A Virginia opossum (Didelphis virginiana) playing dead
A barred grass snake (Natrix helvetica) playing dead

Apparent death[a] is a behavior in which animals take on the appearance of being dead. 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.[1][5][2] Apparent death is separate from the freezing behavior seen in some animals.[1][2]

Apparent death is a form of animal deception considered to be an anti-predator strategy, but it can also be used as a form of aggressive mimicry. When induced by humans, the state is sometimes colloquially known as animal hypnosis. The earliest written record of "animal hypnosis" dates back to the year 1646 in a report by Athanasius Kircher, in which he subdued chickens.[6]


Black house ants attacking a green-head ant which has gone into tonic immobility

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. Some animals use it to attract prey or facilitate reproduction. For example, in sharks exhibiting the behaviour, some scientists relate it to mating, arguing that biting by the male immobilizes the female and thus facilitates mating.[7]

Despite appearances, some animals remain conscious throughout tonic immobility.[8] Evidence for this includes the occasional responsive movement, scanning of the environment and animals in tonic immobility often taking advantage of escape opportunities. Tonic immobility is preferred in the literature because it has neutral connotations compared to 'thanatosis' which has a strong association with death.[1][2]

Difference from freezing

Tonic immobility is different from freezing behavior in animals.[1][2] A deer in headlights and an opossum "playing dead" are common examples of an animal freezing and playing dead, respectively. Freezing occurs early during a predator-prey interaction when the prey detects and identifies the threat, but the predator has not yet seen the prey.[1] Because freezing occurs before detection and is used to better camouflage the prey and prevent the predator from attacking, it is considered a primary defense mechanism.[2]

Tonic immobility occurs after the predator has detected and or made contact with the prey, and is likely used to prevent further attack by the predator or consumption of the prey.[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.[1][2] Although freezing animals become rigid, they often stay upright and do not change their posture while frozen whereas during tonic immobility, animals often become rigid and change their posture.[1][2][4]

Freezing behavior and tonic immobility are similar in that both may induce bradycardia (slowing of the heart rate), but the freezing response may instead be accompanied by rapid or increased breathing rate, increased heart rate, increased blood pressure and inhibition of digestion, depending on whether the sympathetic or parasympathetic nervous system is engaged.[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.[1]


For defensive purposes, thanatosis hinges on the pursuer's becoming unresponsive to its victim, as most predators only catch live prey.[10]

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,[11] which suggests that thanatosis is indeed adaptive.

In the hog-nosed snake, 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 rats try to eat them.[12]

In mammals, the Virginia opossum (commonly known simply as possums) is perhaps the best known example of defensive thanatosis. "Playing possum" is an idiomatic phrase which means "pretending to be dead".[13] It comes from a characteristic of the Virginia opossum, which is famous for pretending to be dead when threatened.[14][15] This instinct does not 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.[16] "Playing possum" can also mean simply pretending to be injured, unconscious, asleep, or otherwise vulnerable, often to lure an opponent into a vulnerable position.[13]

The usual advice for humans attempting to survive an attack by a brown bear is to lie face down, cover the face with one's hands/arms/elbows, and 'play dead'. This is done with the hope that the bear will get bored and wander away after a while.[17]

Thanatosis has also been observed in many invertebrates such as the wasp Nasonia vitripennis,[18] and the cricket, Gryllus bimaculatus.[19]


In the spider species Pisaura mirabilis, male spiders often stage elaborate rituals of gift-giving and thanatosis to avoid getting eaten by female spiders during mating. Studies have shown higher chances of success in mating with females for males who exhibit death-feigning more frequently than for males who do it less.[20]


Cichlids of the genus Nimbochromis use thanatosis as a form of aggressive mimicry, playing dead to attract prey

Nimbochromis (sleeper cichlids), endemic to Lake Malawi in East Africa, are large predatory fish for whom thanatosis is a form of aggressive mimicry. This 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. N. livingstoni then abandons the thanatosis, righting itself again and quickly eating any scavenger unfortunate enough to come too close.[21][22] A similar strategy has also been observed in the African cichlid Lamprologus lemairii from Lake Tanganyika[23] and in the Central American yellowjacket cichlid Parachromis friedrichsthalii.[24]



