Introduction to defensive behavior in vertebrates.

In this introductory chapter we describe the ethological basis of defensive behavior, including tonic immobility (TI). The defensive repertoire activated in response to threatening stimuli, both in natural and experimental conditions, consists of a system of interrelated behaviors influenced by two main dimensions, as distance from the threat and escapable/inescapable context. When the active strategy of escape is not feasible, passive immobility forms are adopted, the latter representing substitutes of actual escape. In an inescapable context, and at very short distance or in contact with the threatening stimulus, TI is adopted, or submissive posture in a social context. Physical restraint represents the strongest stimulus for TI induction. As a result of behavioral flexibility, subsets of animals within a population show a different capacity and modality to cope with aversive stimuli (animal personality). TI can be regarded as a trait of behavioral syndromes in species as mammals and avians.