Tonic immobility
Apparent death, colloquially known as playing dead or playing possum, is an antipredator behavior observed in a wide range of animals in which they take on the appearance of being dead
Death is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....

 to an observer. This could either be an involuntary reflex
A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...

 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
Aggressive mimicry
Aggressive mimicry is a form of mimicry where predators, parasites or parasitoids share similar signals with a harmless model, allowing them to avoid being correctly identified by their prey or host...


Tonic immobility

Tonic immobility is a natural state of paralysis
Paralysis is loss of muscle function for one or more muscles. Paralysis can be accompanied by a loss of feeling in the affected area if there is sensory damage as well as motor. A study conducted by the Christopher & Dana Reeve Foundation, suggests that about 1 in 50 people have been diagnosed...

 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
Catatonia is a state of neurogenic motor immobility, and behavioral abnormality manifested by stupor. It was first described in 1874: Die Katatonie oder das Spannungsirresein ....

 state. The shark remains in this state of paralysis for an average of fifteen minute
A minute is a unit of measurement of time or of angle. The minute is a unit of time equal to 1/60th of an hour or 60 seconds. In the UTC time scale, a minute on rare occasions has 59 or 61 seconds; see leap second. The minute is not an SI unit; however, it is accepted for use with SI units...

s before it recovers. Scientists have exploited this phenomenon
A phenomenon , plural phenomena, is any observable occurrence. Phenomena are often, but not always, understood as 'appearances' or 'experiences'...

 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
Lemon shark
The lemon shark, Negaprion brevirostris, is a shark in the family Carcharhinidae, that can grow to long. It is known as the lemon shark because, at certain depths, light interacting with the local seawater can give this shark a tanned and yellow pitted appearance, much like the surface of a...

 and Reef sharks. With tiger sharks
Tiger shark
The tiger sharks, Galeocerdo cuvier, is a species of requiem shark and the only member of the genus Galeocerdo. Commonly known as sea tigers, tiger sharks are relatively large macropredators, capable of attaining a length of over . It is found in many tropical and temperate waters, and is...

 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
Great white shark
The great white shark, scientific name Carcharodon carcharias, also known as the great white, white pointer, white shark, or white death, is a large lamniform shark found in coastal surface waters in all major oceans. It is known for its size, with the largest individuals known to have approached...

 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.

Tonic immobility also can be somewhat effective on anole lizards, and a loose study was done with tonic immobility with the rabbit
Rabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world...

. 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 invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...

s as well.

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

In an interesting eye witness case in 1997 around the Farallon Islands
Farallon Islands
The Farallon Islands, or Farallones , are a group of islands and sea stacks in the Gulf of the Farallones, off the coast of San Francisco, California, USA. They lie outside the Golden Gate and south of Point Reyes, and are visible from the mainland on clear days...

 off the coast of California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

, a female orca
The killer whale , commonly referred to as the orca, and less commonly as the blackfish, is a toothed whale belonging to the oceanic dolphin family. Killer whales are found in all oceans, from the frigid Arctic and Antarctic regions to tropical seas...

 was seen purposely inducing tonic immobility in a great white shark
Great white shark
The great white shark, scientific name Carcharodon carcharias, also known as the great white, white pointer, white shark, or white death, is a large lamniform shark found in coastal surface waters in all major oceans. It is known for its size, with the largest individuals known to have approached...

. The orca held the shark upside down to induce the tonic immobility, and kept the shark still for fifteen minutes, causing it to suffocate to death. This was the first recorded eye witness case of predation on a great white shark in the wild by a species other than humans. Another case of orcas purposely inducing tonic immobility in fish has been documented with stingray
The stingrays are a group of rays, which are cartilaginous fishes related to sharks. They are classified in the suborder Myliobatoidei of the order Myliobatiformes, and consist of eight families: Hexatrygonidae , Plesiobatidae , Urolophidae , Urotrygonidae , Dasyatidae , Potamotrygonidae The...

s in New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

. In this case, the orcas turn themselves upside down before attacking, trap the stingrays in their mouths, then quickly right themselves, in turn flipping the stingray over, inducing the tonic immobility, rendering the fish helpless and an easy meal.