A Brown widow spider resorting to thanatosis after being shaken from her web

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.[2][20][25][26][27][28]


Tonic immobility has been observed in several species of parasitoid wasp and is considered to be an antipredator behavior in these insects.[25][28] In wasps, tonic immobility can be induced by tapping their antennae, tapping the abdomen repeatedly, or squeezing their abdomen.[25][28] A study in 2020 found that the frequency and duration of tonic immobility was affected by the sex of the wasp and the temperature of the environment, but not the color of the background the wasp was on.[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.[28]

Fire ants

In fire ant colonies, tonic immobility is used by young workers to avoid conflict with competing ants.[27] In the fire ant species Solenopsis invicta, the tendency to exhibit thanatosis decreases with age, with older ants choosing to fight with any workers from neighboring colonies.[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.[27]


In the nuptial gift-giving spider, thanatosis is incorporated into their mating display.[20] A study in 2008 demonstrated that male Pisaura mirabilis spiders who displayed thanatosis were more likely to copulate with females and copulated longer.[20]


Tonic immobility has been observed in a large number of vertebrate taxa, including sharks, fish, amphibians, reptiles, birds, and mammals.


Some sharks can be induced into tonic immobility by inverting them and restraining them by hand, e.g. dogfish sharks, lemon sharks, whitetip reef sharks.[7][29][30][31] 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 ] and muscle contractions become more steady and relaxed. This state persists for an average of 15 minutes before recovery and the resumption of active behaviour. Scientists have exploited this response to study shark behaviour; chemical shark repellent has been studied to test its effectiveness and to more accurately estimate dose sizes, concentrations and time to recovery.[32] Tonic immobility can also be used as a form of mild anesthesia during experimental manipulations of sharks.[33][34]

Scientists also believe that tonic immobility can be a stressful experience for sharks. By measuring blood chemistry samples when the shark is immobile, it has been suggested that tonic immobility can actually put stress on the shark, and reduce breathing efficiency. Others think sharks have a series of compensatory mechanisms that work to increase respiration rates and lower stress.[35]

It has been observed that orcas can exploit sharks' tonic immobility to prey on large sharks. Some orcas ram sharks from the side to stun them, then flip the sharks to induce tonic immobility and keep them in such state for sustained time. For some sharks, this prevents water from flowing through their gills and the result can be fatal.[36]

Teleost fishes

Goldfish, trout, rudd, tench, brown bullhead, medaka, paradise fish, and topminnow have been reported to go limp when they are restrained on their backs.[37] 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.[38] A similar behavior has been reported for convict tangs in the field.[39]

An eastern hog-nosed snake playing dead and regurgitating a toad

Amphibians and reptiles

Tonic immobility can be found in several families of anurans (frogs and toads).[40] In anurans, tonic immobility is demonstrated by most often with open eyes and the limbs sprawled and easy to move, but some species keep their eyes closed.[40] Some species also protrude their tongue.[40]

Tonic immobility has also been observed in several species of lizards and snakes.[41][42] The most common example of tonic immobility in the latter is the North American hog-nose snake, but it has also been observed in grass snakes.[41] Tonic immobility can be reliably induced in iguanas by a combination of inversion, restraint and moderate pressure. During tonic immobility, there are obvious changes in respiration including a decline in respiration rate, the rhythm becomes sporadic, and the magnitude irregular. The prolonged period of tonic immobility does not seem to be consistent with the fear hypothesis, but could be the result of a period of cortical depression due to increased brain stem activity.[43]

Tonic immobility can also be induced in the Carolina anole. The characteristics of this tonic immobility vary as a function of the duration and condition of captivity.[44]


Tonic immobility can be induced in chickens, but the behavior is more colloquially referred to as hypnosis.[45][46] See chicken hypnotism for more information on tonic immobility in chickens.

