      Types of behavioral evolution are often classified according to similarities in biological evolution. Similarities between patterns of muscular contractions are compared among species believed to be related. The degree of difference between presumably homologous behaviours provides a criterion for measuring the degree of evolutionary affinity and for determining the direction of evolutionary changes. Data useful in classification may sometimes also be obtained by observing the appearance or disappearance of behaviours during ontogeny, or individual development.

      Classification based on similarity of function depends on the identification of behaviours with similar evolutionarily adaptive values. Such behaviours may or may not be homologous. The flying behaviour of a bird, for example, is not homologous with that of a butterfly, but both have similar functional and adaptive significance and may be classified together on this basis. A functional classification often closely agrees with a causal classification; this is because evolutionarily functional and causal mechanisms are commonly associated. The more distantly animals are related, the less likely it is that their functional and causal classifications will overlap; the converse also tends to be true: the more closely animals are related, the more likely it is that the two classifications will overlap.

      In the case of behaviour in juveniles and adults of the same species, causal and functional categories may sometimes not overlap; an example is penis erection in juvenile mammals, in which reproduction is not a causal factor, compared with that in adults, in which it is.

The influence of genetics and experience
      Behaviour is sometimes classified according to the nature of the changes occurring during evolution or ontogeny. From these bases groupings such as learned, innate (stereotyped response) (i.e., unlearned, or instinctive (instinct)), and ritualized behaviour are derived. This classification scheme is useful in studies such as those dealing with the acquisition of communication behaviour.

      Every aspect of an animal—behavioral as well as structural—is influenced to some degree by heredity as well as by experience. Although a muscle can become larger, harder, and more vascularized (i.e., supplied with blood vessels) or smaller, softer, and less vascularized depending upon the nature of the experience to which it is subjected, the possibility of such modification is not infinite because of genetic limitations. Some behaviours are inherited in the same sense that an organ is inherited, and they exhibit little or no capacity to be modified by experience. Other types of behaviour achieve definitive form and employment only with experience.

      Various intermediate conditions exist. If the environment is predictable relative to the appropriateness of a given behaviour in an animal, it is biologically advantageous for this behaviour to be innate, because presumably there is no need for a variety of response. Variability would, in fact, be disadvantageous, because an inappropriate response could threaten the animal's chance of survival. On the other hand, if the environment is unpredictable in certain respects, it is biologically advantageous to have some degree of inherent variability based upon response to experience. In this case, however, it is likely that the animal would sometimes respond with the wrong behaviour, resulting in a lowered survival value. In behaviours that may be modified by experience, the nature and extent of the modification are not limitless. The nature, intensity, timing, and duration of experience that may influence a given behaviour vary with the type of behaviour involved. The kind of response to these aspects of experience is determined genetically and has evolved to the extent that the maximum possible survival value is achieved.

      Some behaviours are innate in the same sense that an organ of the body is innate. Their functions are relatively fixed and predictable. Other behaviours have an inherent resiliency and may respond radically to changes in the environment. Genetically determined limits, however, are always imposed upon even the most variable behaviours. An animal would otherwise be capable of limitless modifications of its behaviour, within its structural limitations, and be able to associate arbitrarily any environmental stimulus with any behaviour.

      A general evolutionary trend exists for more and more behaviours to be modified by environmental stimuli as the phylogenetic scale ascends (i.e., as the animal becomes more complex and “advanced”). This tendency has reached its extreme expression in vertebrates, particularly mammals, and especially in humans.

Components of behaviour

Fixed action patterns
      A behaviour that is independent of environmental stimuli for its form is known as a fixed action pattern (FAP). An environmental stimulus may, however, be responsible for the elicitation and proper orientation of the FAP and may have an influence on the completeness of the response. Common examples of FAP's include displays (visible and audible signals), nest-building movements, various food-gathering and food-preparation movements, thermoregulatory movements, and attack and escape movements.

      Because FAP's are often specific for particular species, they are frequently useful in taxonomic and evolutionary studies. The homologous relationship among FAP's of related species is often easily determined, and their qualitative and quantitative differences can be evaluated.

      FAP's are ordinarily quite constant in form, but this stereotypy is not a defining characteristic. Some vary considerably in the degree of completeness, even though the proportions of the components of the response may remain quite constant, relative to one another. Some FAP's share a single environmental stimulus. Sight of a rival male, for example, may elicit flight, attack, or any of a variety of agonistic (attack–escape) displays.

      It is sometimes difficult to distinguish an FAP from its orienting movements. In such cases the eliciting stimulus can be manipulated, and the subsequent effects on the behaviour can be observed. The graylag (greylag) goose (Anser anser), for example, retrieves eggs (egg) displaced from the nest by means of a highly stereotyped behaviour. While sitting on the nest, the goose extends its head beyond the egg so that the undersurface of the bill is against it. The head is then pulled toward the body until the egg is again safely in the nest. While the head is being pulled toward the body, it makes balancing adjustments that compensate for the tendency of the egg to roll to either side. If the egg is removed during retrieval, the head continues its movement toward the nest, but compensatory movements to counter the erratic roll cease. The FAP is elicited by the egg-out-of-nest stimulus and, once triggered, goes to completion whether or not the egg is still balanced against the underside of the bill. The balancing movements, however, depend upon continuing stimulation by the egg itself and are not part of the head-withdrawing FAP. The balancing movements also cease if a smoothly rolling cylinder is substituted for the egg.

      This egg-retrieving FAP illustrates other features of FAP's in general. Every species can perform a finite number of FAP's and have a limited capacity, or none at all, for developing new ones. The limitations may be determined by physical structure: a woodchuck cannot fly, nor can a pelican burrow. Yet neural organization may provide restrictions as ubiquitous and rigid as anatomical ones. The egg-retrieving graylag goose is physically capable of retrieving an egg with its broadly webbed feet or with a wing used as a kind of broom. This never happens, however, because the goose's neural organization permits egg retrieving only in the manner described. Alternative behaviours can sometimes be employed. Parrots (parrot) of the genus Agapornis, for instance, always scratch the head by bringing a foot forward over the wing on the same side; however, when a foot is brought forward for cleaning the bill, it always passes below the wing on the same side. The bird has the physical structure as well as the neural organization necessary for both movements, but each method is specific for its particular stimulus.

Key stimuli
      An animal reacts to relatively few of the stimuli present in its environment. This is a basic characteristic of behaviour. It is seldom known whether an animal actually perceives (perception) but does not react to the various available stimuli or whether it fails to perceive stimuli of no significance to it—at least of no significance within a given context. Both situations are probably true, at least for some species, under given circumstances. The human eye, for instance, focusses on the retina a detailed picture of all that the eye is directed toward. The human observer, however, is unaware of much within his field of vision. The same is true of other sensory modalities. At some point, insignificant stimulation is filtered out. An animal could not function if it had to respond to all of the stimuli its sensory organs are capable of receiving at any moment. Each species has, therefore, evolved responses only to those stimuli significant to itself and at such times that responses to such stimuli are relevant. This simpler world that actually falls within the animal's perception at any particular moment is termed its Umwelt.

