Thanatosis, or tonic immobility, in the Virginia opossum is a reflex triggered by extreme stress rather than a performance. The animal collapses on its side, lips drawn back, tongue protruding, breathing and heart rate slowed, with the eyes open and unblinking. The anal glands discharge a greenish secretion that smells of decay, adding a chemical signal to the visual one.
The episode can last from a few minutes to several hours, and the opossum recovers gradually, often twitching its ears first as it comes round. Because it is involuntary, an opossum in this state can be handled and moved without responding, which is one reason road-struck animals are sometimes assumed dead when they are not.
The defence works on predators that need a struggle to complete their kill, or that avoid carrion. It works poorly on vehicles, and on dogs, which is why the reflex is a poor match for the suburbs the species has colonised so successfully.