involuntary response by an animal to an external stimulus. The concept has come to refer to complex unlearned behaviour that is recognizable and predictable in at least one sex of a species.

Instinctive behaviour is largely heritable. Many of the activities of a species of animal are sufficiently constant and predictable to serve as specific characteristics in the same way, and often to the same degree, as do bodily structures. Examples of such heritable traits include the display movements of birds (e.g., peacocks), the web-spinning movements of spiders, the burrowing habits of marine worms, the prey-catching techniques of weasels or wolves, the food-hoarding movements of squirrels, and the browsing methods of antelope.

The genetic or inherited nature of instinctive behaviour is particularly evident in aggressive and submissive sexual behaviour and in fighting of various kinds. Other instinctive activities of this kind serve nutrition, including methods of obtaining and eating food; care of the body surface, including cleaning, grooming, and scratching movements; escape from predators, including methods of concealment, freezing or “playing dead,” and taking flight; social behaviour, including ways of responding to others both sexually and regardless of sex; and sleep, including the rhythms of rest and wakefulness and bodily positions assumed in sleep.

Many of these relatively fixed, species-characteristic types of behaviour appear to be primarily inherited; at first sight, at least, they may seem little influenced by the particular experiences of the individual animal. But much instinctive behaviour as, for example, playing or exploring, is, nevertheless, modifiable, and the detailed form of the action taken may vary according to the circumstances of the moment and the individual experience previously encountered.

Instinctive activity is not usually a limited response to a simple stimulus but rather is a sequence of behaviour that runs a predictable course; for example, nest-building (nest) behaviour that shows a patterned sequence of acts among many birds and some fishes. Many of these actions are far from being either simple or brief. Extraordinary elaboration may be found, and, although some instinctive behaviours may be complete in seconds, others may take minutes, hours, or days.

Since instinctive behaviour is assumed to be genetically based and therefore shaped by the pressures of natural selection, it follows that most of the consequences of instinctive activity contribute to the preservation of an individual or to the continuity of the species; that is, instinctive activity tends to be adaptive, contributing to the animal's ability to reach maturity and to breed.

Ideally, instinctive behaviour seems not to depend on learning or practice but to emerge in full complexity without rehearsal when appropriate stimuli or circumstances are encountered. Often, such stimuli do not guide or mold the instinctive behaviour but seem simply to trigger or release it. This characteristic gives instinct the appearance of driving the animal endogenously (from within); the quality of instinctive activity thus appears to depend only secondarily on exogenous (external) stimulation.

Experiments in which an animal is raised in a very limited environment (perhaps in isolation, as when a songbird is kept alone in a soundproofed chamber from the moment of hatching) may indicate the extent to which behaviour is spontaneous (or endogenous) or is governed (or triggered) by external circumstances. Rather than seeking to distinguish sharply between instinct and learning, however, it may be more useful to assess the degree to which a given item of behaviour is environmentally stable or unstable (labile). Indeed, aspects of behaviour of a single species may differ greatly in this respect; thus, the nesting activity, calls of alarm, and courting behaviour of birds may be extraordinarily constant under varying conditions. On the other hand, food seeking and feeding may be extremely labile (or susceptible to learning), so that geographically disparate groups of individuals belonging to the same species may have very different feeding habits.

No animal is ever completely isolated from some kind of environment; even in the egg or in the womb it is exposed to environmental variations just as it is after hatching or birth. There is, nevertheless, a sharp distinction between the egg or uterus phase and the free-living, active, sensing animal exposed to environmental stimuli of great diversity. In the case of the egg of an insect enclosed in a largely impervious shell or of a parasitic worm similarly isolated by an impermeable cyst wall, isolation from the environment may be so great that the animal undergoes little more than changes of temperature and oxygen supply. Thus, when such an animal emerges from its egg or cyst, the details of its structure and behaviour would seem maximally attributable to the genetic proteins in its cells; that is, its behaviour appears to be as much inherited as is its bodily structure. This sort of behaviour is environmentally so stable that it is naturally and quite reasonably given the label instinctive.

