Apparent death, colloquially known as playing dead or playing possum, is a behavior observed in a wide range of animals in which they take on the appearance of being dead to an observer. This could either be an involuntary reflex action, as in tonic immobility; or an adaptive behavior as in thanatosis, which is used both as a defense mechanism and as a form of aggressive mimicry.

Tonic immobility is a natural state of paralysis that animals enter, in most cases when presented with a threat. Some scientists relate it to mating in certain animals like the shark.

Some sharks can be placed in a tonic state. The shark remains in this state of paralysis for an average of fifteen minutes before it recovers. Scientists have exploited this phenomenon to study shark behaviour. The effects of chemical shark repellent have been studied to test effectiveness and to narrow down dose sizes, concentrations, and time to awaken.

Sharks may not always respond to tonic immobility by physical inversion of the animal, as has been demonstrated with lemon and Reef sharks.^{[citation needed]} With tiger sharks 3–4 metres (10 to 15 feet) in length, tonic immobility may be achieved by placing hands lightly on the sides of the animal's snout approximate to the general area surrounding its eyes. Great White sharks have been shown to be not as responsive as other species whenever tonic immobility has been attempted. Scientists believe that tonic immobility, displayed by sharks, may be linked with defense, because female sharks seem more responsive than others. During tonic immobility, the dorsal fin(s) straighten, and both breathing and muscle contractions become more steady and relaxed.