Tonic immobility can be induced in chickens through several means, including by gently restraining them on their side, stomach, or back for a short period of time, or by using chalk to draw a line on the ground away from the chicken's beak while restraining them with their head down.[45][47] Chickens have been used in several studies to elucidate the genetic basis of tonic immobility. While early studies focused on determining whether tonic immobility was influenced by genetics, a study in 2019 identified five genes that potentially control tonic immobility in white leghorn chickens and red junglefowl.[46][48][49]


Tonic immobility has been observed in several species of ducks as an effective anti-predatory response. 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.[50] Despite being immobile the ducks remained conscious and were aware of opportunities for escape. 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.[50]


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.[51] As in other prey animals, tonic immobility is considered to be an antipredator behavior in rabbits.[52] Studies on tonic immobility in rabbits focus on the European rabbit Oryctolagus cuniculus, but other species of rabbit have been studied.

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.[53] The study found that how quickly the rabbits "righted" themselves (i.e. how quickly they came out of tonic immobility) depended on how far a predator was away from the rabbit, and how close the rabbit was to their home cage.[53] Rabbits that were closer to their home cage righted themselves more quickly than those that were farther from their home cage. Conversely, when predators were closer to the rabbits, they took longer to right themselves.[53] 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.[53]

A more recent study on European rabbits monitored their heart rate during tonic immobility and found several physiological changes to the cardiovascular system during this state, including a decrease in heart rate.[52]


Main article: Rape paralysis

Tonic immobility has been hypothesized to occur in humans undergoing intense trauma, including sexual assault.[54][55][56]

There is also an increasing body of evidence that points to a positive contribution of tonic immobility in human functioning. Thus, defensive immobilization is hypothesized to have played a crucial role in the evolution of human parent-child attachment,[57] sustained attention and suggestibility,[58][59] REM sleep[60] and theory of mind.[61]


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.[62][63][64] The rationale for the tonic immobility test is that the experimenter simulates a predator, thereby eliciting the anti-predator response. 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.[65]

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.[66] 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,[64] hens on the top tier of tiered battery cages are more fearful than those on the lower levels,[67] hens carried by hand are more fearful than hens carried on a mechanical conveyor,[68] and hens undergoing longer transportation times are more fearful than those undergoing transport of a shorter duration.[69]

Tonic immobility as a scientific tool has also been used with mice,[70] gerbils,[71] guinea pigs,[72] rats,[66] rabbits[73] and pigs.[74]

See also


  1. ^ Alternative names include playing dead, feigning death, playing possum, thanatosis, animal hypnosis, immobilization catatonia, or tonic immobility, the latter of which is preferred in the scientific literature on the subject.[1][2][3][4]


