Tonic immobility

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Many animals, such as sharks, beetles, snakes and the Virginia opossum, are capable of appearing to be dead to an observer, while otherwise alive. This could either be a reflex action, as in tonic immobility, or a defense mechanism for avoiding predators, as in thanatosis, which is probably adaptive, or "playing possum", which is more instinctive. Such actions often prove beneficial in the natural environment but, in the modern world of human intervention, can also be fatal.

Playing dead is an antipredator adaptation.

Tonic immobility

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 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 testing sharks under this "tonic" state, scientists will put a chemical plume in the water awakening the shark.

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 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, displayed by sharks, may be linked with defence, 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.

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.^{[How to reference and link to summary or text]}

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.^{[How to reference and link to summary or text]}

Inducing tonic immobility in common animals requires, in some cases, extreme treatment such as electric shock, along with other elements which produce stress. ^{[How to reference and link to summary or text]}

Thanatosis

File:BatrixNatrixBellyPattern.JPG

A grass snake playing dead

In animal behaviour, thanatosis (from the Greek noun θανάτωσις meaning "putting to death") is the process by which an animal feigns death in order to evade unwelcome attention, such as that of a predator, or a male trying to mate with a female. This hinges on the pursuer 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.

When a Hog-nosed Snake rolls onto its back and appears to be dead when threatened by a predator, a foul-smelling, volatile fluid oozes from its body. Predators, such as cats, then lose interest in the snake, which both looks, and smells, dead. One reason for their loss of interest is that rotten smelling animals are avoided as a precaution against infectious disease, so the snake is, in this case, exploiting that reaction. Newly-hatched young also instinctively show this behaviour when rats try to eat them.

"Playing possum"

File:PlayingPossum.jpg

An injured Virginia Opossum lies incapacitated in front a dog.

"Playing possum" is an idiomatic phrase which means "pretending to be dead". It comes from a characteristic of the Virginia opossum, which is famous for pretending to be dead when threatened. This instinct doesn't always pay off in the modern world; for example, opossums scavenging roadkill may use it in response to the threat posed by oncoming traffic, and subsequently end up as roadkill themselves.

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

See also

• Alarm responses
• Animal defensive behavior
• Antipredator adaptation
• Trout tickling
• Chicken hypnotism
• Tetrodotoxin
• Catatonia

References

• Ways to avoid hitting animals with cars.