      The tick's (tick) response to its relatively simple Umwelt graphically illustrates how an animal selectively responds to only those stimuli pertinent to its immediate requirements. The mature female tick responds to light falling on her photosensitive body surface by moving to the tip of a twig or some suitable substitute, where she waits for a mammal to approach. The tick is capable of waiting for years until that occurs. When a mammal passes close by, she releases her grip on the twig and, if successful, falls onto the body of the mammal. The key stimulus causing her to release her grip is the scent of butyric acid, which naturally issues from the body of any mammal. When the female tick is on the host's body, she reacts to its body heat by inserting her proboscis (feeding organ) into the mammal's skin and sucking herself full of blood. This behavioral sequence depends on three stimuli: light, the odour of butyric acid, and warmth. The tick will relinquish its perch for anything with the scent of butyric acid, sink its proboscis into any warm surface—even a balloon filled with warm water—and fill (feeding behaviour) herself with whatever fluid is within. The taste of blood and mammalian characteristics are not meaningful to the tick.

       aggressive behaviour in the male stickleback fish (Gasterosteus aculeatus) occurs when the fish sees the red belly of other males. Crude dummies are attacked as long as they have red bellies; realistic models or actual fish without red bellies are not attacked.

      Various species of predacious (predation) animals utilize lures that simulate the natural foods of prey species. The alligator snapping turtle (Macrochelys temmincki) has two slender, reddish structures projecting from the tip of its tongue that move like small worms. The turtle, with its mouth open, rests on a river bottom. A fish attracted by the false worm is snapped up by the turtle. Several species of deep-sea anglerfish have a long slender projection from the forepart of the dorsal fin. This “fishing rod” terminates in a wriggling wormlike structure that is dangled close to the anglerfish's mouth. When another fish investigates the lure, it is easily snapped up by the anglerfish. Different species of anglerfish possess different lures that are specific for different prey species.

      The male swordtail characin fish attracts female mates by means of an organ resembling a daphnia, a favourite prey. The gill covers of the male are modified into a long, slender projection terminating in this “daphnia,” which is moved within view of the female. When she has been lured close enough, the male copulates with her.

      Female fireflies of the genus Photurus lure males of other genera by imitating their flashing codes, then seize and eat them. A certain petal in various species of fly orchids is modified to resemble the females of various wasp species, and the odour of these female wasps is also imitated. When a male wasp is attracted to a model female of the same species and attempts to copulate with it, the pollen of the flower adheres to the male and is, in turn, carried to the next flower that attracts the wasp.

      Males of certain African cichlid fish (genus Haplochromis) deceive females by means of a body pattern that resembles cichlid eggs. The females, which are mouth breeders, take into their mouths the eggs they have laid before the male fertilizes them. Visual models of these eggs are part of the pattern of the ventral fins of the male. After the female has placed the eggs in her mouth, the male spreads his ventral fin before her. The male emits sperm as the female snaps at the false eggs, thus permitting the real eggs in her mouth to be fertilized.

      Another mouth-breeder fish, Tilapia macrochir, ensures fertilization by means of a different deception stimulus. The male produces sperm in filament-like packets (spermatophores), which are shed into the water. Later, they are picked up by the female. She may not always find them, however, and the male has evolved long filamentlike spermatophore models that project from his genital region. For some reason, these models exert a stronger stimulus than the real spermatophores. The female takes them into her mouth and at the same time receives real spermatophores that have been placed among the models.

      The eyes (eye, human) of the meadow frog, Rana pipiens (leopard frog), have five types of cells, each of which responds to a different kind of stimulus. One type responds briefly when a light is turned on or off; it also responds to the passing of the leading and trailing edges of an image moving across the retina. It does not respond to a stationary image, however. A second type of cell responds to the passing of straight or curved edges. A third type does not respond to changes in light intensity but to the passage of the image of a small object, in contrast to its background, across the retina. The fourth type of cell measures a decrease in illumination, and the fifth type measures light intensity.

      The frog is capable, therefore, of receiving information about the size, shape, movement, and illumination of objects and is particularly well equipped to perceive small moving objects—its normal food. Much of the stimulus filtering in the frog's vision takes place peripherally at the retinal level. Studies of other subjects such as cats, rabbits, and moths reveal that the processing and integrating of sensory data occurs at various levels in the central nervous system (brain and spinal cord).

      Hundreds of types of responses to a few key stimuli have been identified in various animals. These responses are mediated in the central nervous system by a so-called releasing mechanism (RM) and are responsible for triggering the specific motor response appropriate to the stimulus. If the releasing mechanism is innate, it is termed an IRM. If it is acquired through individual experience, it is termed an ARM. Some innate releasing mechanisms may be modified as a result of individual experience; such a mechanism is termed an IRMe.

      A given stimulus does not always prompt the same response in the same individual. Such differences are due to internal factors. Some changes are seasonal and are brought about by internal conditions that may, for example, be related to reproduction and associated aggressive behaviour. In the spring, a male wood thrush (Hylocichla mustelina) responds to a female with courtship behaviour and to another male with aggression. In the winter the same thrush fails to respond in this way. Relatively short-term changes in responsiveness also occur. An animal that has just fed, for example, shows no further interest for a time in food. In such an instance a short-term internal change has taken place.

      The strength of a stimulus necessary to evoke a response of standard intensity also varies with time. The longer an animal is deprived of food, for example, the more unappetizing the food can be and still be accepted. The converse is also true. The more recently an animal has eaten, the more appetizing the food must be to be accepted.

      The intensity of a noxious stimulus or the degree of difficulty of an obstacle that an animal will attempt to surmount varies with time. The longer an animal has been without food (short of physical debilitation), the more difficult an obstacle can be and still be surmounted by the hungry animal. Again, the converse is true.

      Internal changes that initiate behavioral changes are commonly termed drive or motivation. These terms are usually applied to short-term reversible changes in response to a constant stimulus. They are not applied to long-term changes that are the result of learning or to short-term changes that result from muscular fatigue, sensory accommodation, and sensory adaptation.

      When internal conditions are intense enough to initiate a particular drive, an animal commonly behaves as if it were searching for the correct environmental stimulus necessary to trigger the appropriate response. Such searching, or appetitive, behaviour is often highly variable. The true nature of a particular appetitive behaviour can, as a rule, be ascertained only as the act progresses. The drive that motivates a robin to search a lawn, for example, cannot be determined until the search nears culmination. If the robin seizes an earthworm, it is evident that hunger was the activating drive. If it picks up mud or grass, the appetitive phases of nest-material gathering are apparent. Appetitive behaviour tends to become less and less variable as the appropriate terminating situation becomes more and more likely to occur. A hunting falcon flies a search pattern until a potential prey is sighted. The bird dives upon it, after which the exact flight path is determined by whatever evasive action the prey may take. If the falcon is successful, the prey is struck and killed, carried to a perch, and systematically pulled apart and eaten. When the hunger is satisfied, the drive state no longer exists, and some other activity follows. The closer the appetitive sequence is to termination, the more stereotyped (stereotyped response) the falcon's behaviour becomes. The consummatory act is the most stereotyped behaviour of all.

      Courtship behaviour culminating in successful copulation provides many examples of characteristic appetitive–consummatory chains of behaviour. In such cases copulation itself is the terminal appetitive behaviour and is highly stereotyped. Ejaculation, or discharge of sperm by the male, in certain species is completely stereotyped and is followed by temporary cessation of the sex drive.

      Learned behaviour is important in appetitive sequences in many animals. A jay, for instance, quickly discovers the best places to find particular foods, and it learns to begin its search in such places rather than search at random until a suitable forage area is found.

“Supernormal” stimuli
      A major subject of investigation in animal behaviour has been the determination of key stimuli necessary to trigger particular behaviours. In order to determine those characteristics of an egg by which an incubating bird identifies it as such, a selection of model eggs can be presented to the bird, each model differing in one respect from a normal egg. The reactions to variations in colour, pattern, shape, size, and texture vary according to the species. Generally, differences in shape do not seem important to an incubating bird; but models with more rounded contours appear to be favoured. Differences in colour, pattern, and size are important, but differences in texture do not seem to be.

