Apparent death

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.[2][4][5][6] 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 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.[7] Tonic immobility can also be used as a form of mild anesthesia during experimental manipulations of sharks.[8][9]

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. However, it has also been proposed that sharks have a series of compensatory mechanisms that work to increase respiration rates and lower stress.[10]

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.[11]

Goldfish, trout, rudd, tench, brown bullhead, medaka, paradise fish, and topminnow have been reported to go limp when they are restrained on their backs.[12] 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.[13] A similar behavior has been reported for convict tangs in the field.[14]

Tonic immobility can be reliably induced in iguanas by a combination of inversion, restraint and moderate pressure. During TI, there are obvious changes in respiration including a decline in respiration rate, the rhythm becomes sporadic, and the magnitude irregular. The prolonged period of TI 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.[15]

Tonic immobility can also be induced in the Carolina anole. The characteristics of this TI vary as a function of the duration and condition of captivity.[16]

In 2006, a single study in rabbits tried to find a link between a fear-motivated stress state and some of the physiological responses induced by tonic immobility. Researchers observed an increase in heart rate and breathing immediately following periods of tonic immobility, and while these comprise an expected physiologic reaction to sudden position change,[17] the data were nonetheless used to advance a conclusion that despite the appearance of deep relaxation, the rabbits are actually in fear for their lives.[18] While the study failed to explore why rabbits seem to enjoy self-induction of tonic immobility and are sometimes found sleeping on their backs, it is now routinely cited to support a notion that rabbits actually hate what they appear to enjoy. The researchers did allow for induction of tonic immobility in rabbits as appropriate for certain veterinary procedures, as it holds less risk than anesthesia.[18]

See Chicken hypnotism.

Tonic immobility has been viewed in some duck species.[19]

Tonic immobility has been hypothesized to occur in humans undergoing intense trauma, including sexual assault.[20][21][22]

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,[23] sustained attention and suggestibility,[24][25] REM sleep[26] and theory of mind.[27]

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.[28][29][30] The rationale for the TI 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; TI in quail reduces the probability of the birds being predated by cats.[31]

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.[32] 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,[30] hens on the top tier of tiered battery cages are more fearful than those on the lower levels,[33] hens carried by hand are more fearful than hens carried on a mechanical conveyor,[34] and hens undergoing longer transportation times are more fearful than those undergoing transport of a shorter duration.[35]

Tonic immobility as a scientific tool has also been used with mice,[36] gerbils,[37] guinea pigs,[38] rats,[32] rabbits[39] and pigs.[40]

For defensive purposes, thanatosis hinges on the pursuer's becoming unresponsive to its victim, as most predators only catch live prey.[41]

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,[42] 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.[43]

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".[44] It comes from a characteristic of the Virginia opossum, which is famous for pretending to be dead when threatened.[45][46] 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.[47]

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'; hopefully the bear will get bored and wander away after a while.[48]

"Playing possum" can also mean simply pretending to be injured, unconscious, asleep, or otherwise vulnerable, often to lure an opponent into a vulnerable position him or herself.[44]

Thanatosis has also been observed in some invertebrates such as the wasp Nasonia vitripennis,[49] and the cricket, Gryllus bimaculatus.[50]

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.[51]

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 pretense, righting itself again and quickly eating any scavenger unfortunate enough to come too close.[52][53] A similar strategy has also been observed in the African cichlid Lamprologus lemairii from Lake Tanganyika [54] and in the Central American yellowjacket cichlid Parachromis friedrichsthalii.[55]