Tonic immobility -- also known as thanatosis or a cataleptic state -- is a natural state of paralysis that some animals enter under various stimuli, and can mimic lifelessness. Sharks display tonic immobility as a response to only one well-documented stimulus: being inverted. Some species of shark may also display tonic immobility after being handled in other ways.
During Tonic Immobility
During tonic immobility -- usually after being inverted for less than a minute -- a shark will relax, its breathing will slow and become even, and it may be touched or otherwise stimulated with virtually no response. This state may last up to fifteen minutes, after which the shark will typically right itself and simply swim away, with no apparent harm done. The predominant theory is that the state of inversion causes some physiological imbalance in sharks which causes tonic immobility, and definitive research is in short supply.
Tonic Immobility Caused By Humans
Tonic immobility in sharks is often caused by divers and researchers. Researchers and scientists in particular, often invert sharks to study certain shark behaviors and physiology, and to record and tag individual animals. Such research has discovered that female sharks are far more susceptible to tonic immobility than males.
Tonic Immobility Caused By Predators
Eye witness accounts of other animals forcing sharks into the inverted position to induce tonic immobility to make them easier targets of predation. Accounts off the coast of New Zealand and California claim that orcas have maneuvered sharks and other fish species -- such as stingrays -- into inverted positions before devouring them.
Not all species are known to go into tonic immobility during inversion. Lemon sharks, black-tip sharks, great whites, nurse sharks, blue sharks, sandbar sharks and tiger sharks are all confirmed to experience tonic immobility -- but this state does not always last long.
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