      A model in which the key stimulus has been exaggerated to an extreme degree may be chosen in preference to a normal model. The oystercatcher, for example, prefers a “supernormal” egg, several times the usual size. It also prefers an abnormal clutch of five eggs to the normal clutch of three.

      Chicks of the herring gull (Larus argentatus) are stimulated by a red spot on the lower bill of the adult. When the chick pecks at this spot, the adult regurgitates food for it. By presenting the chick with various models of beaks, it has been found that differences in the colour of the head and bill are not significant; but the red spot, narrowness of the bill, movement, low position of the head, and a downward pointing of the bill are all important in eliciting a response. A thin rod with a red band near the tip moved in a low position provides a supernormal set of stimuli, which elicit a positive response.

      When more than one stimulus elicits a given response, the stimuli may supplement one another. If two or more stimuli are required to evoke a response, a weakness of one stimulus may be counterbalanced by the strength of another. Such a compensatory effect is termed the law of heterogenous summation. In higher animals, learned behaviour may play an important role in this phenomenon. A response may originally depend upon one or a few key stimuli, but, as a result of experience, an animal may come to regard previously irrelevant conditions as among those stimuli necessary for the response, resulting in a kind of gestalt response, in which several stimuli are perceived as an integrated whole. Animals lower on the phylogenetic scale may be more apt to respond to heterogeneous summation, and those higher on the scale may be more apt to respond to a gestalt, however acquired.

      The supernormal stimuli discussed above have been observed experimentally. The tendency of an animal to respond more vigorously to enhanced stimulation may be of evolutionary significance, because animals with a genetically based tendency to prefer more advantageous variants of a stimulus would tend to have a higher probability of survival. In social situations in which the relevant stimuli are part of an animal's body, those structures offering a favourable departure from the normal would enhance the probability of survival in the offspring.

      The form of movement of a response is not determined by either the eliciting stimulus or by the properties of the musculature involved. It is possible, however, that the form of movement may be determined by either of two other factors. First, the sequence in which muscles (muscle) contract to produce a movement may be determined solely by the properties of central nervous system mechanisms responsible for the movement. In this case the movement, once initiated, is independent of further sensory stimulation.

      Second, the muscle contractions near the completion of a movement may also be influenced through a feedback mechanism provided by the earlier contractions. The form of the movement would thus be continuously monitored by sensory control. Fixed action patterns, although independent of further stimulation once elicited, may depend upon such internal feedback mechanisms.

Distinction between external and internal movement
      The way by which animals are able to distinguish between movements of the environment and movements of the sense organs is not fully understood. When the human eye views a moving object, the object appears to move. If the eye is moved while looking at a stationary object, the object appears stationary, though in both cases the image moves across the retina. But if a stationary object is viewed while the eye is displaced slightly by pushing with a fingertip, the object appears to move.

      If a resting fish is tilted to one side, the statolith (organ of equilibrium) on that side shifts position, thereby activating sensory endings; these set in motion muscular action that restores the fish to an upright position. A fish often deliberately tilts sideways, however, and, in this case, the automatic reflex does not pull the fish upright. It was formerly believed that the righting reflex is blocked during spontaneous movement. Studies have shown, however, that such blocking does not occur. If a fish is whirled in a centrifuge, the deliberate tilting movements made by the free-swimming fish are of lower intensity. The tilting movements become less because the statoliths are made heavier. The righting reflex is not blocked during deliberate tilts, therefore, but is dependent upon the feedback caused by the tilts.

      During spontaneous movement, the stimuli that otherwise release postural reflexes are not inactivated but must be neutralized in another way. The principle of reafference has been hypothesized to account for this. By this hypothesis the functional system is visualized as a feedback loop, whereby afferent nerves carry impulses toward the central nervous system and efferent ones carry impulses away from the central nervous system to the motor areas. Afferences can be divided into receptor excitations caused by internal changes in the musculature (reafference) and those produced passively by external stimulation (exafference). Reafference and exafference are integrated in some manner in the higher centres of the nervous system. The reafference hypothesis postulates that, with each voluntary movement, a copy of the efferent motor impulse is stored in a subordinate nervous centre. The efferent impulse continues to the effector, and movement results. The sense organs then report the result of this movement as a reafference—a feedback of information. This reafference is matched with the efferent copy and is cancelled. If the total afference is too much or too little, as the result of external stimulation, there remains a plus or minus value as compared to the efferent copy stored in the subordinate centre. The discrepancy is reported to the higher centre, which then strengthens or weakens the initial command.

Behavioral chains
      When an animal responds to a stimulus, the releasing situation is often altered because the animal has progressed to a new position, in which other stimuli are effective. For example, when a female three-spined stickleback enters the territory of a male, he performs a zigzag dance. She responds to this with a signal of her own, which, in turn, releases a behaviour in the male that causes her to follow him. The male shows her the nest opening, which she enters. The male trembles with his snout against her tail, stimulating her to spawn, after which she leaves the nest. The male then fertilizes the eggs. Each of these behaviours depends upon the appropriate stimulus. If one is omitted, the chain terminates without a productive conclusion.

      The behaviour of the bee-hunting wasp Philanthus triangulum illustrates another such chain. This wasp flies from flower to flower as it searches for bees. It responds initially to the visual stimulus afforded by any moving bee-size object; during this time it is indifferent to bee scent. After the wasp perceives the visual stimulus, it hovers about 10 to 15 centimetres (four to six inches) downwind of the bee and then is sensitive to bee scent (odour); if the scent is appropriate, the wasp attacks the bee and seizes it. Following seizure, bee scent is no longer an effective stimulus. Moving models of the appropriate size attract the wasp, but they will not be seized unless they have bee scent. The behaviour depends upon a succession of stimuli that must occur in a precise sequence.

      Many such behavioral chains are known for vertebrates as well as invertebrates. They are not always precisely ordered, and variations may occur. Many such behavioral chains do not exhibit a succession of stimuli made available as a result of the responses. Single causal factors may stimulate several responses. Activities occurring near the end of a chain may require a higher intensity of stimulus than earlier ones. If a causal factor proves inadequate at some point, the behaviour reverts to an earlier stage in the sequence. This is probably true in the courtship of the three-spined stickleback in which all the activities of both sexes depend upon common endocrine factors (i.e., hormones), on short-term states of heightened responsiveness, and on the nature of the external stimuli.

Conflict resolution
Simultaneous stimulation
      Causal factors for many types of behaviour are usually present at any given moment. A male stickleback may be simultaneously confronted with several stimuli: a ripe female, food, a rival male, and a predator. Animals usually respond to one stimulus at a time and according to a certain priority of sequence. In the male stickleback, escape from a predator almost always takes priority over concurrent stimuli.

      More than one drive is often activated simultaneously by the same situation. A conflict between the drives occurs, and the situation must be resolved. The resolution may occur in any one of several ways. Sometimes two drives may be expressed simultaneously. Pecking and head turning, when activated together, often occur simultaneously in chickens (chicken). Chickens that are in conflict between watching out with an elongated neck and making wide sweeping movements of the head as they peck, elongate the neck even further but reduce the extent of the head sweeping. Such conflicts can also be resolved by alternately performing the activities appropriate to the conflicting drives. The separate activities are often incomplete. A housewife, for example, may perform in this manner if the telephone begins ringing just as something begins to boil over on the stove.