  1. ^ a b c d e f g h i j Humphreys, Rosalind K.; Ruxton, Graeme D. (2018-01-15). "A review of thanatosis (death feigning) as an anti-predator behaviour". Behavioral Ecology and Sociobiology. 72 (2): 22. doi:10.1007/s00265-017-2436-8. ISSN 1432-0762. PMC 5769822. PMID 29386702.
  2. ^ a b c d e f g h i j Sakai, Masaki, ed. (2021). Death-Feigning in Insects: Mechanism and Function of Tonic Immobility. Entomology Monographs. Singapore: Springer Singapore. doi:10.1007/978-981-33-6598-8. ISBN 978-981-336-597-1. S2CID 232415330.
  3. ^ Rogers, Stephen M.; Simpson, Stephen J. (2014). "Thanatosis". Current Biology. 24 (21): R1031–R1033. doi:10.1016/j.cub.2014.08.051. PMID 25517363. S2CID 235311966.
  4. ^ a b Rusinova, E. V.; Davydov, V. I. (2010-05-21). "Dynamics of Changes in Electrical Activity in the Rabbit Cerebral Cortex during Sequential Sessions of "Animal Hypnosis"". Neuroscience and Behavioral Physiology. 40 (5): 471–478. doi:10.1007/s11055-010-9283-7. ISSN 0097-0549. PMID 20490695. S2CID 20118773.
  5. ^ Miyatake, T.; Katayama, K.; Takeda, Y.; Nakashima, A.; Sugita, A.; Mizumoto, M. (2004-11-07). "Is death–feigning adaptive? Heritable variation in fitness difference of death–feigning behaviour". Proceedings of the Royal Society of London. Series B: Biological Sciences. 271 (1554): 2293–2296. doi:10.1098/rspb.2004.2858. ISSN 0962-8452. PMC 1691851. PMID 15539355.
  6. ^ Gilman, T.T.; Marcuse, F.L.; Moore, A.U. (1960). "Animal hypnosis: a study of the induction of tonic immobility in chickens". Journal of Comparative Physiology and Psychology. 43 (2): 99–111. doi:10.1037/h0053659. PMID 15415476.
  7. ^ a b Henningsen, A.D. (1994). "Tonic immobility in 12 elasmobranchs - use as an aid in captive husbandry". Zoo Biology. 13 (4): 325–332. doi:10.1002/zoo.1430130406.
  8. ^ Jones, R.B (1986). "The tonic immobility reaction of the domestic fowl: a review". World's Poultry Science Journal. 42 (1): 82–96. doi:10.1079/WPS19860008.
  9. ^ Roelofs, Karin (2017-02-27). "Freeze for action: neurobiological mechanisms in animal and human freezing". Philosophical Transactions of the Royal Society B: Biological Sciences. 372 (1718): 20160206. doi:10.1098/rstb.2016.0206. ISSN 0962-8436. PMC 5332864. PMID 28242739.
  10. ^ Pasteur, G (1982). "A classificatory review of mimicry systems". Annual Review of Ecology and Systematics. 13: 169–199. doi:10.1146/annurev.es.13.110182.001125.
  11. ^ Miyatake, T; Katayama, K.; Takeda, Y.; Nakashima, A.; Mizumoto, M.; Mizumoto, M (2004). "Is death-feigning adaptive? Heritable variation in fitness difference of death-feigning behaviour". Proceedings of the Royal Society B. 271 (1554): 2293–2296. doi:10.1098/rspb.2004.2858. PMC 1691851. PMID 15539355.
  12. ^ Triumph of Life (2006). Alexandria, VA: PBS Home Video.
  13. ^ a b The Chambers Dictionary. Allied Publishers. 1998. p. 1279. ISBN 978-81-86062-25-8.
  14. ^ Francq, E. (1969). "Behavioural aspects of feigned death in the opossum Didelphis marsupialis". American Midland Naturalist. 81 (2): 556–568. doi:10.2307/2423988. JSTOR 2423988.
  15. ^ Ann Bailey Dunn. "Playing Possum". Wonderful West Virginia. Archived from the original on October 1, 2011. Retrieved May 11, 2011.
