The main reason why thanatosis is such a good piece of evidence of predators’ concept of death has to do with the particular complexity exhibited by this behaviour. In order to see this, we need to begin by distinguishing thanatosis from a similar – and probably evolutionarily related – behaviour. Many animals, when they feel threatened, go into a kind of paralysis that reduces the probability of being preyed upon. This is known as tonic immobility and can be found in a wide range of species, from insects to humans. While tonic immobility is a simple behaviour that operates at a superficial level – in some species it can even be accompanied by an increase in heart rate – in thanatosis, the animal not only stays still but actively imitates the characteristics of a corpse. Although thanatosis might have evolved from tonic immobility, it’s much more than a mere paralysis: the animal is feigning death.
The Virginia opossum is probably the animal with the most elaborate thanatosis display (hence the expression ‘to play possum’), but not the only one to exhibit a behaviour deserving this label. Some frogs engage in thanatosis, whereby they stop responding to all interactions while lying still, with their eyes open and their limbs extended and flaccid. Their tongue might also hang from their mouth, and their breath will give off an ammonia-like smell. Hognose snakes precede their thanatosis display with a dramatic ‘death’, during which the animal writhes in an erratic and violent manner until she finally lies still with her belly facing up, her mouth open, her tongue hanging out, displaying no signs of breathing. She even secretes blood from her mouth. You could poke her with a stick or lift her up and she would not react.