Passive avoidance is achieved by the inhibition of a previously exhibited response. Thus, after a laboratory animal has learned to approach a food dish, it may then be punished by an electric shock whenever a selected visual or auditory stimulus is present. In passive avoidance, the animal may freeze as soon as the stimulus is given; in active avoidance, the animal is given the opportunity of fleeing.

Freezing proper entails general motor inhibition, which, if sustained, may pass into signs of reduced arousal. States of considerable loss of muscle tonus, of eye closure, and many signs of deep sleep have been variously termed feigning death or animal hypnosis. In very young fowl, such signs can be induced simply by holding the animal firmly if the experience is novel (and thus presumably frightening). Such states tend to occur as an alternative to fleeing when the apparently frightening stimulus is difficult to locate or to escape. Among social mammals (e.g., cats or dogs) the status and confidence of an animal may be inferred from its degree of leg extension, arched back (vertebral tonus), and cocky tail elevation. Threat from which there seems no escape may induce a progressive approach to immobility and to general motor inhibition.

Punishment

Inhibitory interconnections have been postulated between the punishment and reward systems within the brain. One line of evidence suggesting a single punishment system rather than a number of them includes behavioral and neurological resemblances in the responses of animals to fear-inducing and to frustrating circumstances. If either fear or frustration is induced during conditioning, both produce resistance to extinction. Both are specifically opposed by barbiturate drugs.

Whatever its physiological basis, punishment can induce in an animal both the inhibition of the response that produced the punishment and the avoidance of the location at which it occurred. Sometimes the tendency to show avoidance behaviour develops further with time, even without additional training. Thus, when being conditioned to discriminate between stimuli (e.g., two tones), some breeds of dog (e.g., Alsatians), if made to wait for food reward or given an impossible discrimination to perform, will howl and show great excitement. On later days, they may first resist mounting the conditioning stand and finally resist approaching the room to which earlier they ran eagerly, presumably for the rewards of food. Stimuli associated with the training room are sometimes said to act as conditioned stimuli in such a case.

Even a piece of cockroach nervous system (metathoracic ganglion) and the leg it controls have been shown to be capable of avoidance conditioning. If each contact of the leg with a water surface is paired with an electric shock, the leg comes to be retracted on contact with the water; no such change occurs in a control leg receiving the same number of shocks at random. The conditioning is accompanied by a very marked decrease in a chemical (acetylcholinesterase) found in the nervous system; since it greatly facilitates transmission of some nerve impulses, such a chemical may well be basic to this primitive kind of learning.

Hormonal effects

Male hormones (androgens) cause the performance of new mobbing calls in the breeding season by many male passerine birds (e.g., chaffinch) and also some other birds (e.g., farmyard cock); it is not certain whether the effect is specific to the vocalization or whether the hormone produces a general change in responsiveness to frightening stimuli. Female hamsters are initially faster than males to emerge from a box and also move about more in a strange place; perhaps females innately tend to be less nervous. Females behave more like male hamsters if given a small injection of male hormone (testosterone) in the second day of life; the adult difference survives castration, so it probably rests on sexual differentiation of the nervous system rather than on adult hormone levels.

The adrenocorticotropic hormone (ACTH) from the pituitary glands of many animals may facilitate avoidance behaviour. ACTH has other direct effects on the nervous system (e.g., facilitating male sexual behaviour).

Fleeing and escape

Most animals capable of locomotion show a rapid locomotor reflex to painful or startling stimuli. Such a reflex is very ancient in an evolutionary sense; it is present even in such primitive marine animals as the slender, tiny, translucent amphioxus. The rapid propulsion of an octopus or squid by its own jet of water or of a crayfish by a blow of its tail, the sudden leap and flight of a grasshopper, and the retraction of a worm into its hole—all are examples of such avoidance behaviour.

Many invertebrates commonly compete in speed against their vertebrate predators, which tend to have faster conducting individual nerve cells; in order to compete successfully, the invertebrates seem to have evolved giant nerves (bundle of individual cell fibres), for the broader a nerve is, the faster it conducts. Among such lower animals, perhaps one-third or more of the nerve cord running the length of the body is made up of fibres responsible for initiating the escape response of the species. The fibres of a cockroach, for example, activate a mechanism that produces rapid running when the rear end (anal cerci) is disturbed by air movements. Bony fishes also have such structures, the Mauthner cells, that initiate escape swimming when stimulated.

Escape may be facilitated not only by speed of response but also by its explosive onset (e.g., after a period of shamming death), making it difficult for a predator to predict the behaviour of a prospective meal. Escape movements may stop as suddenly as they start. Many animals may even be especially conspicuous in escape (e.g., showing coloured hind wings, as do some grasshoppers and moths), so that their disappearance appears even more sudden. Presumably, the predator, engaged in pursuing and tracking a moving prey, finds it difficult to shift quickly enough to a different kind of search and so is unable to localize the exact point of disappearance.

In many instances, rapid locomotion is enough to frustrate a predator; in others, direction is crucial (e.g., a fish moving upward to the water surface or downward to the bottom or, among birds, a more elaborate celestial orientation). Under threat, insects such as pond skaters (Vellia) flee toward the nearest shore; beach fleas (amphipods) flee to the sea; and particular populations of ducks have a preferred compass direction for escape (so-called nonsense orientation).

Freezing

Immobility usually makes detection less likely. For stick insects and other animals resembling twigs or leaves, when immobility itself becomes conspicuous against moving foliage, the animals’ compensatory swaying increases the camouflage effect. There seems to be an evolutionary conflict between camouflage and the need for conspicuous signals in communication. Social groups commonly keep in touch by calls or by movements such as tail flicks, which are inhibited during freezing or even under incipient immobility. Movements may be made conspicuous by patches of white or colour on a bird’s outer tail feathers or under a mammal’s tail. The well-known white rear patch of hair among antelopes, for example, is hidden when the tail is folded or lowered under conditions of safety.