  16. ^ "Virginia Opossum". Mass Audubon. Archived from the original on December 29, 2010. Retrieved May 11, 2011. Opossums are frequently encountered as corpses along highways. Some biologists believe that many die as they feed on road-killed animals – a favorite food. Others believe that the opossums' small brain (5 times smaller than that of a raccoon) suggests that they may just be too dumb to get out of the way of vehicles!
  17. ^ "How to survive a bear attack".
  18. ^ King, B.; H. Leaich (2006). "Variation in propensity to exhibit thanatosis in Nasonia vitripennis (Hymenoptera: Pteromalidae)". Journal of Insect Behavior. 19 (2): 241–249. CiteSeerX doi:10.1007/s10905-006-9022-7. S2CID 26623855.
  19. ^ Nishino, H. (2004). "Motor output characterizing thanatosis in the cricket Gryllus bimaculatus". Journal of Experimental Biology. 207 (22): 3899–3915. doi:10.1242/jeb.01220. PMID 15472021.
  20. ^ a b c d Line Spinner Hansen; Sofia Fernandez Gonzales; Søren Toft; Trine Bilde (2008). "Thanatosis as an adaptive male mating strategy in the nuptial gift–giving spider Pisaura mirabilis". Behavioral Ecology. 19 (3): 546–551. doi:10.1093/beheco/arm165.
  21. ^ Gene S. Helfman; Bruce B. Collette; Douglas E. Facey (1997). The diversity of fishes. Wiley-Blackwell. p. 324. ISBN 978-0-86542-256-8.
  22. ^ McKaye, K.R. (1981). "Field observation on death feigning: a unique hunting behavior by the predatory cichlid, Haplochromis livingstonii, of Lake Malawi". Environmental Biology of Fishes. 6 (3–4): 361–365. doi:10.1007/bf00005766. S2CID 24244576.
  23. ^ Lucanus, O (1998). "Darwin's pond: Malawi and Tanganyika". Tropical Fish Hobbyist. 47: 150–154.
  24. ^ Tobler, M (2005). "Feigning death in the Central American cichlid Parachromis friedrichsthalii". Journal of Fish Biology. 66 (3): 877–881. doi:10.1111/j.0022-1112.2005.00648.x.
  25. ^ a b c d Amemiya, Mio; Sasakawa, Kôji (2021-01-10). "Factors Affecting Thanatosis in the Braconid Parasitoid Wasp Heterospilus prosopidis". Insects. 12 (1): 48. doi:10.3390/insects12010048. ISSN 2075-4450. PMC 7826778. PMID 33435169.
  26. ^ Martinez, Alexander; Ritzi, Christopher M. (2020-03-31). "Duration of Thanatosis is Based on Temperature in Estigmene acrea1". Southwestern Entomologist. 45 (1): 289. doi:10.3958/059.045.0130. ISSN 0147-1724. S2CID 214718486.
  27. ^ a b c d Cassill, Deby L.; Vo, Kim; Becker, Brandie (2008-04-05). "Young fire ant workers feign death and survive aggressive neighbors". Naturwissenschaften. 95 (7): 617–624. Bibcode:2008NW.....95..617C. doi:10.1007/s00114-008-0362-3. ISSN 0028-1042. PMID 18392601. S2CID 2942824.
  28. ^ a b c d King, B. H.; Leaich, H. R. (2006). "Variation in Propensity to Exhibit Thanatosis in Nasonia vitripennis (Hymenoptera: Pteromalidae)". Journal of Insect Behavior. 19 (2): 241–249. doi:10.1007/s10905-006-9022-7. ISSN 0892-7553. S2CID 26623855.
  29. ^ Whitman, P.A.; Marshall, J.A.; Keller, E.C.Jr (1986). "Tonic immobility in the smooth dogfish shark, Mustelus canis (Pisces, Carcharhinidae)". Copeia. 1986 (3): 829–832. doi:10.2307/1444973. JSTOR 1444973.
  30. ^ Watsky, M.A.; Gruber, S.H. (1990). "Induction and duration of tonic immobility in the lemon shark, Negaprion brevirostris". Fish Physiology and Biochemistry. 8 (3): 207–210. doi:10.1007/bf00004459. PMID 24221983. S2CID 6763380.
  31. ^ Davie, P.S.; Franklin, C.E.; Grigg, G.C. (1993). "Blood pressure and heart rate during tonic immobility in the black tipped reef shark, Carcharhinus melanoptera". Fish Physiology and Biochemistry. 12 (2): 95–100. doi:10.1007/bf00004374. PMID 24202688. S2CID 19258658.
