When gripped by a predator, many animals stop struggling and apparently feign death. This behaviour has been described in mammals, birds, fish, reptiles, crustaceans and especially insects. But its evolutionary benefit — or adaptive significance — has generally not been considered, except to speculate that predators somehow ‘lose interest’ in prey that they mistakenly think are dead.

As reported in Proceedings of the Royal Society, Atsushi Honma and colleagues1 provide a demonstration of an anti-predator benefit to such behaviour. In laboratory experiments, they found that grasshoppers (Criotettix japonicus) adopted a characteristic static pose when grabbed by frogs, such that three body parts were stretched in different directions, making the grasshoppers more difficult to swallow. Frogs often abandoned prey that struck this attitude. In further experiments, the grasshoppers did not adopt this pose when captured by other predators that do not swallow them whole. This is both a demonstration of an advantage to predator-induced immobility and a warning that we are probably mistakenly interpreting other behaviours as feigning death.

The adoption of an immobile pose when an animal is physically restrained is also called tonic immobility, thanatosis and (more colloquially) playing possum. This response is quite different from freezing to make detection by predators more difficult, because it generally happens only after the predator has grasped the prey. In poultry, the duration of tonic immobility induced by human manual restraint is used as a measure of stress2. And next time you see a beetle scuttling across the ground, you can attempt to induce tonic immobility yourself by tapping the beetle lightly on the back.


The usual explanation is that this behaviour fools the predator into thinking that the prey is dead, causing the predator to lose interest. But there is no convincing explanation for this apparently maladaptive loss of interest. Feigning death might be adaptive if the predator is attempting to subdue several prey in rapid succession before returning to consume them, with prey immobility causing the predator to prematurely release one victim and move on to subduing the next3. Even if this explanation is true, however, it cannot explain the impressive diversity of this behaviour throughout the animal kingdom.

Another potential mechanism is that tonic immobility serves as a signal that an animal possesses defensive chemicals. Miyatake et al.4 have shown that tonic immobility is a heritable trait in Tribolium flour beetles, and that it is used to good effect when a beetle is attacked by spiders. They report that, when preying on flies, a spider never set a fly free after grabbing it, and always ate it immediately. In contrast, a spider always relinquished its hold on a flour beetle during the first attack. Miyatake et al. found that if the beetle struggled, it was attacked again and often consumed. But they say that, “If, however, the prey feigned death and remained immobile after the first attack, the spider often lost interest and the prey survived”.

I suggest that, rather than feigning death, the beetles may be signalling that because they have a strong chemical defence, which the spider may have already sampled in the first attack, they have no need to struggle or flee. This is similar to the characteristically slow flying adopted by butterflies with chemical defence, signalling that they have no need to fear capture5. This hypothesis could be tested: I would predict that beetle lines bred for tonic immobility might also be expected to have greater chemical defence than control lines.

So much for speculation. By contrast, Honma et al.1 have produced what is to my knowledge the first logically and empirically supported theory to account for tonic immobility: making physical handling of prey more challenging. It is unlikely that this mechanism can explain more than a subset of cases of tonic immobility. But their paper will do the valuable service of shaking us from the rut of interpreting such behaviours uncritically as feigning death.