Enjoying useless actually works to assist bugs keep away from being eaten by birds

By Christa Leste-Lasserre

The European ant lion (Euroleon nostras) can survive predators by pretending to be dead

Blickwinkel / Alamy

Playing dead could help keep prey alive as the tactic leaves predators prone to their attention being diverted elsewhere.

Nigel R. Franks of the University of Bristol, UK, and colleagues conducted a study of how the beetle-like larvae of flying ant lions (Euroleon nostras) use grains of sand to build pitfalls to catch food. They noticed that the insects would freeze if they dropped the 12 millimeter long larvae on a microbalance to weigh them.

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Fascinated, Franks and his colleagues observed the behavior repeatedly and found that the insects would remain immobile on the microbalance for a few seconds to more than an hour.

The researchers hypothesized that this was a last digging survival mechanism when various species of birds of prey, such as dunnocks (Prunella modularis), accidentally dropped ant lions after retrieving them from their sand pits.

They modeled behavior using computers in hopes of understanding how playing the dead – what the scientists called post-contact immobility (PCI) – keeps a prey alive.

Their models took into account various predator prey factors such as the number of pits in a given patch of sand, the distance between them, the time it takes birds to travel between pits, aspects of the birds’ behavior – the likelihood of a bird dropping you Antlion for example – and the time the Antlion remains in PCI.

The models were also informed by the limit theorem, which describes the optimal way an animal should feed in order to maximize efficiency. This weighs the costs and benefits of having an animal in one place to eat the last bit of food that is available or instead take the time to move to another high food location when the Supplies are running low at the original location.

Their results suggest that the dead can really help an insect survive if it lives in the same place with many other people of the same age. This appears to be because there are so many other targets nearby that it is more efficient for the bird to pick up another insect.

The researchers speculate that this efficiency may be due to the way birds hunt. If they do this primarily by seeking movement, a motionless “dead” insect is a difficult target. Any moving insect nearby will be easier to spot and pick up, making the dead-game strategy a winning strategy.

“I just find the phenomenon extremely fascinating and extremely confusing to tell the truth,” says Franks.

A previous study by the same research group suggested that the game-dead trick works best when immobile time differs from individual to individual. This means that birds cannot learn to accurately predict when a “dead” insect will come back to life, making them even more open to being distracted by nearby insects.

“It’s the concept of ‘I’m going to hide from you by staying still and you won’t be able to guess how long I’m going to do this,” and it’s really nice strategy, “says Franks.

He says the tactic is almost similar to the techniques a magician uses to distract an audience’s attention while performing a magic trick, although there are obvious differences: a dead-playing insect relies on other insects to distract a bird rather than its attention actively diverting the bird.

There is no benefit in staying dead too long, however. The models show that longer periods of immobilization would not provide better protection. In practice, they would likely put the larvae at real risk of being attacked by scavengers.

“What we are seeing here with the play dead strategy is really an arms race between prey and predator, and the ant lions have taken this to an absolute extreme where they would get no further advantage,” says Franks.

Journal reference: Biology Letters, DOI: 10.1098 / rsbl.2020.0892

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