Why do animals play dead?

Playing dead, technically tonic immobility or thanatosis, works because it exploits two predator tendencies at once. Many predators trigger their attack sequence on movement, so a motionless animal may simply stop registering as prey. And many avoid eating animals they did not kill, since carrion carries disease risk, so an apparently dead animal becomes unattractive even after being found. The behavior appears independently across insects, spiders, fish, amphibians, reptiles, birds, and mammals, which is a strong signal of convergent evolution toward a strategy that pays.

The opossum is the famous case and the most misunderstood, since playing possum is not a decision. Extreme threat triggers an involuntary physiological shock state: the animal collapses, its breathing and heart rate slow, its lips draw back, it drools, and it secretes a foul greenish fluid from anal glands that smells like decay. The state can persist from minutes to several hours and the animal cannot end it at will, which means an opossum in this condition on a roadside should be left alone rather than assumed dead. Other notable users include hognose snakes, which perform elaborately and will flip themselves back over if you right them, spoiling the effect, and some ducks that go limp when caught by a fox and escape when the predator relaxes its grip to reposition.