When physically restrained, many animals adopt a relatively immobile state that can last after the constraint has been released. The lack of movement is suggested to inhibit further attack by the predator and reduce the perceived need of the predator to continue to attack or constrain the prey. This phenomenon has been variously called death feigning, animal hypnosis, tonic immobility, and playing dead. However, we will use the term that the scientific literature seems to be coalescing around: thanatosis. Thanatosis is distinct from immobility used to reduce the risk of detection or tracking by a predator, since it acts later in the sequence of a predation event (generally after the prey has been detected and contacted by the predator). Although some instances of thanatosis are strongly suggestive of mimicry of death, some are less so, and there is potential for thanatosis to offer protection from predators through other mechanisms than feigning death. However, except for some very specialist situations considered briefly here, the function of thanatosis does appear to be employed as a means of protection from predators. Thanatosis is widespread taxonomically, and has been well known for a long time, but only recently has it received purpose-designed study.
Behaviour and metabolism during tonic immobility (death-feigning) in Eucryptorrhynchus scrobiculatus and E. brandti (Coleoptera: Curculionidae)