Tonic immobility as a scientific tool

According to Gilman et al. the investigation of ‘animal hypnosis’ dates back to the year 1646 in a report by Kircher. As a scientific tool, tonic immobility is considered to be a fear-potentiated response induced by physical restraint and characterised by reduced responsiveness to external stimulation. It has been used as a measure in the assessment of animal welfare, particularly hens, since 1970. The rationale for the tonic immobility test is that the experimenter simulates a predator thereby eliciting an anti-predator response - “death feigning”. The precept is that the prey animal 'pretends' to be dead to be able to escape when/if the predator relaxes its concentration. Death feigning birds often take advantage of escape opportunities; tonic immobility in quail reduces the probability of the birds being predated by cats.

To induce tonic immobility, the animal is gently restrained on its side or back for a period of time, e.g. 15 seconds. This is done either on a firm, flat surface or sometimes in a purpose-built ‘V’ or ‘U’-shaped restraining cradle. In rodents, the response is sometimes induced by additionally pinching or attaching a clamp to the skin at the nape of the neck. Scientists record behaviours such as the number of inductions (15 second restraining periods) required for the animal to remain still, the latency to the first major movements (often cycling motions of the legs), latency to first head or eye movements and the duration of immobility, sometimes called the ‘righting time’.

Tonic immobility has been used to show that hens in cages are more fearful than those in pens, hens on the top tier of tiered battery cages are more fearful than those on the lower levels, hens carried by hand are more than hens carried on a mechanical conveyor, and hens undergoing longer transportation times are more fearful than those undergoing transport of a shorter duration.

Tonic immobility as a scientific tool has also been used with mice
-Fiction:*Mice , alien species in The Hitchhiker's Guide to the Galaxy*The Mice -Acronyms:* "Meetings, Incentives, Conferencing, Exhibitions", facilities terminology for events...

, gerbils, guinea pigs, rats
RATS may refer to:* RATS , Regression Analysis of Time Series, a statistical package* Rough Auditing Tool for Security, a computer program...

, rabbits and pigs
PIGS is a four letter acronym that can stand for:* PIGS , Phosphatidylinositol glycan anchor biosynthesis, class S, a human gene* PIGS , the economies of Portugal, Italy , Greece and Spain...



In animal behaviour
Animal behaviour
Animal behaviour is the subject of:* The field of Ethology* Animal Behaviour, a scientific journal...

, thanatosis (from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 noun θανάτωσις meaning "putting to death") is the process by which an animal feigns
Deception, beguilement, deceit, bluff, mystification, bad faith, and subterfuge are acts to propagate beliefs that are not true, or not the whole truth . Deception can involve dissimulation, propaganda, and sleight of hand. It can employ distraction, camouflage or concealment...

Death is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....

 in order to evade unwelcome attention. It can be for various reasons, such as that of a prey evading a predator, a male trying to mate
In biology, mating is the pairing of opposite-sex or hermaphroditic organisms for copulation. In social animals, it also includes the raising of their offspring. Copulation is the union of the sex organs of two sexually reproducing animals for insemination and subsequent internal fertilization...

 with a female, or a predator trying to lure potential prey closer. Pasteur classifies it as a form of self-mimesis, a form of camouflage
Camouflage is a method of concealment that allows an otherwise visible animal, military vehicle, or other object to remain unnoticed, by blending with its environment. Examples include a leopard's spotted coat, the battledress of a modern soldier and a leaf-mimic butterfly...

 or mimicry in which the "mimic" imitates itself in a dead state.

For defense

For defensive purposes, thanatosis hinges on the pursuers' becoming unresponsive to its victim, as most predators only catch live prey.

In beetles, artificial selection experiments have shown that there is heritable variation for length of death-feigning. 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.

In the Hog-nosed Snake
Heterodon is a genus of harmless colubrids found in North America. They are stout with upturned snouts and are perhaps best known for their characteristic threat displays. Three species are currently recognized.-Description:...

, a threatened individual rolls onto its back and appears to be dead when threatened by a predator, while 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 rat
Rats are various medium-sized, long-tailed rodents of the superfamily Muroidea. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, Rattus norvegicus...

s try to eat them.