Redirection and displacement
      Sometimes an animal has a drive to perform a particular behaviour but is prevented from doing so and directs the behaviour to another object. If an animal is prompted to attack another but is prevented by fear of the opponent or by a reluctance to leave its territory, it might attack a harmless companion, the ground, vegetation, or even itself. Such behaviour is termed redirection. Displacement (displacement activity) is the resolution of a conflict situation in which a seemingly irrelevant activity is performed. When an animal is obviously in conflict between, for example, sex and aggression or between aggression and fear, it will often perform an apparently irrelevant activity such as grooming, feeding, or scratching, or the animal may go to sleep. It is as if the two activated drives neutralize one another and the surplus energy is fed into another system. In disinhibition, two drives that appear to independently inhibit a third mutually inhibit each other in a conflict situation and lose their inhibiting effect on the third, which then becomes free to activate its own behaviour.

      The probable reason that displacement activities are so often comfort behaviours (e.g., preening) is that such behaviours have a typically low threshold of performance; ordinarily, they do not have a particularly high priority and are, therefore, easily elicited when more urgent demands are not being made upon the animal. This explanation is consistent with the fact that, though the frequent performance of these displacement activities is essential to survival, they are seldom associated with any condition of urgency.

      The nature of the displacement activity may also depend upon the immediate environment or upon the effects of autonomic nervous activation on the animal; sometimes the activity depends on both factors. A wood thrush caught, while sitting in a horizontal position, by a strong attack–escape conflict will wipe its bill on its perch. The same bird caught by the same conflict, while sitting almost vertically, will make perfunctory preening movements directed at its upper breast. The relationship of the bird to its immediate environment makes it easier to perform a particular comfort activity. Autonomic responses—the result of fear or aggression in man, for example—may cause an expansion or constriction of blood vessels, a tingling sensation in the skin, a tendency to urinate or defecate, and so on. Scratching and temperature-adjustment behaviours may sometimes be prompted by such physiological changes.

Transitional activity
      In transitional activity, another type of conflict resolution, the animal is stimulated to perform a particular behaviour, but the required environmental stimulus becomes unavailable during the course of the response. The animal discontinues its initial behaviour and substitutes another behaviour that it initially had not “intended” to perform.

      The common grackle demonstrates a transitional activity in the form of a threat behaviour termed a spread–squeak; the bird ruffles its plumage, then utters a squeak as it compresses its feathers. If a rival approaches, prompting a spread–squeak, and turns away only at the plumage-ruffling stage, the bird will then, instead of squeaking, often shake its body—a normal comfort activity customarily preceded by plumage ruffling. When a man offers his hand to be shaken and it is not accepted, he often behaves as if he had really intended to gesture or perform a similar action. Transitional activities may be cases of displacement in which the nature of the behaviour is determined by the immediate environment. A grackle involved in the ruffling stage of a spread–squeak, after the rival leaves, may simply shake its body because, in the absence of further stimuli for fear and aggression, it is left with the stimulus for shaking, which may be ruffling. Conflict situations are of great interest from an evolutionary standpoint, because they are often the raw material from which signals have evolved.

Behavioral evolution and development
      Behaviours are believed to evolve in the same way as structures. The recombination of genes afforded by sexual reproduction ensures that each individual differs in some degree from all others of its species. Even slight variations from the norm increase or decrease the probability of survival. Advantageous features tend to be conserved and disadvantageous ones eliminated. Marked changes are the result of the slow accumulation of small variations over long periods of time, representing many generations. A species never becomes totally adapted to its environment because the environment is constantly changing. As a result, selective pressures always exist, and the process of evolution continues.

Selection in domestic animals
      Animals can be selectively bred for specific behavioral changes. Many domestic animals differ markedly in behaviour from their wild progenitors. Domestic breeds such as fighting cocks and Siamese fighting fish are hyperaggressive, but most domestic animals tolerate greater crowding and are more docile than their wild ancestors. House (house mouse) mice have been selectively bred in the laboratory to produce unusually aggressive, as well as unusually timid, strains. Mating selection in domestic animals is usually less restrictive than in their wild counterparts. Reciprocal signalling systems between animals become less precise with domestication, and behavioral components may be omitted or lost altogether. Promiscuity may replace pairing in certain animals.

      Marked differences in courtship displays occur between wild pigeons (pigeon) (Columba livia) and domestic breeds. Wild males loudly clap the wings over the back in flight and then glide with the wings held well above the horizontal position. In pouter pigeons (a breed of C. livia), the wings are clapped so frequently that two-thirds of the length of the primary wing feathers may thus be worn off, with the result that flying becomes very difficult. The elevation of the wings during the glide phase becomes so exaggerated that the wing tips touch, and the bird quickly loses altitude. In roller pigeons, another breed, the gliding flight has become a series of backward somersaults. On the ground the male wild pigeon, while cooing, twirls and makes a small hop when the female walks away. The German ringbeater breed elaborates on this behaviour by performing a wing-clapping flight around the female, who remains on the ground.

      Domesticated zebra finches (Poephila guttata) show marked loss of specificity in their mating interactions and in care of the young, when compared with their wild counterparts. Wild chickens will kill their own chicks that lack specific colour patterns. Domestic chickens, on the other hand, will care for almost any chick regardless of colour and pattern; yet, they retain specific reactions to the species-specific calls of chicks so that they do not ordinarily accept other young birds, such as ducklings. Highly domesticated chicken breeds, such as Plymouth Rocks and barred Plymouth Rocks, however, will rear even ducklings. The least domesticated breeds, such as certain game bantams, still show much of the specificity characteristic of wild birds. Wild graylag (greylag) geese form pairs only after a very long courtship period and remain monogamous. Domestic derivatives, on the other hand, pair quickly with any member of the opposite sex and are not monogamous. All of these differences between wild animals and their domesticated derivatives have a genetic basis.

Behaviour in hybrids (hybrid)
      Two closely related species of small African parrots, the peach-faced lovebird (Agapornis roseicollis) and Fischer's lovebird (A. personata fischeri), have completely different methods of carrying nesting material. The females of both species prepare nesting material by cutting long, narrow strips of bark, leaves, or paper. The peach-faced lovebird tucks each strip, after she cuts it, into the feathers of the lower back, or rump. When she has accumulated about six strips, she flies to the nest cavity, retrieves the strips, and places them in her nest. Fischer's lovebirds carry each strip in the bill, one at a time, to the nest cavity.

      Female hybrids between these two species initially tuck nest material into their rump feathers, but the strips fall out before the birds reach the nest. The birds gradually develop, through learning, an increased tendency to carry each strip singly in the bill. About four months after the onset of the tucking behaviour, they are utilizing both behaviours about equally. Although the tendency to carry in the bill continues to increase after this point and the tendency to tuck continues to decrease, the rate of divergence between the two methods becomes much slower. By the end of the third year the hybrids carry all strips in the mouth, but they make small intention movements to tuck. These intention movements consist of little ticlike, side movements of the head just before the bird flies off to the nest.

      The courtship behaviour of male hybrids, paired with female hybrids of this same cross, is intermediate between that of the two parental-species males. When the hybrid males are paired with parental-species females, their courtship behaviour, in most cases, is closer to that of the parental species of the female, although it sometimes remains intermediate. The species-typical behaviour of the females is thus seen to influence the pattern of male courtship. The courtship behaviours of some bird hybrids are not so greatly modified; for them, no permissible variability has been inherited.

      Two cricket species, Gryllus campestris and G. bimacularis, are so similar morphologically that they can be distinguished from one another only with great difficulty. Their behaviours on the other hand, differ markedly. If the two species are crossed, however, the inheritance patterns may be traced by means of behaviour. Four behaviour patterns—antennal vibration in the post-courtship period, pendulum movements of the thorax, stridulation (rubbing one body part against another to produce sound), and fighting by young adults—have been investigated in particular detail. It has been found that antennal vibration and juvenile fighting in the hybrids have a monofactorial inheritance (i.e., are caused by a single gene). The pendulum-like movement of the thorax during mating is found only in G. campestris and has a polygenic basis (i.e., is caused by more than one gene). The stridulating sounds preceding courtship, performed only by G. bimacularis, are seemingly based on one pair of alleles (i.e., different forms of a single gene).