  32. ^ "Tonic immobility". Shark defense: Chemical repellents. Archived from the original on June 14, 2006. Retrieved January 28, 2006.
  33. ^ Heithaus, M.R.; Dill, L.M.; Marshall, G.J.; Buhleier, B. (2002). "Habitat use and foraging behavior of tiger sharks (Galeocerdo cuvier) in a seagrass ecosystem". Marine Biology. 140 (2): 237–248. doi:10.1007/s00227-001-0711-7. S2CID 83545503.
  34. ^ Holland, K.N.; Wetherbee, B.M.; Lowe, C.G.; Meyer, C.G. (1999). "Movements of tiger sharks (Galeocerdo cuvier) in coastal Hawaiian waters". Marine Biology. 134 (4): 665–673. doi:10.1007/s002270050582. S2CID 7687775.
  35. ^ Brooks, E. J., et al. "The Stress Physiology of Extended Duration Tonic Immobility in the Juvenile Lemon Shark, Negaprion Brevirostris (Poey 1868)." Journal of experimental marine biology and ecology 409.1-2 (2011): 351-60.
  36. ^ Lauren Smith (2017-11-16). "Orcas vs great white sharks: in a battle of the apex predators who wins?". The Guardian. Retrieved 2017-11-18.
  37. ^ Table; Whitman, P.A.; Marshall, J.A.; Keller, E.C.Jr (1986). "Tonic immobility in the smooth dogfish shark, Mustelus canis (Pisces, Carcharhinidae)". Copeia. 1986 (3): 829–832. doi:10.2307/1444973. JSTOR 1444973.
  38. ^ Crawford, F.T. (1977). "Induction and duration of tonic immobility". The Psychological Record. 27: 89–107. doi:10.1007/bf03394435. S2CID 149316109.
  39. ^ Howe, J.C. (1991). "Field observations of death feigning in the convict tang, Acanthurus triostegus (Linnaeus), with comments on the nocturnal color pattern in juvenile specimens". Journal of Aquariculture and Aquatic Sciences. 6: 13–15.
  40. ^ a b c Toledo, Luís Felipe; Sazima, Ivan; Haddad, Célio F.B. (2010-07-12). "Is it all death feigning? Case in anurans". Journal of Natural History. 44 (31–32): 1979–1988. doi:10.1080/00222931003624804. ISSN 0022-2933. S2CID 84462802.
  41. ^ a b Gregory, Patrick T.; Isaac, Leigh Anne; Griffiths, Richard A (2007). "Death feigning by grass snakes (Natrix natrix) in response to handling by human "predators."". Journal of Comparative Psychology. 121 (2): 123–129. doi:10.1037/0735-7036.121.2.123. ISSN 1939-2087. PMID 17516791.
  42. ^ Santos, Maurício Beux dos; Oliveira, Mauro Cesar Lamim Martins de; Verrastro, Laura; Tozetti, Alexandro Marques (2010). "Playing dead to stay alive: death-feigning in Liolaemus occipitalis (Squamata: Liolaemidae)". Biota Neotropica. 10 (4): 361–364. doi:10.1590/s1676-06032010000400043. hdl:10183/29497. ISSN 1676-0603.
  43. ^ Prestrude, A.M.; Crawford, F.T. (1970). "Tonic immobility in the lizard, iguana iguana". Animal Behaviour. 18 (2): 391–395. doi:10.1016/s0003-3472(70)80052-5.
  44. ^ Hennig, C.W; Dunlap, W.P. (1978). "Tonic immobility in Anolis carolinensis: Effects of time and conditions of captivity". Behavioral Biology. 23 (1): 75–86. doi:10.1016/s0091-6773(78)91180-x.
  45. ^ a b Gallup, Gordon G.; Nash, Richard F.; Wagner, Alan M. (1971). "The tonic immobility reaction in chickens: Response characteristics and methodology". Behavior Research Methods & Instrumentation. 3 (5): 237–239. doi:10.3758/bf03208389. ISSN 0005-7878. S2CID 143472517.
  46. ^ a b Gallup, Gordon G. (1974). "Genetic influence on tonic immobility in chickens". Animal Learning & Behavior. 2 (2): 145–147. doi:10.3758/bf03199142. ISSN 0090-4996. S2CID 144313260.
  47. ^ Gilman, Thelma T.; Marcuse, F. L.; Moore, A. U. (1950). "Animal hypnosis: a study in the induction of tonic immobility in chickens". Journal of Comparative and Physiological Psychology. 43 (2): 99–111. doi:10.1037/h0053659. ISSN 0021-9940. PMID 15415476.
