Sometimes, when an animal comes into contact with a predator, its last attempt to survive consists of lying completely still or ‘playing dead’. But what happens later? Scientifically referred to as post-contact immobility, the steps some animals take after playing dead seems very individualized and random. A study on small insects was published in the journal on August 22 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, and adjust their response accordingly.
Why animals play dead
Some animals lie completely still to trick a predator into thinking it is a rotting corpse that is dangerous to eat. Some snakes even secrete blood and feces, making them appear as if they have deteriorated even further and are no longer tasty at all. Opossums are perhaps the most famous animals to use this tactic, but it has also been observed in sharks go with your stomach up when you are approached by a predator, domestic chickens, and more.
[Related: These snakes play dead, bleed, and poop to avoid being eaten.]
This new study was looked at antlion larvae (Euroleon nostras) – a group of more than 2,000 species of small fluffy insects known for their burrowing and eating ants. The insects tend to seek safety by immersing themselves in the material they live in. A 2021 study A study of antlion larvae from the same team revealed that the insects become completely silent after being handled individually by scientists when it was time to weigh them. It can be a real challenge to get accurate measurements of these little bugs because they can move all over the scale. However, the team saw that they were antlion larvae remained completely silent for more than enough time to take their weight.
Context is important
Building on that research, the team observed how antlions react to a perceived predator. They used automated video tracking of the insects’ intermittent movements in different places substrates–or the different ones layers of sand, earth or other material where they live.
They found that an antlion’s actions after it finishes playing dead depend on what escape routes are available, including digging deeper or burying itself in a top layer. The awakening and escape behavior was highly individual and unpredictable, with one of the antlions remaining completely still for over an hour.
“We chose to investigate this so-called ‘death feigning’ behavior and we found that the amount of time individual antlions remain still is completely unpredictable for each individual. University of Bristol in England, said in a statement.
Their silence after contact was completely random, but could be broadly predicted for all insects. This is similar to how radioactive atoms move.
“Just like with radioactive atoms, it is unpredictable when an individual changes state, while the population pattern is completely predictable,” says Franks.
[Related: Female frogs appear to play dead to avoid mating.]
The team believes this means playing dead is considered a adaptive action. With this unpredictability, a predator would not know how long to wait for its potential prey to move again and become more recognizable. The antlions seem to be able to adapt by waiting until they sense it is safe before moving. However, it is also possible for a predator carrying an antlion to drop an antlion onto a harder surface, such as concrete, that it cannot burrow into as easily. Further research is needed to test these scenarios.
“Our study could be the first to determine what animals do after playing dead, and we show that what they do is context dependent. It is a trade-off,” said Franks. “Our work thus opens the field of studying the afterlife in a large number of animals that exhibit death appearance, thanatosis, or what we prefer to call immobility after contact.”