      Two races of honeybees (honeybee) are distinguished from one another by the presence or absence of hygienic behaviour. The race exhibiting hygienic behaviour opens comb cells containing dead pupae, which are removed. The nonhygienic race leaves dead pupae in their cells. The first generation (F1) of hybrids contain only nonhygienic bees. One F1 queen produces four kinds of drones, or males. When the F1 is backcrossed with the hygienic form, a second generation (F2) is obtained, which is made up of four different types of bees. One group is hygienic; one group opens the cells of dead pupae but does not remove the dead pupae; one group does not open the cells of dead pupae but removes the dead pupae if the cells are open; and the remaining group is nonhygienic.

The influence of experience on behaviour
      The adaptive change of behaviour as the result of experience—usually known as learned behaviour or experience-dependent behaviour—may be observed in all higher vertebrate forms. Several types of such learning (animal learning) in animals are recognized: habituation, classical conditioning (CR Type I), trial and error (CR Type II, instrumental conditioning, or operant conditioning), latent learning, insight learning, and imprinting.

      Habituation is learning to disregard stimuli that are without significance to the animal. In many respects it is the simplest form of learning, and it is sometimes regarded as a fundamental property of all living matter. Most animals inherit a response to be frightened by sudden and strong stimuli such as loud sounds, flashes of light, and the sudden intrusion of anything foreign into the animal's sensory field. Yet, if an animal reacts nonselectively to all phenomena such as rustling leaves, thunder, snapping twigs, and the sudden appearance of harmless animals, those phenomena that are significant to the animal's well-being will not receive the intensity of response that they often require. All animals, then, quickly habituate to such harmless stimuli, but the adaptation is highly specific. An animal that habituates to one type of sound does not, as a consequence of this habituation, become habituated to other sounds. Habituation is distinct from failing to respond to stimulation as a result of fatigue, sensory adaptation, or injury. The effects of habituation are generally long lasting. If an animal is repeatedly exposed to a potentially harmful stimulus (such as to a predator) without being harmed, habituation does not generally occur. Responses to dangerous stimuli often seem to have an inherited resistance to habituation—a mechanism of obvious survival value.

      Classical conditioning was studied early in the 20th century by the Russian physiologist Ivan Pavlov (Pavlov, Ivan Petrovich), who observed that dogs (dog) salivate when food is placed in their mouths. He gave dogs food and, at the same time, provided another stimulus such as a flashing light or the sound of a bell. After a few such pairings of stimuli, a dog would salivate upon seeing the light or hearing the bell but without the presence of food. The dog had learned to associate the flashing light or the sound of a bell with food. A previously irrelevant stimulus that assumes significance as a result of association with a relevant stimulus is called a conditioned stimulus (CS). Salivation in response to such a stimulus is termed a conditioned response (CR). Prior to the learning experience, only the food (unconditioned stimulus) was effective in producing salivation (unconditioned response).

      Such conditioned responses have been observed in a wide variety of animals, from lower invertebrates to man. Birds learn to avoid noxious insects in this manner; the distasteful monarch butterfly (Danaus plexippus) or a species of stinging wasp provide effective stimuli that quickly become associated with the appearance of such insects. This kind of association together with the conditioned response relevant to it is also the basis for Müllerian mimicry, in which palatable insects and other animals evolve to resemble noxious ones, thus enhancing their chances of survival.

      In trial-and-error learning, an animal learns to behave in a particular way by associating something it does with a desired effect. If a dog's foot is lifted by the experimenter and food then given, the dog, after a few such trials, will spontaneously lift its foot in anticipation of food. Both classical conditioning (CR Type I) and trial-and-error learning (CR Type II) are termed associative learning, because in both cases an unconditioned response is associated with a conditioned stimulus. In natural situations an animal probably learns to associate certain spontaneous activity of its own with certain desired results, thus fixing the conditioned stimulus and response. Learning of this type often occurs when animals modify their behaviour during appetitive sequences such as those involving feeding and mating.

Latent learning
      Latent learning is the association of indifferent stimuli or situations with one another without reward. The phenomenon is clearly exemplified in exploratory behaviour. Animals finding themselves in unfamiliar environments or among unfamiliar objects, but in familiar surroundings, show exploratory behaviour. The animal uses its sense organs on all that is novel, shows much ambivalence between approach and avoidance, and, finally, as the hesitancy to approach wanes, will test the novelty. A mammal may, at this point, sniff, nudge, or handle a strange object; a bird may peck at it. The first contact often results in abrupt avoidance, but, typically, the object or situation is, at last, thoroughly explored and finally abandoned if neutral. A mouse sniffs and pokes about most of a new environment. A bird, having sight and hearing, rather than smell and touch, as dominant sensory modalities does not have to occupy physically as much of a novel environment. Instead, it places itself successively at several vantage points and then carefully peers about and listens.

      Subsequent behaviours of an animal can reveal that it has learned much about its environment during such an exploratory phase. It may learn, for example, the physical features of the environment and their spatial relationships to one another, the location of food and water, and the location of places safe from predators (predation). Animals apparently learn all of these things early in their exposure to an environment, even though the information acquired may become significant only at some time in the future. The learning takes place without being associated with immediate reward (unlike that in conditioned response Types I and II) unless a need to know is postulated as a kind of immediate self-reward.

Insight learning (insight)
      Insight learning is believed to be an advanced type of learning. Insight involves the spontaneous combination of a number of isolated experiences; the result is a new experience that is effective for gaining a desired result. Humans are able to exercise insight; it is extremely difficult, however, to identify such behaviour in most other animals. Animals are believed to use insight when they solve a problem too rapidly for normal trial and error to occur. It is possible that such an animal is carrying out trials in its brain; this implies reasoning ability. The higher primates (primate) are probably capable of insight learning at times, but further down the phylogenetic scale the evidence of such learning becomes progressively less conclusive.

      Chimpanzees, to get food out of reach, will pile boxes to make a stand for themselves or will fit sticks together to knock the food down. These solutions may come quickly, an obvious result of prior experience (latent learning). Much trial-and-error learning is demonstrated, however, when they actually pile boxes or fit sticks together. In humans, insight is probably often aided by latent learning and trial and error.

      Imprinting, a learning process observed in young birds and mammals, is the identification of an animal with another animal. Normally, it is a relationship between members of the same species, but it can occur, for example, between a bird and a human. Imprinting can take place only during a particular period of the animal's development—a time span that is specific for each species.

      In 1935 the Austrian ethologist Konrad Lorenz (Lorenz, Konrad) first observed the process in ducklings and goslings. After goslings hatch and become dry, they follow their parents. The adults provide warmth, safety, and shelter and bring the goslings food. The more uncomfortable a gosling becomes (e.g., cold, frightened, hungry), the more intensely it follows. If goslings are reared by a human, they become imprinted to humans; thus, they ignore geese.

      The development of this response occurs during a sensitive period, before and after which the response cannot be learned; if the response is not acquired during the sensitive period, it will never occur. Zebra finches that are isolated from their own species before they are 35 days old are never able to distinguish males and females of their own species. This is because their sensitive period for imprinting occurs before they are 35 days old.

