The power to play dead

Anteater larvae are notorious for their ferocity: they dig pits in the sand to catch unsuspecting prey, stab it with their sharp mandibles, and digest it from the inside out.

But everything is relative. For their own predators, anteater larvae are just small snacks. And a colony’s cunning traps, clustered and conspicuous in the sand, are like flashing “DRIVE-THRU” signs.

Rather than mobilizing an active defense, the larvae militarized passivity – responding to provocation by remaining totally still, for unpredictable time intervals that could range from a few seconds to over an hour.

In an article published this week in Biology Letters, researchers show how this behavior can give ants a head start over impatient predators – and in so doing, sheds new light on dead play, a widespread behavior that the effectiveness has been a mystery to researchers.

Nigel R. Franks and Ana Sendova-Franks, married biologists at the University of Bristol in England, have been studying insects together for over 30 years. In 2016, during an independent investigation of anteater larvae, they decided to weigh some of them. “I thought it was going to be a nightmare” of impatient bugs, said Dr. Franks. Instead, when tipped over on the scale, the larvae quickly froze.

The researchers took out their stopwatches. Under their gaze, a larva remained motionless for 61 minutes. “Watching the paint dry would have been fun in comparison,” said Dr. Franks. “But it was incredibly impressive.”

Many animals behave in the same way when faced with danger. Larger vertebrates, like opossums, can emit bad smells to reinforce the illusion that they are dead and therefore not tasty.

But with some creatures like ant larvae – eaten by animals, like birds, that are in tune with movement – the key may not be false putridity, but stillness, Dr Franks said. , who for this reason prefers the term “post-contact immobility.” “Maybe they’re not playing dead, but hiding in plain sight.

On their next research trip, the researchers timed the length of time that all ants in a population were played dead and found this to be extremely variable., even when the same larva has been tested twice.

Such inconstancy could be part of the survival strategy. If the downtime of a given anthill larva was predictable, predators “might be able to learn some patterns,” said Dr Sendova-Franks. But if there’s no pattern – and there’s more food nearby – a predator might just move on, like someone with a bag full of Lays who accidentally drops a single chip. in the sofa.

To test this idea, the two biologists and their colleague Alan Worley simulated a hypothetical community of anteaters threatened by a bird. In their scenario – based on the marginal value theorem, often used in ecology to model foraging behavior – the bird visits a patch of anthill pits and pecks at the larvae, but sometimes drops them.

If a simulated larva is released, it remains motionless for a variable amount of time, informed by the researchers’ actual observations. The bird waits for a spell, and if the ant doesn’t move, it abandons the virus to look for a meal that is still moving.

After running the simulation thousands of times, the researchers found that “playing possum” did indeed help the anteaters: it increased the survival rate in a given patch by about 20%.

And although reducing the length of periods of larval immobility in the model significantly lowers survival rates, increasing them has virtually no effect – suggesting that the ants have taken their strategy “to the extreme.” “Said Dr Franks. “Which is kinda cute, I think.”

This model will allow researchers to compare the benefits of playing possum between different species and populations, helping scientists understand the effectiveness of the behavior and its evolution, said Kennan Oyen, a postdoctoral fellow at the University of Cincinnati who does did not participate in this work.

The next step is to “field confirm some key assumptions of the model,” including that immobility actually helps prevent ants from being detected by predators, said Alejandro G. Farji-Brener, professor of ecology at the National University of Comahue. in Argentina.

Anteaters remain formidable adversaries to all those around them in the food chain.

“Playing dead is a great way to waste the predator’s time,” Dr. Franks said. “It changes the game.”