  48. ^ CRAIG, J.V.; KUJIYAT, S.K.; DAYTON, A.D. (1984). "Tonic Immobility Responses of White Leghorn Hens Affected by Induction Techniques and Genetic Stock Differences". Poultry Science. 63 (1): 1–10. doi:10.3382/ps.0630001. ISSN 0032-5791. PMID 6701136.
  49. ^ Fogelholm, Jesper; Inkabi, Samuel; Höglund, Andrey; Abbey-Lee, Robin; Johnsson, Martin; Jensen, Per; Henriksen, Rie; Wright, Dominic (2019-05-07). "Genetical Genomics of Tonic Immobility in the Chicken". Genes. 10 (5): 341. doi:10.3390/genes10050341. ISSN 2073-4425. PMC 6562468. PMID 31067744.
  50. ^ a b Sargeant, Alan B.; Eberhardt, Lester E. (1975). "Death Feigning by Ducks in Response to Predation by Red Foxes (Vulpes fulva)". American Midland Naturalist. 94 (1): 108. doi:10.2307/2424542. ISSN 0003-0031. JSTOR 2424542.
  51. ^ Whishaw, Ian Q.; Previsich, Nick; Flannigan, Kelly P. (1978). "Tonic immobility in feral and domestic dutch rabbits (Oryctolagus cuniculus), mountain cottontail (Sylvilagus nuttalli), and whitetail jackrabbit (Lepus townsendi) as a function of posture". Behavioral Biology. 24 (1): 88–96. doi:10.1016/s0091-6773(78)92941-3. ISSN 0091-6773.
  52. ^ a b Giannico, Amália Turner; Lima, Leandro; Lange, Rogério Ribas; Froes, Tilde Rodrigues; Montiani-Ferreira, Fabiano (2014-02-11). "Proven cardiac changes during death-feigning (tonic immobility) in rabbits (Oryctolagus cuniculus)". Journal of Comparative Physiology A. 200 (4): 305–310. doi:10.1007/s00359-014-0884-4. ISSN 0340-7594. PMID 24515628. S2CID 12719656.
  53. ^ a b c d Ewell, Albert H.; Cullen, John M.; Woodruff, Michael L. (1981). "Tonic immobility as a predator-defense in the rabbit (Oryctolagus cuniculus)". Behavioral and Neural Biology. 31 (4): 483–489. doi:10.1016/s0163-1047(81)91585-5. ISSN 0163-1047.
  54. ^ "Archived copy" (PDF). www.aic.gov.au. Archived from the original (PDF) on 21 September 2013. Retrieved 17 January 2022.((cite web)): CS1 maint: archived copy as title (link)
  55. ^ Hopper, James W. "Why many rape victims don't fight or yell" – via www.washingtonpost.com.
  56. ^ Kalaf, Juliana; Coutinho, Evandro Silva Freire; Vilete, Liliane Maria Pereira; Luz, Mariana Pires; Berger, William; Mendlowicz, Mauro; Volchan, Eliane; Andreoli, Sergio Baxter; Quintana, Maria Inês; De Jesus Mari, Jair; Figueira, Ivan (March 7, 2017). "Sexual trauma is more strongly associated with tonic immobility than other types of trauma – A population based study". Journal of Affective Disorders. 215: 71–76. doi:10.1016/j.jad.2017.03.009. PMID 28319694.
  57. ^ Porges S W (2003). "Social engagement and attachment: a phylogenetic perspective". Annals of the New York Academy of Sciences. 1008 (1): 31–47. Bibcode:2003NYASA1008...31P. doi:10.1196/annals.1301.004. PMID 14998870. S2CID 1377353.
  58. ^ Hoskovec, J; Svorad, D (1969). "The relationship between human and animal hypnosis". American Journal of Clinical Hypnosis. 11 (3): 180–182. doi:10.1080/00029157.1969.10402029. PMID 5764620.
  59. ^ Cortez, C. M.; Silva, D (2013). "Hypnosis, tonic immobility and electroencephalogram". Jornal Brasileiro de Psiquiatria. 62 (4): 285–296. doi:10.1590/S0047-20852013000400006. ISSN 0047-2085.
  60. ^ Tsoukalas I (2012). "The origin of REM sleep: A hypothesis". Dreaming. 22 (4): 253–283. doi:10.1037/a0030790.