      The duration and time of onset of the sensitive period depend on the species and on the type of behaviour involved. Some animals imprinted to animals of another species will mate with members of their own species but, if given a choice, will prefer the animal to which they have been imprinted. Many species refuse social contact with any animal except the one to which they are imprinted. Male golden pheasants (Chrysolophus pictus) imprinted to humans will court females of their own species but immediately transfer this behaviour to a human, should one appear. The same is true for budgerigars (budgerigar) (Melopsittacus undulatus) and turkeys (Meleagris gallopavo). Mallard ducks imprinted to humans, on the other hand, will not associate with members of their own species (conspecifics) and will continue throughout their lives to treat humans as conspecifics. Imprinting is fixed for life, in contrast to other types of learning, in which forgetting is common. Imprinting of motor patterns, such as birdsong (vocalization), also occurs. Exposure to a particular birdsong may be relatively brief and still be permanently fixed in the bird's memory. Chaffinches (chaffinch) (Fringilla coelebs) learn their songs during the first 13 months of life, although they do not sing until nearly a year later. A 12-day-old nightingale (Erithacus megarhynchos) was kept in the same room with a singing black-capped warbler (Sylvia atricapilla) for about one week. The following spring the nightingale sang a typical black-capped warbler song.

      Little is known about imprinting in mammals, but hoofed animals, such as sheep and horses, that are imprinted to man express this response by following him about. Dogs from four to six weeks old develop normal social responses to dogs or to another species such as man. Imprinting apparently also occurs in humans. An infant deprived of its mother for a short period during its first year may develop serious mental retardation. A separation of several months—particularly during the seventh to the 12th month—will frequently result in irreparable damage; under such conditions, death may result.

play behaviour and curiosity behaviour
      Play and curiosity are exhibited by many mammals and by some birds and figure importantly in the learning of numerous activities. Play is especially characteristic of young animals, but the adults of many species also engage in it. Spontaneous curiosity, in which the animal actively seeks out novel situations for exploration, is exhibited by the young of mammals and some birds; indeed, they seem to be under the compulsion of some drive to do so. Carnivores and primates exhibit more curiosity than rodents, which gnaw novel objects and may hoard them. Monkeys inspect and manipulate such objects.

      The curiosity drive implements the development of new motor skills and ensures the acquisition of new perceptual impressions, thereby resulting in new knowledge. The only reward, however, seems to be the performance of the activities themselves. Rhesus monkeys (rhesus monkey) will learn a puzzle game without any reward except its successful solution. Among rats, it has been observed that the nerve cells in the lateral hypothalamus and preoptic regions of the brain are more active in those rats that explore. Electrical stimulation of these brain areas is rewarding to rats, and they will learn to press a lever that activates this stimulation.

      Such curiosity behaviour seems linked to play behaviour. Play is difficult to define; it is usually easy, however, to distinguish a playing animal from one that is seriously occupied. An animal plays only when it is satiated and not preoccupied with other tasks. Play seems not to be dictated by immediate need but is extremely important in behavioral development. Only animals that spontaneously seek new situations on their own initiative play in the true sense. Invertebrates, fish, and amphibians do not seem to play. The taxonomic distribution of play among mammals and birds suggests that play is related to learning. Play involves interactions with the environment; this leads to the acquisition of knowledge about environmental features, including information about conspecifics and the animal's own possibilities of movement. Play behaviour occurs only at particular times; progression to a second play activity takes place only after a certain level of skill has been achieved in the first.

      Much play appears to be fighting or fleeing behaviour, and usually it is easily identified as such. An animal that is play escaping or play attacking does not actually escape or attack. A rodent play fleeing into a hole, for example, quickly reappears. If a rodent's flight is truly an effort to escape, it reappears only after a much longer interval. Play-fleeing animals often reverse roles quickly, and the pursuer becomes the pursued. Threat behaviour that is associated with real attack is missing, and there is strong reluctance to bite. Play tends to be highly repetitive. A dog may retrieve a stick many times or play fight until it is exhausted or until a more interesting activity distracts it.

      Such play behaviour could mistakenly be postulated as the performance of immature instinctive activities. In many instances, however, this is known not to be the case. Much playful behaviour occurs at a time in an animal's life when it is fully capable of serious activity. Play also involves the use of species-typical patterns of behaviour (agonism) in various sequences that do not occur in serious activity.

Modification of instinctive behaviour by experience
      Behaviours based on both instinct and learning are commonly intercalated into functional wholes. In the peach-faced lovebird, for example, the cutting of nest-material strips is partly instinctive and partly the result of experience. The propensity for cutting is instinctive. This includes punching holes in the sheet of material from which the strips are fashioned and a “knowledge” of the proper width, length, and straightness of strips. The spacing of punch holes in such a manner as to form a strip is learned through experience. It is as if the animal has an instinctive picture in its central nervous system and persistently tries punching holes, in various relationships to one another, until the right pattern is made. The bird tends to repeat punching patterns that most closely approach the ideal and, thus, gradually, through progressively more satisfying feedbacks, approach the definitive functional technique of cutting strips. Idiosyncratic techniques develop—some birds stand on the sheet while cutting, and others stand off the sheet; some cut to the left, others to the right, and still others cut in various combinations of these directions. Birds developing their techniques try all of the directions and places of standing but gradually act with less and less variation. They do not need to observe experienced birds in order to develop a technique. The stimuli necessary for learning response are intrinsic to the birds' individual activities.

      If a meadowlark (Sturnella magna) is exposed to an alien song during its sensitive period for learning song, it will learn the alien song. Meadowlarks, then, learn their species-typical songs rather than inherit the capacity for particular melodies. But, if they are exposed during the sensitive period to alien songs along with meadowlark songs, they will learn only the normal species-typical song. Although the song must be learned, the bird instinctively learns the species-typical song if there is a choice. The bullfinch (Pyrrhula pyrrhula) instinctively learns the male parent's song rather than the species-typical song. Bullfinches raised by foster parents of another species will learn the song of the male foster parent, even though the normal bullfinch song is also audible to it during the same period. In both types of song acquisition, learned and instinctive elements combine in the development of the species-typical song. Some species, however, do not learn their songs and do not need the experience of hearing others sing during their development. Different vocalizations in the same species are commonly acquired in more than one way. Some are purely instinctive; others are learned.

      The behaviour of an individual animal is the result of a genotype that has developed over millions of years of evolution—a genotype that also permits a certain degree of variability through experience.

Hormonal and nervous control of behaviour

Interaction of endocrine (endocrine system) and nervous systems
      Physiological (physiology) changes within an animal are largely the direct or indirect result of nervous and endocrine (hormonal (hormone)) changes and their interactions. These changing states are also responsible for a changing responsiveness to internal and external environmental stimuli.

      The endocrine system and the nervous system are probably of equal evolutionary age; the two systems may be evolutionarily linked. Certain nerve cells, or neurons, that are highly modified produce substances that pass through the axons (threadlike extensions of neurons) and into the bloodstream. These neurosecretory cells are sometimes clustered, forming glands that have connections with both the nervous system and the bloodstream. The endocrine glands are similarly distributed. Some may have evolved from clusters of neurosecretory cells. The vertebrate pituitary gland has evolved as a fusion between neural tissue and epithelial tissue (the lining, or covering, of organs) and is intimately associated with the hypothalamus, a ventral portion of the brain. The pituitary may be regarded as a master gland that regulates, with its secretions, all the other endocrine glands. The nervous system, in turn, has a regulatory effect upon the pituitary, which may also be influenced by feedback effects stemming from the secretions of other endocrine glands. This relationship permits the outside and inside environments to exert influences on the endocrine system. Seasonal changes in day length, for example, influence the nervous system by means of visual stimuli. The endocrine system is then activated through stimulation of the pituitary gland by the hypothalamus; the pituitary, in turn, secretes hormones appropriate to the most adaptive response. The ultimate effect of a seasonal change in day length may be migration or reproduction (see endocrine system (endocrine system, human)).

