AVOIDANCE BEHAVIOUR

type of activity seen in animals exposed to adverse stimuli, in which the tendency to act defensively is stronger than the tendency to attack. The underlying implication that a single neural mechanism is involved (such as a specific part of the brain, which, under electrical stimulation, seems to inflict punishment) remains only a hypothesis. Clearly, the same kinds of avoidance behaviour might result from different underlying physiological mechanisms. Thus, although the various dichotomies, or polarities, of behaviour such as positive and negative, psychoanalytic life and death instincts, and approach and withdrawal concepts may be logical or philosophical conveniences, they seem, nevertheless, to lack clear meaning physiologically. Alternative usage defines avoidance behaviour by describing a number of patterns: active avoidance (fleeing), passive avoidance (freezing stock-still or hiding), and a pattern of protective reflexes, as seen in the startle response. There is good reason to suppose that, in cats, for example, each of these patterns is coordinated separately by the brain. One kind of fleeing, in which the cat moves continuously and shows much upward climbing, is produced by electrical stimulation of specific parts of the brain (hypothalamic sites). Stimulation of other sites (in the thalamus) generates other types of fleeing movements, causing the animal to crouch, look around, move, slink close to the floor, and hide, if possible. In general, among birds and mammals, brain sites for fleeing of the first type occur in hypothalamic and mesencephalic zones. Protective reflexes in mammals include ear retraction to a position of safetypressed against and somewhat behind the skullas when a horse is seen to lay its ears back. Among the monkey-like bush babies (Galagos) the outer ear folds up laterally and longitudinally at the same time, under threat. The eyes are closed, and the muscles around the eye are contracted, adding to the protection. During this so-called startle reflex, breathing is checked, and the mouth corners are pulled back to expose the teeth; this prepares both for biting in defense and also for movements of the tongue and for head shaking to free the mouth of any dangerous or distasteful substance that may have been taken in. In most mammals, the limbs flex as if ready for a leap; in the human startle reflex, the arms are thrust outward as if ready to grasp at a support. It is helpful to consider avoidance behaviour in terms of factors that elicit it (e.g., specific stimuli) and regulate it (e.g., hormones). type of activity seen in animals exposed to adverse stimuli, in which the tendency to flee or to act defensively (e.g., to hide or to freeze movement) is stronger than the tendency to attack. Vision is the sense that most often produces avoidance behaviour. Many small birds, for example, will react to the sight of an owl or, more particularly, to its characteristic wide-set eyes. This reaction is innate; birds raised in laboratories also display an avoidance reaction at the sight of an artificial owl with prominent eyes. The reaction of many people to the sight of snakes suggests that this too is an instinctive avoidance. Sound may also produce a strong avoidance response. An animal that spies a predator can warn its fellows with a cry that will produce in them instinctive avoidance responses even before the enemy is sighted. Also, what is instinctive to one species can be learned by another; a mixed colony of birds will react to the alarm cry of any of its constituent species. The chemical response of some individuals can produce avoidance behaviour in others of the same species; ants, for example, emit a hormonelike substance into the air that induces other members of the nest either to take up defensive positions or to flee. Once a warning has been received, avoidance behaviour may take one of many forms, ranging from rapid flight to feigning death in expectation that the predator is interested only in living prey. A cat may end up on a high branch so insecure that it has difficulty descending when the danger is past. A burrowing animal will never lose sight of or stray too far from one of the many entrances to its underground warren, and some spiders make trapdoors they can close behind them. A squid will propel itself away from the source of danger, leaving behind an inky cloud to hinder pursuit. Noises and visual signs that serve to keep a group in contact will also be suppressed during flight. Thus, an antelope in flight will fold its tail to conceal the normally visible fluffy white patch. When an animal cannot run away from danger, it may turn to face the predator with a threatening posture. A monkey, for example, will grimace, showing as many teeth as possible. Hair may stand on end to increase an animal's apparent size; an arched backas seen in a cornered catfurther exaggerates the threat. An animal may discourage pursuit in other ways, as when a skunk leaves behind a cloud of noisome odour. When no form of active avoidance or discouragement is available, the animal may have to rely on one of the totally passive means that nature has devisedi.e., the shell of a clam, the carapace of a turtle, the plates of an armadillo, or the quills of a porcupine. Additional reading Sebastian P. Grossman, A Textbook of Physiological Psychology (1967), contains valuable material on several aspects of avoidance behaviour. David L. Evans and Justin O. Schmidt (eds.), Insect Defenses: Adaptive Mechanisms and Strategies of Prey and Predators (1990), focuses on behaviours involving avoidance, escape, and counterattack in terrestrial arthropods.