Playing dead really works to help insects avoid being eaten by birds - New Scientist News

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Playing dead might help prey animals stay alive because the tactic leaves predators vulnerable to having their attention diverted elsewhere.

Nigel R. Franks at the University of Bristol, UK, and his colleagues were running a study on how the beetle-like larvae of flying antlions (Euroleon nostras) use grains of sand to build pitfall traps to catch food. They noticed that when they dropped the 12-millimetre-long larvae onto a microbalance to weigh them, the insects would freeze.

Fascinated, Franks and his colleagues observed the behaviour repeatedly, noting that the insects would stay immobile on the microbalance for anywhere from a few seconds to more than an hour.

The researchers suspected this was a last-ditch survival mechanism for when various kinds of predatory birds, like dunnocks (Prunella modularis), accidentally drop antlions after grabbing them out of their sandpits.

They modelled the behaviour using computers in the hope of understanding how playing dead – what the scientists called post-contact immobility (PCI) – keeps a prey animal alive.

Their models considered various predator-prey factors like 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’ behaviour – the likelihood that a bird will drop an antlion, for instance – and the amount of time that the antlion remains in PCI.

The models were also informed by marginal value theorem, which describes the optimal way an animal should feed to maximise efficiency. This weighs the costs and benefits of an animal staying in one spot to eat every last morsel of food available there, or instead taking the time to move to another food-rich spot when supplies at the initial location begin to run low.

Their results suggest that playing dead really can help an insect survive, if it lives in a patch with many other peers. This seems 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 down to the way birds hunt. If they do so primarily by looking for movement, then a motionless “dead” insect is a difficult target. Any nearby moving insect is easier to detect and pick up, making the playing-dead strategy a winning one.

“I just find the phenomenon utterly entrancing and utterly bewildering, to tell you the truth,” says Franks.

An earlier study by the same research group suggested that the playing-dead trick works best when the time spent immobile varies from individual to individual. This means birds can’t learn to anticipate exactly when a “dead” insect will become “alive” again, leaving them even more open to being distracted by nearby moving insects.

“It’s this concept of, ‘I’m going to hide from you in plain sight by keeping still, and you’re not going to be able to guess how long I’m going to do this for’, and it’s a really beautiful strategy,” says Franks.

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

However, there is no benefit in staying dead for too long. The models indicate that lengthy periods of immobilisation wouldn’t give better protection. Practically speaking, they would probably put the larvae at real risk of predation by scavengers.

“What we’re seeing here with the play-dead strategy is really an arms race between prey and predator, and the antlions have carried this to an absolute extreme, beyond which they would gain no further advantage,” says Franks.

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

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