      The nervous system and the endocrine system interact with and complement each other. The nervous system sends information with great speed but of short duration along its pathways. Its messages can change rapidly. Hormones, secreted by the endocrine system into the bloodstream, travel much more slowly than nervous impulses. The endocrine system can keep a message constantly available for many months if necessary. The nervous system generally affects only muscles and glands, but hormones can reach every cell in the body. Adrenaline (epinephrine and norepinephrine) is a hormone that acts with relative speed. It is secreted by the two adrenal glands, which are attached to the kidneys. The adrenals consist of two portions that differ from one another both in origin and in function. The inner portion is the adrenal medulla, and the outer portion is the adrenal cortex. During stress, such as occurs in fighting, mating, and fear, the adrenal medulla is stimulated by the autonomic nervous system to release adrenaline into the bloodstream. Some of the changes that occur throughout the body under the stimulus of adrenaline include hair erection, sweating, and acceleration of heartbeat and breathing; adrenaline also causes the blood to be diverted from the digestive tract to the muscles. All of these changes, and others, help prepare the animal for extreme effort. During brief stressful periods the bloodstream is quickly flushed with adrenaline, but the hormone is quickly dissipated. If the stress situation persists, other events take place—the adrenal cortex becomes involved, and its hormones are released. The cortex, unlike the medulla, is not under direct nervous control but is stimulated by another hormone, the adrenocorticotropic hormone, or ACTH, produced by the pituitary gland. Prolonged stress stimulates cells in the hypothalamus, which, in turn, stimulates the pituitary gland to produce ACTH. The cortical hormones in turn stimulate various responses to prolonged stress. Some of these responses are concerned with the metabolism of glucose, a sugar that may be associated with the utilization of food reserves. The effects of cortical hormones are, in any case, profound, and continuing stress will result in enlargement of the adrenal cortex, which leads to increased production of the cortical hormones. Chronic stress may cause severe illness and even death. It has been shown that rats confined to the territories of other rats will die as the result of overproduction of cortical hormones in response to the stressful situation. Stress as a result of overcrowding may also cause death in animals. Such stress may, in fact, be the cause of a decline in numbers of mice after they have reached a certain population level in a given area.

Sex hormones (sex hormone)
      The pituitary (pituitary gland) hormones that have a direct effect upon reproductive behaviour are the follicle-stimulating hormone (FSH), the luteinizing hormone (LH), and prolactin (lactogenic hormone or luteotropic hormone). FSH and LH are called gonadotropins because they stimulate the gonads (ovaries (ovary) and testes (testis)) to produce germ cells and gonadal tissue; gonadal tissue, in turn, secretes other hormones. Prolactin has a variety of effects. In different species of vertebrates, prolactin affects different target organs. In female mammals, for example, it stimulates growth of the mammary glands and the secretion of milk. It also stimulates the corpora lutea—glandular bodies of the ovary—causing them to produce another hormone, progesterone. In pigeons and doves, prolactin causes the characteristic modification of the crop (stomach) associated with the production of so-called pigeon's milk—a soft white substance that is passed from the mouth of the adult pigeon to that of the young. A slow change of colour in some fish is caused by the influence of prolactin on pigment cells. (Rapid colour changes are under nervous control.)

      The gonads secrete hormones from special cells when stimulated by FSH and LH from the pituitary gland. Collectively, the female hormones are termed estrogens (estrogen), and the male hormones are called androgens (androgen). Both androgens and estrogens belong to a chemical group known as steroids (steroid). All steroids have closely related chemical structures; the different vertebrate groups have slightly different steroid hormones that seem to be largely interchangeable in function. Removal of the testes or ovaries ( castration) in vertebrates causes profound changes in behaviour and structure, especially if done early in life. Among many invertebrates castration has no such profound consequences. Apparently, therefore, only in the vertebrates are the gonads important endocrine organs.

      Androgens and estrogens control the development of the secondary sexual characteristics; they also effect the production of eggs and sperm from the gonads. Secondary sexual characteristics tend to be relatively permanent throughout life, but many are temporary, occurring only during the breeding season. Examples of temporary characteristics include the special breeding plumages and songs (vocalization) of many birds, the antlers of deer, colour changes in some fish, and all the behaviour associated with the formation of fertilized eggs (zygotes).

       progesterone, the production of which is stimulated by prolactin, is produced in mammals by the ovary. After an egg is shed, the empty follicle enlarges, forming the corpus luteum, a conspicuous yellowish structure, which secretes progesterone; progesterone, in turn, stimulates changes in the uterus preparatory to its receiving the fertilized egg. Progesterone also inhibits the contraction of uterine muscles. The females of other vertebrate groups produce structures similar to the corpora lutea of mammals. Birds, which have rather inconspicuous corpora lutea, also produce progesterone.

      The amount of any hormone normally present in the bloodstream is minute. Artificially high levels often have marked effects; the introduction of massive doses of testosterone into females, for example, causes male sexual behaviour. It will also cause female sexual behaviour in both sexes, in which case testosterone may be acting as a general stimulant.

      Sometimes a massive dose of hormone has an effect opposite to the one expected. Female canaries (canary) (Serinus canarius) given overdoses of estrogen would be expected to show enhanced sensitivity of their brood patches (highly vascularized areas of skin in close contact with the eggs during incubation); instead, overdoses of estrogen may result in some desensitization of these areas.

      It is tempting to conclude that the sex hormones have a direct effect on all the structures and behaviours concerned with the formation and nourishment of zygotes. Much is not known, however, about the hormonal control of behaviour. There may be feedback effects after the hormones have initiated a particular response; for instance, estrogen in conjunction with progesterone in some birds causes increased thickening and vascularization of the brood-patch area. The target tissues in this case (skin, blood vessels, and feather follicles) all have a nerve supply, and feedback through them to other parts of the nervous system could initiate further behavioral modifications. Physiological changes resulting from hormonal action may render an animal more or less responsive to its environment and thus modify its behaviour.

      There is evidence that hormones directly affect behaviour by acting directly on neurons in the hypothalamus. If estrogen is injected into certain areas in the hypothalamus of castrated female cats (feline), they develop strong estrous behaviour, even though the reproductive system remains underdeveloped.

      Behaviour may stimulate hormone production. Female cats, rabbits, and some other mammals are “induced ovulators.” In other words, copulation stimulates the hypothalamus via the nervous system, and the pituitary gland is then stimulated to produce luteinizing hormones (LH), which in turn affects the ovaries. A few hours after copulation ovulation occurs at about the time the sperm have reached the upper levels of the reproductive tract; the germ cells meet, and fertilization takes place. In deer, sheep, and weasels, stimulation of the pituitary gland (resulting in the production of follicle-stimulating hormones [FSH]) is achieved when the animal senses a change in day length. The females of other mammal species (e.g., the house mouse [Mus musculus]) seem to have an internally based clock that periodically triggers the release of FSH regardless of environmental changes. They may still be influenced, however, by the presence of a male.

      Complex interactions between hormones and behaviour are known to occur in birds. The sight of a courting male stimulates the release of FSH and LH in a female dove (Streptopelia risoria). Physical contact between the birds is not necessary for this response to take place.

Nervous system and behaviour
      The nervous system receives information about the external environment through sensory receptors and about the internal environment through hormones, internal neural responses, and other physiological events. This information, regardless of the source, is processed in the brain or spinal cord, and appropriate responses are initiated by outgoing (efferent) nerve impulses leading to muscles or glands.

