Animals that play dead

Why do some animals play dead when threatened by a predator? The commonly held belief is that many animals including snakes, bison, chickens, rabbits, and, of course, opposum act dead to discourage those who would eat them. Recent research suggests that this isn't always the case. For example, the Parachromis friedrichsthalii fish acts dead in order to hunt. Smaller fish move in to take a nibble and wham! Other animals, such as hognose snakes, do exhibit what scientists call "tonic immobility" upon sensing a predator, sometimes delivering an Oscar-worthy performance. From Science News:
When a hognose snake that's facing a predator flips belly-up, its mouth opens and stays agape, sometimes oozing drops of blood. And the snake defecates or otherwise releases an unappetizing smell. "It's spectacular," says Gordon Burghardt of the University of Tennessee in Knoxville.

Grossly dead as the animal may look, Burghardt and Harry Greene, now at Cornell University, found that it's paying attention. Even snakes just 2 weeks old resurrect themselves sooner when a nearby human is looking away from them rather than directly at them.

Europe's grass snake puts on an even more realistic death act, says Patrick Gregory of the University of Victoria in British Columbia. When he caught his first grass snake years ago in France, it went limp. "I thought I'd accidentally killed it," he says. A death-feigning grass snake stays in character, not flopping back to its original position after it is turned over.

The relationships among the great range of freezing behaviors have yet to be clarified. "I see them as part of a continuum," Burghardt says.

Many questions remain: Are some of the activities seizures? A mental meltdown in response to disorientation? And how do some feigners remain conscious of vital details such as the gazes of observers?