  61. ^ Tsoukalas, Ioannis (2018). "Theory of Mind: Towards an Evolutionary Theory". Evolutionary Psychological Science. 4 (1): 38–66. doi:10.1007/s40806-017-0112-x.Pdf.
  62. ^ Gallup, G.G. Jr.; Nash, R.F.; Potter, R.J.; Donegan, N.H. (1970). "Effect of varying conditions of fear on immobility reactions in domestic chickens (Gallus gullus)". Journal of Comparative and Physiological Psychology. 73 (3): 442–445. doi:10.1037/h0030227. PMID 5514679.
  63. ^ Gallup, G.G. Jr (1979). "Tonic immobility as a measure of fear in the domestic fowl". Animal Behaviour. 27: 316–317. doi:10.1016/0003-3472(79)90159-3. S2CID 53271327.
  64. ^ a b Jones, B.; Faure, J.M. (1981). "Tonic immobility ("righting time") in laying hens housed in cages and pens". Applied Animal Ethology. 7 (4): 369–372. doi:10.1016/0304-3762(81)90063-8.
  65. ^ Forkman, B.; Boissy, A.; Meunier-Salaün, M.-C. Canali; Jones, R.B. (2007). "A critical review of fear tests used on cattle, pigs, sheep, poultry and horses". Physiology & Behavior. 92 (3): 340–374. doi:10.1016/j.physbeh.2007.03.016. PMID 18046784. S2CID 15179564.
  66. ^ a b Zamudio, S.R.; Quevedo-Corona, L.; Garcés, L.; De La Cruz, F. (2009). "The effects of acute stress and acute corticosterone administration on the immobility response in rats". Brain Research Bulletin. 80 (6): 331–336. doi:10.1016/j.brainresbull.2009.09.005. PMID 19772903. S2CID 24347387.
  67. ^ Jones, R.B. (1987). "Fearfulness of caged laying hens: The effects of cage level and type of roofing". Applied Animal Behaviour Science. 17 (1–2): 171–175. doi:10.1016/0168-1591(87)90018-9.
  68. ^ Scott, G.B.; Moran, P. (1993). "Fear levels in laying hens carried by hand and by mechanical conveyors". Applied Animal Behaviour Science. 36 (4): 337–345. doi:10.1016/0168-1591(93)90131-8.
  69. ^ Cashman, P.; Nicol, C.J.; Jones, R.B. (1989). "Effects of transportation on the tonic immobility fear reactions of broilers". British Poultry Science. 30 (2): 211–221. doi:10.1080/00071668908417141.
  70. ^ Bazovkina, D.V.; Tibeikina, M.A.; Kulikov, A.V.; Popova, N.K. (2011). "Effects of lipopolysaccharide and interleukin-6 on cataleptic immobility and locomotor activity in mice". Neuroscience Letters. 487 (3): 302–304. doi:10.1016/j.neulet.2010.10.043. PMID 20974218. S2CID 6789654.
  71. ^ Griebel, G.; Stemmelin, J.; Scatton, B. (2005). "Effects of the cannabinoid CB1 receptor antagonist rimonabant in models of emotional reactivity in rodents". Biological Psychiatry. 57 (3): 261–267. doi:10.1016/j.biopsych.2004.10.032. PMID 15691527. S2CID 18429945.
  72. ^ Donatti, A.F.; Leite-Panissi, C.R.A. (2011). "Activation of corticotropin-releasing factor receptors from the basolateral or central amygdala increases the tonic immobility response in guinea pigs: An innate fear behaviour". Behavioural Brain Research. 225 (1): 23–30. doi:10.1016/j.bbr.2011.06.027. PMID 21741994. S2CID 1566034.
  73. ^ Verwer, C.M.; van Amerongen, G.; van den Bos, R.; Coenraad, F.M.H. (2009). "Handling effects on body weight and behaviour of group-housed male rabbits in a laboratory setting". Applied Animal Behaviour Science. 117 (1–2): 93–102. doi:10.1016/j.applanim.2008.12.004.
  74. ^ Hessing, M.J.C.; Hagelsø, A.M.; Schouten, W.G.P.; Wiepkema, P.R.; van Beek, J.A.M. (1994). "Individual behavioral and physiological strategies in pigs". Physiology and Behavior. 55 (1): 39–46. doi:10.1016/0031-9384(94)90007-8. PMID 8140172. S2CID 24787960.