Apparent death

Overview

Animals have several mechanisms for giving the appearance of being dead.

Tonic immobility

Tonic immobility is a natural state of paralysis that animals enter, in most cases when presented with a threat.

Some sharks can be placed in a tonic immobility state by turning them upside down. 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. Usually when being tested by scientists under this "tonic" state the scientists will put a chemical plume in the water awakening the shark.

Tonic immobility also can be somewhat effective on anole lizards and a loose study was done with tonic immobility with the rabbit. Both were inconsistent examples of tonic immobility.

Tonic immobility has also been used to describe the paralysis which often immobilizes animals such as rodents or birds when they feel threatened by a predator. It can be argued that tonic immobility plays a role in survival if it helps a hunted animal to blend in with its surroundings by remaining as motionless as an inanimate object. This tonic state is common with invertebrates as well.

Inducing tonic immobility in common animals requires, in some cases, extreme treatment such as electric shock along with other elements that produce stress.

Contrary to popular belief, theories regarding "putting a lobster to sleep" seem to be rumors without empirical data to confirm their validity.

Note: Sharks may not always respond to tonic immobility by physical inversion of the animal as has been done with lemon and reef sharks. With tiger sharks 10 to 15 feet in length, tonic immobility may be achieved by placing hands lightly on the sides of the animal's snout approximately 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, 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 contraction becomes more steady and relaxed.

Thanatosis

In animal behavior, thanatosis (from the Greek adjective θανàτωσιs meaning "putting to death") is the process by which an animal feigns death in order to evade an enemy, such as a predator, or a male trying to mate with a female. This hinges on the animal pursuing it becoming unresponsive to its victim, as most predators only catch live prey. Pasteur classifies it as a form of self-mimesis, a form of camouflage or mimicry in which the 'mimic' imitates itself in a dead state, such that its pursuer no longer takes notice of it. It is a phenomenon known in invertebrates such as the Hymenoptera Nasonia vitripennis and the Orthoptera Gryllus bimaculatus, but also in vertebrates such as Didelphis marsupialis. Artificial selection experiments have shown that there is heritable variation for length of death feigning in beetles and that those selected for longer death feigning durations are at a selective advantage to those at shorter durations, when a predator is introduced, which suggests that thanatosis is indeed adaptive.

Playing possum

Playing possum is a phrase that, taken literally, means to pretend to be dead.

It comes from a characteristic of the Virginia opossum, which is famous for pretending to be dead when threatened. This natural instinct does not always pay off in the modern world: for example, opossums scavenging roadkill may use it in response to the threat of approaching traffic, and end up as roadkill themselves.

"Playing possum" can also mean simply pretending to be injured, unconscious, or otherwise vulnerable, often to lure an opponent into a vulnerable position himself.

See also

• Trout tickling
• Chicken hypnotism
• Tetrodotoxin
• Catatonia

References

• Ways to avoid hitting animals with cars.