Apparent death
It is an immobile state most often triggered by a predatory attack and can be found in a wide range of animals from insects and crustaceans to mammals, birds, reptiles, amphibians, and fish.
The earliest written record of "animal hypnosis" dates back to the year 1646 in a report by Athanasius Kircher, in which he subdued chickens.
Tonic immobility (also known as the act of feigning death, or exhibiting thanatosis) is a behaviour in which some animals become apparently temporarily paralysed and unresponsive to external stimuli.
Tonic immobility is most generally considered to be an anti-predator behavior because it occurs most often in response to an extreme threat such as being captured by a (perceived) predator.
[8] Evidence for this includes the occasional responsive movement, scanning of the environment and animals in tonic immobility often taking advantage of escape opportunities.
[1][2] Because tonic immobility occurs later in the predator attack sequence, it is considered a secondary defense mechanism and is therefore distinct from freezing.
[9] In contrast, along with bradycardia, vertebrates in tonic immobility often reduce their breathing rate or protrude their tongue, further distinguishing this behavior from the freezing response.
One reason for their loss of interest is that rotten-smelling animals are instinctively avoided as a precaution against infectious disease, so the snake's adaptions exploit that reaction.
[20] Nimbochromis (sleeper cichlids), endemic to Lake Malawi in East Africa, are large predatory fish for whom thanatosis is a form of aggressive mimicry.
[24] Within the invertebrates, tonic immobility is widespread throughout phylum Arthropoda and has been demonstrated to occur in beetles, moths, mantids, cicadas, crickets, spiders, wasps, bees, and ants.
[25] These results were consistent with a study in 2006 that found no effect of background color on tonic immobility in a different wasp species, Nasonia vitripennis.
[27] By using tonic immobility to evade conflict, the researchers found that the young ants were four times more likely to survive an attack compared to their older counterparts, despite being more vulnerable due to their softer exoskeletons.
[29] Tonic immobility has been observed in a large number of vertebrate taxa, including sharks, fish, amphibians, reptiles, birds, and mammals.
[7][30][31][32] 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[disputed – discuss] and muscle contractions become more steady and relaxed.
[37] Goldfish, trout, rudd, tench, brown bullhead, medaka, paradise fish, and topminnow have been reported to go limp when they are restrained on their backs.
[38] Oscars seem to go into shock when they are stressed (when their aquarium is being cleaned, for example): they lie on their side, stop moving their fins, start to breathe more slowly and deeply, and lose colour.
A study by Sargeant and Eberhardt (1975) determined that ducks who feigned death had a better chance at surviving a fox attack than those who resisted and struggled.
Although the researchers concluded that tonic immobility was an effective anti-predator response, they conceded that it would not be useful against predators that kill or fatally injure prey immediately after capture.
[52] Tonic immobility occurs in both domestic and wild species of rabbit and can be induced by placing and restraining the animal for a short period of time.
A laboratory experiment by Ewell, Cullen, and Woodruff (1981) provided support to the hypothesis that European rabbits use tonic immobility as an anti-predator response.
[55] These results were consistent with those found in studies on chickens, lizards, and blue crabs at the time, and provided support that rabbits use tonic immobility as an antipredator response.
Thus, defensive immobilization is hypothesized to have played a crucial role in the evolution of human parent-child attachment,[61] sustained attention and suggestibility,[62][63] REM sleep[64] and theory of mind.
