Antlion larvae are famously ferocious: 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. To their own predators, antlion larvae are simply little snacks. And a colony’s cunning traps, grouped together and quite visible in the sand, are like flashing “DRIVE-THRU” signs.
Rather than muster an active defense, the larvae have weaponized passivity — responding to provocation by lying totally still, for unpredictable intervals of time that can range from seconds to over an hour.
In a paper published this week in Biology Letters, researchers model how this behavior may give antlions a leg up against impatient predators — and in the process shed new light on playing dead, a widespread behavior whose effectiveness has been mysterious 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 unrelated investigation into antlion larvae, they decided to weigh some of them. “I thought it was going to be a nightmare” of impatient bugs, Dr. Franks said. Instead, when tipped onto the scale, the larvae quickly froze.
The researchers took out their stopwatches. Under their gaze, one larva remained motionless for 61 minutes. “Watching paint dry would have been fun by comparison,” Dr. Franks said. “But it was so incredibly impressive.”
Many animals behave similarly when faced with danger. Larger vertebrates, like opossums, may emit bad smells to enhance the illusion that they are dead, and thus not tasty.
But with some creatures like antlion larvae — eaten by animals, like birds, that are attuned to movement — the key may not be fake putridity, but stillness, said Dr. Franks, who for this reason prefers the term “post-contact immobility.” Perhaps they’re not playing dead, but hiding in plain sight.
On their next research trip, the researchers timed the length of time all antlions from a population played dead, and found it to be extremely variable, even when the same larva was tested twice.
Such fickleness could be part of the survival strategy. If the downtime of a given antlion larva was predictable, predators “could perhaps learn some patterns,” Dr. Sendova-Franks said. But if there is no pattern — and more food is nearby — a predator might just move on, like someone with a full bag of Lays who accidentally drops a single chip into the couch.
To test this idea, the two biologists and their colleague Alan Worley simulated a hypothetical antlion community menaced 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 antlion pits and pecks the larvae out, but sometimes drops them.
If a simulated larva is dropped, it stays immobile for a variable amount of time, informed by the researchers’ real observations. The bird waits for a spell, and if the antlion doesn’t budge, it abandons the bug to seek a meal that is still moving.
After running the simulation thousands of times, the researchers found that “playing possum” indeed helped the antlions: It increased the survival rate in a given patch by about 20 percent.
And although reducing the lengths of the larva’s immobility spells in the model lowered survival rates appreciably, increasing them had hardly any effect — suggesting that the antlions have taken their strategy “to an absolute extreme,” Dr. Franks said. “Which is kind of cute, I think.”
This model will allow researchers to compare the benefits of playing possum among different species and populations, helping scientists understand the behavior’s efficacy and how it evolved, said Kennan Oyen, a postdoctoral fellow at the University of Cincinnati who was not involved in this work.
The next step is to “confirm in the field some key assumptions of the model,” including that immobility actually helps keep the antlions from being detected by predators, said Alejandro G. Farji-Brener, a professor of ecology at the National University of Comahue in Argentina.
The antlions remain formidable opponents to all who surround them on the food chain.
“Playing dead is such a great way of wasting the predator’s time,” Dr. Franks said. “It changes the game.”
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