If you could manage to grab a typical Didelphis virginian by the tail, it would probably soon seem dead: body stiff, eyes blank, tiny clutched fingers frozen. And it's no act: This Virginia (also known as North American) opossum is really and truly out cold. Touch its glazed eyeballs and there will be no response. This near-coma state can last up to four hours, during which time the animal's tongue hangs out and a green, putrid-smelling fluid emits from its anus.

What's actually going on is called thanatosis (a Greek word meaning "putting to death"). In order to escape notice—either by a predator or, in some cases, by a male dead-set on mating at the wrong time—certain animals and insects can feign death. Because most predators (and suitors) are more interested in live prey (and mates), they will generally stop their pursuit.

Scientist Georges Pasteur wrote in a 1982 Annual Review of Ecology and Systematics article that thanatosis, or "playing 'possum," is a kind of "self-mimesis"—the opossum, like the wasp and cricket, can mimic itself in a dead state. This is different from the reflex phenomena that some other mammals and fish experience. Take, for example, the intriguing fainting goats. These have myotonia, a condition in which muscles rapidly stiffen under stress and can cause them to keel over. During the 15 seconds or so this lasts (and it can happen many times a day), the goat lies on the ground, but remains conscious and fully aware of its surroundings. Thanatosis is also different from the shark adaptation known as "tonic immobility." When threatened, sharks can go into paralysis for as long as a quarter of an hour. They can also be induced into this state by being rolled over onto their backs while underwater, a practice that shark researchers often use in their studies.

Thanatosis requires a good deal of stress before it occurs. If a dog is threatening, for instance, the opossum will first bare its teeth, growl, and sometimes bite to protect itself. If these strategies don't work, then the response of losing consciousness kicks in. As opossums are poorly equipped for fighting they must rely on temporary catatonia to keep themselves alive.

While it does work to keep them safe from predators, this isn't the safest behavior in the modern world. A dead-looking opossum in the road runs a serious risk of getting hit by drivers who assume the critter has already met its maker. And some misguided people have reportedly shot comatose opossums, thinking they are injured creatures that need to be put out of their misery.