Tonic immobility as a survival, adaptive response and as a recovery mechanism.

In this conclusive chapter, we review findings giving support to the hypothesis that TI represents an adaptive, survival response to threatening situations. In models of prey-predator interactions, in vertebrates and invertebrates, there are evidence that immobility per se contributes to survival, as the predator loses interest for a prey in which TI is experimentally induced. TI duration is also reciprocally modulated by the evaluation of the risk factors in the environment, and by the opportunity to reach a safe refuge. This supports the adaptive value of TI and suggests that, during TI, the animal may be transiently aware of the environmental situation. As for the adaptive value of TI, genetic correlations with other behavioral systems contributing to fitness (e.g., mating) are taken into account. Moreover, neurophysiological and endocrine findings in mammals support our hypothesis that TI activates the mechanisms responsible for recovery from disruptive experiences and body lesions.