      There is much evidence to support the view that the central nervous system (CNS) is hierarchically organized. Its organization is thought to consist of a system of centres, each with the function of collecting stimuli and appropriately redispatching them. Reproduction in the peach-faced lovebird depends first upon the activation in the proper sequence of a number of subordinate centres. These involve formation of a pair bond between a male and female; selection of a nest site; conduction of courtship; laying and incubating of eggs (egg); and caring for the young. Nest building occurs throughout courtship, egg laying, incubation, and care of the young.

      Subordinate events such as those mentioned above control, in turn, other neurally controlled events. nest building, for example, consists of various subordinate activities that have already been described. Each of these subordinate activities also has subordinate activities. Tucking a strip of nesting material consists of several activities: simultaneously turning the head back over the rump, lowering the unfolded wing on the same side, and erecting the rump feathers; pushing the strip into the feathers, performing rapid hooking movements, which seem to function as an anchoring behaviour; releasing the strip; and, finally, simultaneously bringing the head, wing, and rump feathers back to the normal position.

      It has been proposed by investigators that the smallest irreducible neuromuscular coordinations of an activity be thought of as acts. Each species is capable of performing a finite number of such acts, and these are combined in various ways to produce all of the behaviours of which an animal is capable. Each act is thought to have an act centre in the CNS. The act centres are subordinate to the behavioral centres, which coordinate them; behavioral centres are, in turn, subordinate to their initiating and coordinating centres and so on. The term centre in each case refers to a functional rather than to an anatomical locus. Portions of the CNS responsible for mediating a given response may be quite diffuse anatomically; typically, there may be a great deal of redundancy—a condition that frequently enables an animal to regain normal behaviour following damage to the CNS. Brain tissue, however, does not regenerate.

      Detailed implementation of various subordinate activities may depend upon the details of environmental feedback, as in the taxis components of many fixed action patterns (see above Fixed action patterns (animal behaviour)). Such detailed implementation may also depend upon long-term changes in behaviour that result from experience. Various releasing mechanisms may also be altered as a result of experience, as may the significance of various environmental stimuli. Experience may therefore exert modifying effects upon the input of information, its mediation in the CNS, and the details of its implementation through muscle coordination and glandular activity. These experiential effects may be of long or brief duration.

      For many years the classical reflex was thought to explain adequately the mechanism of various behaviours. Such a reflex consists, in its simplest form, of an afferent neuron carrying information to the CNS, excitation then being carried to an effector (muscle or gland) via an efferent neuron. Intermediate neurons often occur between the afferent and efferent neurons. This entire mechanism is termed a reflex arc. In certain reflexes, the excitations activate the same muscles or glands from which the stimuli originate (monosynaptic reflexes). Most reflexes have intermediate neurons, and the stimulation of a single receptor may activate many effectors; similarly, the stimulation of many receptors may activate but a single effector. Chain reflexes occur as a result of one reflex triggering another. Additional reflex arcs may become established as a result of experience (that is, conditioned reflexes), and some reflexes are thought to have facilitatory or inhibitory effects on others.

      It is now known that not all behaviours are the result of afferent impulses. Early in the 20th century it was found that cats' leg muscles (muscle) with all afferent nerves removed still demonstrate (feedback) rhythmic movement. Afferent impulses are not necessary for coordinated response; the swimming movements of eels and other fish, the crawling of earthworms, and the flying movements of grasshoppers are some examples. It is now known that spontaneously generated stimulation of the central nervous system initiates and controls much behaviour.

      The evolutionarily older portions of the CNS, such as the spinal cord and the medulla and hypothalamus of the brain, seem to be concerned mainly with inborn behaviour such as heartbeat, breathing, and reflexes and with instinctive (drive) behaviour. The evolutionarily newer portions of the brain, such as the cerebrum of mammals, seem to be concerned either with new behaviours resulting from experience or with the modification of inborn behaviours. The conspicuous cerebrum of mammals often comprises a major portion of the brain. Although it is generally accepted that mammals, as a class of vertebrates (vertebrate), are the most intelligent of all animals, many birds seem to be capable of greater modification of behaviour through experience than are some mammals. Many other vertebrates, such as certain reptiles and fish, are capable of learning new behaviours rather easily. It was long believed that, because birds and other nonmammalian vertebrates have little or no discernible cerebral tissue, their behaviours were instinctive. It is now known, however, that such animals are often capable of much behavioral modification as a result of experience and that some other portion of the brain must be involved. Different portions of the brain in different animal groups seem to have been selected for the development of learning ability.

William C. Dilger

Additional Reading

General works
Konrad Lorenz, King Solomon's Ring: New Light on Animal Ways (1952, reissued 1991; originally published in German, 1949), is a highly recommended popular treatment. Peter Marler and William J. Hamilton III, Mechanisms of Animal Behavior (1968), is also of interest. Semipopular works include Niko Tinbergen, Curious Naturalists, rev. ed. (1974, reprinted 1984), highly recommended to all interested in behaviour; and Niko Tinbergen et al., Animal Behavior, rev. ed. (1980), an excellent résumé of ethology. Marc Bekoff and Dale Jamieson (eds.), Interpretation and Explanation in the Study of Animal Behavior, 2 vol. (1990), is a collection of interdisciplinary essays covering a wide range of topics. Bonnie V. Beaver, The Veterinarian's Encyclopedia of Animal Behavior (1994), is easy to read and describes normal and abnormal animal behaviours and specific subjects in alphabetical order.

Nature and patterns of animal behaviour
Studies include Claire H. Schiller (trans. and ed.), Instinctive Behavior (1957); Anne Roe and George Gaylord Simpson (eds.), Behavior and Evolution (1958); Kenneth D. Roeder, Nerve Cells and Insect Behavior, rev. ed. (1967); Irenäus Eibl-Eibesfeldt, Ethology: The Biology of Behavior, 2nd ed. (1975; originally published in German, 1966); J.E.R. Staddon, Adaptive Behavior and Learning (1983); Jeffrey M. Camhi, Neuroethology: Nerve Cells and the Natural Behavior of Animals (1984); Matthew H. Nitecki and Jennifer A. Kitchell (eds.), Evolution of Animal Behavior: Paleontological and Field Approaches (1986); and John Alcock, Animal Behavior: An Evolutionary Approach, 5th ed. (1993). More advanced treatments are Eugene L. Bliss (ed.), Roots of Behavior: Genetics, Instinct, and Socialization in Animal Behavior (1962); Konrad Lorenz, Evolution and Modification of Behavior (1965, reprinted 1986); Robert A. Hinde, Animal Behaviour: A Synthesis of Ethology and Comparative Psychology, 2nd ed. (1970); Peter H. Klopfer (compiler), Behavioral Ecology (1970); W. Sluckin, Imprinting and Early Learning, 2nd ed. (1972); Peter H. Klopfer, Behavioral Aspects of Ecology, 2nd ed. (1973); J. Balthazart, E. Pröve, and R. Gilles (eds.), Hormones and Behaviour in Higher Vertebrates (1983); John L. Fuller and Edward C. Simmel (eds.), Behavior Genetics: Principles and Applications (1983); and Katherine Albro Houpt, Domestic Animal Behavior for Veterinarians and Animal Scientists, 2nd ed. (1991). Works that deal with animal cognitive behaviour include Donald R. Griffin, The Question of Animal Awareness: Evolutionary Continuity of Mental Experience, rev. and enlarged ed. (1981), Animal Thinking (1984), and Animal Minds (1992); and James L. Gould and Carol Grant Gould, The Animal Mind (1994).

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