Perhaps one of the most peculiar behaviours witnessed in the animal kingdom, tonic immobility is an involuntary reflex where a creature experiences total paralysis and, essentially, appears dead. Often demonstrated by sharks and some bony fish when turned on their backs, the animal enters a cataleptic state – much like the proverbial ‘rabbit caught in the headlights’. Whether it’s your world being flipped 180 degrees or a car hurtling towards you, it’s believed this condition is the result of some form of sensory overload. Left to their own devices, most fish will ‘come to’ within 15 minutes and return to normal, however certain chemicals can be used to speed up the process.
Although an animal might look dead when in a tonic state, there’s a distinction to be made between tonic immobility – which is outside a creature’s control – and thanatosis, which is an instinctual behaviour where death is actively feigned. This is seen across mammals (eg opossum – hence ‘playing possum’) to reptiles (eg grass snake) and insects (eg pselaphid beetle). Most use the technique as a defence to deceive predators, or members of their own species, into thinking they’re already dead, but a few actually use it as a means of predation. For example, the pselaphid beetle tricks ants into carrying it back to their nest, where it will dine on the colony’s eggs and larvae.
There has been some research into whether humans can experience tonic immobility and recent studies suggest we can in extremely traumatic situations, where we essentially ‘switch off’ from a life-threatening situation. Most likely our brains trigger this response to try and reduce psychological damage.