Freezing and Tonic Immobility: Their Definitions and Naming

Abstract

Defensive immobility behaviors, the so-called freezing and tonic immobility (death-feigning), are discussed. Freezing occurs when the prey encounters a predator, while tonic immobility occurs at the time of the predator’s attack. Since these two immobility reactions have some similarity in appearance, they have sometimes been used incorrectly in reports. To clarify the difference between these two immobility reactions, their characteristics, definitions, mechanisms, and functions are summarized. As for the naming of the two immobility reactions, the term “freezing” has no obvious problem for usage because what it indicates is easily understood. On the other hand, “tonic immobility (TI)” is a bit complicated because many different terms including TI have been used to indicate the same or a similar immobility reaction. In this book, the most appropriate naming is considered.