Tonic immobility

Apparent death, colloquially known as playing dead, feigning death, or playing possum, is a behavior in which an animal takes on the appearance of being dead. This form of animal deception is an adaptive behavior also known as tonic immobility or thanatosis. Apparent death can be used as a defense mechanism or as a form of aggressive mimicry, and occurs in a wide range of animals.

When induced by humans, the state is sometimes colloquially known as animal hypnosis. According to Gilman et al., the investigation of "animal hypnosis" dates back to the year 1646 in a report by Athanasius Kircher.

Tonic immobility (TI) is a behaviour in which some animals become apparently temporarily paralysed and unresponsive to external stimuli. In most cases, this occurs in response to an extreme threat such as being captured by a (perceived) predator, although in sharks exhibiting the behaviour, some scientists relate it to mating, arguing that biting by the male immobilizes the female and thus facilitates mating.

Despite appearances, the animal remains conscious throughout tonic immobility. Evidence for this includes the occasional responsive movement, scanning of the environment and animals in TI often taking advantage of escape opportunities.

Some sharks can be induced into tonic immobility by inverting them and restraining them by hand, e.g. dogfish sharks, lemon sharks and reef sharks. For tiger sharks (measuring 3–4 metres in length), tonic immobility can be induced by humans placing their hands lightly on the sides of the animal's snout in the area surrounding the eyes. During tonic immobility in sharks, the dorsal fins straighten, and both breathing and muscle contractions become more steady and relaxed. This state persists for an average of 15 minutes before recovery and the resumption of active behaviour. Scientists have exploited this response to study shark behaviour; chemical shark repellent has been studied to test its effectiveness and to more accurately estimate dose sizes, concentrations and time to recovery. Tonic immobility can also be used as a form of mild anesthesia during experimental manipulations of sharks.

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