A variety of what may be called simple instinctive behaviour has long been known as reflex action. When this term was introduced, it meant the simple and almost invariable response of a simple organ system (e.g., a single muscle) to a simple stimulus, such as a touch or a flash of light. In its most elementary versions, this activity has been seen as the function of an idealized mechanism that has been called the reflex arc. The primary components of the reflex arc have been identified as the sensory-nerve cell (or receptor) that receives the stimulation, in turn connecting (hence the term arc) to another nerve cell that activates the muscle cell (or effector).

Although such a reflex arc might be the simplest imaginable mechanism for inflexibly automatic behaviour, it is, as noted above, a theoretical minimum rather than an actually observed functional arrangement of cells in the body of the animal; nevertheless, a mechanism but little more complicated than this helps to account for the locomotion of such animals as millipedes. In some insects, for example, the stepping movement of one limb or muscle provides stimuli that set off another limb or muscle on a similar course of movement, providing a kind of feedback system or chain of reflex arc activities. In most cases of this sort, however, the basic physiological mechanism is more complicated than the simple arc theory would suggest. Additional nerve cells capable of communicating with other parts of the body (beyond the receptor and effector) are invariably present in reflex circuits. Such connections are what make possible the conditioning of reflex responses.

Among higher animals, and perhaps many others (such as insects), what once were thought to be chain reflexes are not systems simply linked or chained together; they are systems under the precise control of coordinated complexes of nerve cells in particular parts of the nervous system, such as the spinal cord and brain. Even without evidence of a chain of feedback (or reafferent) stimuli, performance may be smoothly integrated. This is well illustrated by the complex movements of swallowing in mammals; in the dog, for example, 11 separate muscles or muscular systems are found to discharge one after the other, precisely timed to a matter of milliseconds, and all under the control of the central nervous system (CNS: brain and spinal cord). Such complexes of precisely controlled movement, known as fixed action patterns (FAP's), are thought to form the hard core of the inborn movement forms of instincts. When such sequences of uniform stereotyped responses seem to constitute an end point or goal-directed climax of some sort, they are known as consummatory acts.

Some male spiders (spider) perform elaborate courtship actions that affect selectively females that are ready to respond sexually. The male, in testing for a receptive female, first stands out of reach and goes through elaborate precise gestures with limbs, pedipalps, and other body parts that are distinctively shaped or patterned in a manner characteristic of the species. Perhaps even more remarkable FAP's are found in the displays of male fiddler crabs (fiddler crab) of the genus Uca, about 40 species of which are distributed over the Earth's tropical ocean beaches. One of the two claws is enormously enlarged, seemingly having been evolved primarily for sexual and aggressive displays, and its ritualistic gestures are quite characteristic of the species. The timing and form of movement are so invariable from one individual to another that an expert can distinguish by this alone the species of crab, whether it comes from the shores of Panama, Tahiti, or Bali.

There must be some very precise, built-in mechanism, presumably some integration of hormonal and nervous-system controls, which ensures that each individual in a species exhibits the distinctive FAP at the right speed, amplitude, and intensity. Beyond those noted for fiddler crabs and spiders, innumerable examples of such instinctive patterns are found among the elaborate display movements of many other animals (animal communication), such as insects, fishes, and birds. Among the latter three groups, these movements also are part of the species communications mechanism, serving to permit members of a species to distinguish the signals of their fellows and prospective mates from all other visual stimuli. Thus, there is a system of instinctive perceptual abilities at least as complex as the inborn tendencies to exhibit patterns of motor behaviour. Likewise, the perceptual instincts are of primary importance for the survival of the species.

Some types of instinctive behaviour, while showing a rigid core of fixed action pattern, are still modifiable by conditioning and other learning processes (see above). A good example is provided by the nest-building behaviour of many birds: after the breeding female has chosen a nest site, she finds and deposits sticks or twigs or pieces of grass there. A jackdaw (Corvus monedula) or rook (C. frugilegus) standing on a potential nest locality with twigs held in its beak performs a downward and sideward sweeping movement that brings the material into contact with the ledge or the branches on which the nest is to be built. The moment the twig or branch carried by the bird meets resistance, the sideways movements become more vigorous and merge into a series of quick trembling thrusts (so-called tremble shoving). When the twig is in a position that offers even more resistance, the efforts become more intense until the twig wedges fast. After this consummatory achievement the bird apparently loses interest in his activities for the moment.

