For many animals, tonic immobility is a natural state that can best be described as a kind of temporary paralysis. You've heard the expression "playing possum." Opossums can easily enter a state of tonic immobility, which evolved as a defense mechanism long before roads and fast-moving vehicles came on the scene.
In nature, playing dead can be advantageous for prey when the predator prefers its meals alive. Sometimes, staying alive depends on the ability to not move a muscle. That ability, unfortunately, isn't quite as effective when it's a 2014 Honda Odyssey barreling down the freeway.
Besides opossums, sharks and chickens are well known for the ease in which they can slip into states of tonic immobility. But they're far from the only ones. Snakes, orcas, pigs, iguanas, rabbits, rats, deer, many types of fish (including goldfish and trout) and even humans can play dead when the time calls for it, which might not surprise anyone who has found themselves frozen in the face of extreme danger.
Rabbits are not generally known for being brave in the face of danger. (Photo: Martyn Fletcher/flickr)
Orca whales are enormously intelligent predators who roam the seas doing pretty much whatever they want. They go where they please and eat what they please. One of the things that orca whales have figured out over the eons is that sharks can be put into a state of tonic immobility by being flipped upside down. Using this knowledge, predatory orca whales will grab tasty sharks, flip them on their backs and wait for them to drown before chomping down. This works because sharks need moving water flowing over their gills in order to extract oxygen. If you hold a shark still for 15 minutes, it will suffocate.
Tonic immobility in sharks can be induced in a less violent manner with some gentle strokes using the right kind of glove. Some divers are able to calm certain species of sharks by stroking their snouts with chain metal gloves to stimulate a network of electrosensors called the ampullae of Lorenzini.
Here's a great video of that in action:
Tonic immobility is easy to induce in chickens, a fact used by many humane farmers to lessen the stresses on the birds when harvesting them for meat. If you pick up a chicken or turkey and hold it upside down, stroking it gently down its beak and neck, you will soon put the bird into a state of tonic immobility. If you have the stomach to handle seeing it, YouTube is awash in examples of birds being humanely slaughtered after being put upside down and into a state of tonic immobility. For a more family-friendly video, check out this chicken being "hypnotized" by being held upside down and gently stroked:
A particularly fascinating phenomenon can be affected by holding down a chicken and drawing a straight line from its beak going away from its body. The combination of being held down while looking at the line being drawn will put the chicken straight into paralysis.
They're faking it. (Photo: PapaPiper/flickr)
Thanatosis describes a particular subset of tonic immobility where the animal pretends to be dead, usually as a mechanism for avoiding becoming actually dead. This is the behavior exhibited in possums as well the hog-nosed snake, which curls up in a ball and exudes a foul-smelling fluid when threatened, hoping whatever predator was sniffing around would be dissuaded by its apparent deadness.
A hog-nosed snake curls up in a ball and exudes a foul smelling liquid to play dead when threatened by a predator. (Photo: Hunter Desportes/flickr)