Sometimes, when an animal comes into contact with a predator, its last attempt to survive is to lie completely still or “play dead.” But what happens afterward? Scientifically referred to as post-contact immobility, the steps some animals take after playing dead seem highly individualized and random. A study of small insects published Aug. 22 in the journal PLOS ONE discovered that it is virtually impossible to predict how long someone will play dead or what they will do when they feel the coast is clear. They can adapt their response to the situation.

Why Animals Play Dead

Some animals lie motionless to trick a predator into thinking it’s a rotting corpse that’s dangerous to eat. Some snakes will even excrete blood and feces to show that they’re even more decayed and not at all appetizing. Possums are probably the most famous animals to use this tactic, but it’s also been observed in sharks that lie on their stomachs when approached by a predator, pet chickens, and more.

(Related: These snakes pretend to be dead, bleed and defecate to avoid being eaten.)

This new study looked at antlion larvae (Euroleon our website)–a group of more than 2,000 species of small, furry insects known for burrowing and eating ants. The insects seek safety by submerging themselves in the material in which they live. A 2021 study of antlion larvae by the same team revealed that the insects become completely still after being individually handled by scientists when it was time to weigh them. Getting accurate measurements from these tiny insects can be a real challenge, since they can move around the scale’s pan. However, the team saw that the antlion larvae remained completely still for more than enough time to determine their weight.

Context matters

Building on that research, the team observed how antlions respond to a perceived predator. They used automated video tracking of the insects’ intermittent movements across different substrates, or the different layers of sand, soil, or other material in which they live.

They found that an antlion’s actions after it has finished playing dead depend on what escape routes are available, including digging deeper or burying itself in an upper layer. The awakening and escape behavior was highly individual and unpredictable, with one antlion remaining completely still for over an hour.

“We chose to investigate this so-called ‘death-prone behavior’ and found that the amount of time individual ant lions remain still is completely unpredictable for each individual,” evolutionary biologist Nigel Franks, co-author of the study and professor emeritus at the University of Bristol in England, said in a statement.

Their post-contact silence was completely random, but could be predicted broadly across the insects, similar to how radioactive atoms move.

“As with radioactive atoms, it is unpredictable when an individual will change state, but the population pattern is completely predictable,” Franks said.

(Related: Female frogs appear to play dead to avoid mating.)

The team suggests that this means that playing dead is considered an adaptive action. With this unpredictability, a predator would not know how long to wait until its potential prey moves again and becomes more recognizable. The antlions seem to have adapted the ability to wait until they see that it is safe before moving. However, it is also possible that a predator carrying an antlion might drop it on a harder surface such as concrete where it cannot easily dig itself in. Further research is needed to test these scenarios.

“Our study may be the first to document what animals do after they play dead, and we show that what they do depends on the context. It’s a trade-off,” Franks said. “So, our work opens up the field of studying life after death in the huge range of animals that exhibit death appearances, thanatosis or what we prefer to call post-contact immobility.”