An inexperienced jackdaw at first will try any objects small enough to be handled, even pieces of ice and the metal ends of small electric bulbs. None of these ever becomes lodged firmly enough by the tremble shoving to result in a stimulus that is sufficiently consummatory to ensure successful nest building. Such failure quickly extinguishes the bird's tendency to fetch inadequate objects; equally rapid positive conditioning is effected, and the jackdaw learns to be a twig connoisseur, coming to use only those that are just right in shape and flexibility. Indeed, it has been observed that the nests of entire groups of such birds are predominantly constructed of twigs and other pieces that are taken from only one kind of tree, even though there are other building materials that are readily available.

In contrast to jackdaws, many small songbirds do apparently have an inborn tendency to select the kinds of materials that are appropriate for different phases of nest construction. This innate predilection for suitable building materials has been shown dramatically among canaries (canary) reared in man-made nests of felt. Even though female canaries thus reared have never encountered anything long and flexible before, when nest-building time comes, they can select materials appropriately. As soon as pieces of grass, bits of string, cotton, or any long flexible objects are placed in the cage, these female canaries display interest and, within seconds, carry the objects to the nest place and commence weaving movements. Once the proper state of construction has been reached, and not before, the birds display an innate tendency to line their nests with feathers, plucking out their own when no others are to be found.

A caged female canary long deprived of nesting material may take hold of one of its own feathers in its beak and, without detaching it, go through the motions of lining the nest with it again and again without, of course, actually lining the nest. Another striking example of complex nest building is found in the extraordinarily complicated movements and responses by which weaver birds build their elaborate hanging nests with such architectural features as roof, egg chamber, antechamber, and entrance tunnel. Research has identified an elaborate system of relations among external stimuli, internal hormonal conditions, neural function, and reproductive development in the behaviour of female canaries during the course of their breeding cycle. Evidence of an elaborate combination of innate physiological activity and individual experience also comes from studies concerning the development of songs and call notes among birds.

A young bird isolated from other members of its kind, or even more rigorously from all patterned auditory stimulation, produces an extremely limited, basic sound pattern. If the young bird is allowed to develop and sing along with other members of the species, however, the instinctive tendencies seem to be more fully realized through fine adjustments added by imitative learning.

With more complexity in brain structure and function as is found in many mammals, behaviour is relatively flexible, and fixed action patterns tend to be overlaid by learned patterns and to that extent obscured; nevertheless, an inexperienced brown rat deprived of nest material tends to use its own tail instead, carrying it about in behaviour that is reminiscent of the behaviour shown by a feather-deprived canary. After gathering materials, brown rats typically heap them up in a more or less circular wall and then begin to tap down and smooth the inner surface of the nest cavity. When a very inexperienced rat is offered paper strips or some other soft material for the first time, it goes into a random frenzy in which the sequence (gathering, heaping, and smoothing) is decidedly confused. Yet, each of the three phases is performed to perfection, not differing even on analysis by slow-motion films from those of an experienced rat. After having placed two or three paper strips together flat on the ground, however, the novice rat will perform heaping-up movements in the empty air above them, then apparently pat down a nest wall that is not yet in existence! Only later, after the animal has had more experience as a builder, does the rat seem to learn to inhibit the instinctive heaping and patting movements until the appropriate stages of construction have been reached.

The catalog of instinctive behaviour among animals is much richer than the few examples offered here. Despite the clear differences observable in the detailed manifestations of instincts in comparing frog, goldfish, pigeon, cat, rabbit, and man, for example, all of these behaviours may be seen to hinge on genetically transmitted physiological structures and functions. Indeed, many animal instincts may profitably be compared with some of the built-in forms of behavioral tendency among plants.