In mammals, the Virginia opossum is perhaps the best known example of defensive thanatosis. "Playing possum" is an idiomatic phrase which means "pretending to be dead". It comes from a characteristic of the Virginia opossum
Virginia Opossum
The Virginia opossum , commonly known as the North American opossum or tlacuache in Mexico, is the only marsupial found in North America north of Mexico. A solitary and nocturnal animal about the size of a domestic cat, and thus the largest opossum, it is a successful opportunist...

, which is famous for pretending to be dead when threatened. This instinct
Instinct or innate behavior is the inherent inclination of a living organism toward a particular behavior.The simplest example of an instinctive behavior is a fixed action pattern, in which a very short to medium length sequence of actions, without variation, are carried out in response to a...

 does not always pay off in the modern world; for example, possums scavenging roadkill
Roadkill is an animal or animals that have been struck and killed by motor vehicles. In the United States of America, removal and disposal of animals struck by motor vehicles is usually the responsibility of the state's state trooper association or department of transportation.-History:During the...

 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
Unconsciousness is the condition of being not conscious—in a mental state that involves complete or near-complete lack of responsiveness to people and other environmental stimuli. Being in a comatose state or coma is a type of unconsciousness. Fainting due to a drop in blood pressure and a...

, asleep, or otherwise vulnerable, often to lure an opponent into a vulnerable position him or herself.

Thanatosis has also been observed in some invertebrates such as the wasp, Nasonia vitripennis, and the cricket, Gryllus bimaculatus
Gryllus bimaculatus
Gryllus bimaculatus is one of many cricket species known as the Field cricket. Also known as the African or Mediterranean field cricket or as the two-spotted cricket, it can be discriminated from other Gryllus species by the two dot-like marks on the base of its wings.This species of cricket is...


For reproduction

In the spider species Pisaura mirabilis
Pisaura mirabilis
The nursery web spider Pisaura mirabilis is a spider species of the family Pisauridae.Males of this species offer food gifts to potential female mates. Some Pisaura mirabilis have also been observed to feign death, remaining still while holding the food gift in their mouths...

, male spiders often stage elaborate rituals of gift-giving and thanatosis to avoid getting eaten by female spiders during mating. Studies have a shown higher chances of success in mating with females for males who exhibit death-feigning more frequently than for males who do it less.

For predation

In the cichlid
Cichlids are fishes from the family Cichlidae in the order Perciformes. Cichlids are members of a group known as the Labroidei along with the wrasses , damselfish , and surfperches . This family is both large and diverse. At least 1,300 species have been scientifically described, making it one of...

 Haplochromis livingstoni, thanatosis serves an aggressive purpose. The large predatory fish will lie down on its side on the bottom sediments and assume a blotchy coloration. Scavengers, attracted to what seems like a dead fish, will approach the predator to investigate. H. livingstoni then abandons the pretense, righting itself again and quickly eating any scavenger unfortunate enough to come too close.

See also

  • Antipredator adaptation
  • Chicken hypnotism
    Chicken hypnotism
    A chicken can be hypnotized, or put into a trance, by holding its head down against the ground, and continuously drawing a line along the ground with a stick or a finger, starting at its beak and extending straight outward in front of the chicken...

  • Aggressive mimicry
    Aggressive mimicry
    Aggressive mimicry is a form of mimicry where predators, parasites or parasitoids share similar signals with a harmless model, allowing them to avoid being correctly identified by their prey or host...

  • Tetrodotoxin
    Tetrodotoxin, also known as "tetrodox" and frequently abbreviated as TTX, sometimes colloquially referred to as "zombie powder" by those who practice Vodou, is a potent neurotoxin with no known antidote. There have been successful tests of a possible antidote in mice, but further tests must be...

  • Trout tickling
    Trout tickling
    Trout tickling is the art of rubbing the underbelly of a trout using fingers. If done properly, the trout will go into a trance-like state after a minute or so, and can then easily be thrown onto the nearest bit of dry land.- History :...

  • Autohaemorrhaging
    Autohaemorrhaging or reflex bleeding is the action of animals deliberately ejecting haemolymph or blood from themselves. If the animal has toxic compounds in its blood then it may be an effective chemical defence mechanism....

External links

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