Apparent death
Apparent death 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. Apparent death is separate from the freezing behavior seen in some animals.
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.
Description
Tonic immobility 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 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.Despite appearances, some animals remain conscious throughout tonic immobility. 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.
Difference from freezing
Tonic immobility is different from freezing behavior in animals. A deer in headlights and an opossum "playing dead" are common examples of an animal freezing and tonic immobility, 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. 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.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. Because tonic immobility occurs later in the predator attack sequence, it is considered a secondary defense mechanism and is therefore distinct from freezing. 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.
Freezing behavior and tonic immobility are similar in that both may induce bradycardia, 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. 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.
Defensive
For defensive purposes, thanatosis hinges on the pursuer's 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, 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 instinctively avoided as a precaution against infectious disease, so the snake's adaptions exploit that reaction. Newly hatched young also instinctively show this behaviour when rats 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, which is famous for reacting with a death-like posture when threatened. This instinct does not always pay off in the modern world; for example, opossums scavenging roadkill may react with the death-like posture 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.
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'.
Thanatosis has also been observed in many invertebrates such as the wasp Nasonia vitripennis, and the cricket, Gryllus bimaculatus.
Reproductive
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.Predatory
Nimbochromis, 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. A similar strategy has also been observed in the African cichlid Lamprologus lemairii from Lake Tanganyika and in the Central American yellowjacket cichlid Parachromis friedrichsthalii.Examples
Invertebrates
Amongst 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.Wasps
Tonic immobility has been observed in several species of parasitoid wasp and is considered to be an antipredator behavior in these insects. In wasps, tonic immobility can be induced by tapping their antennae, tapping the abdomen repeatedly, or squeezing their abdomen. 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. 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.''Fire ants
In fire ant colonies, tonic immobility is used by young workers to avoid conflict with competing ants. 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. 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.Spiders
In the nuptial gift-giving spider, thanatosis is incorporated into their mating display. A study in 2008 demonstrated that male Pisaura mirabilis spiders who displayed thanatosis were more likely to copulate with females and copulated longer.Green Lacewings
Larvae of Chrysoperla plorabunda engage in tonic immobility when they come into close proximity with a predator. Usage of tonic immobility as an antipredator strategy has been shown to vary with energy availability and within-population genetic variation, with lacewings under energetic stress being more likely to engage in tonic immobility.Vertebrates
Tonic immobility has been observed in a large number of vertebrate taxa, including sharks, fish, amphibians, reptiles, birds, and mammals.Sharks
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. For tiger sharks, 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. Tonic immobility can also be used as a form of mild anesthesia during experimental manipulations of sharks.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.
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.