Apparent death

Apparent death, colloquially known as playing dead, feigning death, or playing possum, is a behavior in which animals take on the appearance of being dead. This form of animal deception is an adaptive behavior also known as tonic immobility or thanatosis. Apparent death can be used as a defense mechanism or as a form of aggressive mimicry, and occurs in a wide range of animals. Apparent death, colloquially known as playing dead, feigning death, or playing possum, is a behavior in which animals take on the appearance of being dead. This form of animal deception is an adaptive behavior also known as tonic immobility or thanatosis. Apparent death can be used as a defense mechanism or as a form of aggressive mimicry, and occurs in a wide range of animals. When induced by humans, the state is sometimes colloquially known as animal hypnosis. According to Gilman et al., the investigation of 'animal hypnosis' dates back to the year 1646 in a report by Athanasius Kircher. Tonic immobility (TI) is a behaviour in which some animals become apparently temporarily paralysed and unresponsive to external stimuli. In most cases, this occurs in response to an extreme threat such as being captured by a (perceived) predator, although 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, the animal remains conscious throughout tonic immobility. Evidence for this includes the occasional responsive movement, scanning of the environment and animals in TI often taking advantage of escape opportunities. 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 (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. 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. However, it has also been proposed that 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. Goldfish, trout, rudd, tench, brown bullhead, medaka, paradise fish, and topminnow have been reported to go limp when they are restrained on their backs. 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 similar behavior has been reported for convict tangs in the field. 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.