Antlion larvae are known to be wild: they dig pits in the sand to catch unsuspecting prey, stab them with their sharp mandibles, and digest them inside out.
But everything is relative. Antlion larvae are just small snacks for their own predators. And the cracked traps of a colony, grouped and clearly visible in the sand, are like flashing “DRIVE-THRU” signs.
Instead of building an active defense, the larvae have armed passivity – they respond to provocations by lying completely still for unpredictable time intervals ranging from seconds to over an hour.
In an article published this week in Biology Letters, researchers are modeling how this behavior can give ant lions a leg against impatient predators – shedding new light on the playing of the dead, a common behavior the effectiveness of which is mysterious to researchers was.
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 study of Antlion larvae, they decided to weigh some of them. “I thought it was going to be a nightmare,” said Dr. Franks. Instead, the larvae quickly froze when tipped on the scales.
The researchers took out their stopwatches. Under their gaze, a larva remained motionless for 61 minutes. “Seeing the paint drying would have been fun by comparison,” said Dr. Franks. “But it was so incredibly impressive.”
Many animals behave similarly when exposed to danger. Larger vertebrates like possums can give off bad smells to reinforce the illusion that they are dead and therefore not palatable.
But with some creatures like Antlion larvae that are eaten by animals like birds and tuned to move, the key may not be false putrefaction, but silence, said Dr. Franks, who for this reason prefers the term “immobility after contact”. “Maybe they are not playing dead, but are hiding in sight.
On their next research trip, the researchers determined how long all the ant lions in a population had been dead and found that they were extremely variable, even if the same larva was tested twice.
Such impermanence could be part of the survival strategy. If the downtime of a given Antlion larva could be foreseen, predators “might learn some patterns,” said Dr. Sendova Franks. But when there isn’t a pattern – and more food is around – a predator might just move on, like someone with a full bag of lays who accidentally drops a single chip on the couch.
To test this idea, the two biologists and their colleague Alan Worley simulated a hypothetical ant lion community that was threatened by a bird. In their scenario – based on the limit theorem, which is often used in ecology to model forage foraging behavior – – The bird visits a piece of the ant lion den and picks out the larvae, but sometimes drops them.
When a simulated larva is dropped, it remains immobile for a variable length of time, as determined by the researchers’ actual observations. The bird is waiting for a spell, and if the ant lion does not move, it will abandon the beetle to look for a meal that is still moving.
After running the simulation thousands of times, the researchers found that “playing opossum” actually helped the ant lions: it increased the survival rate in a given patch by about 20 percent.
And while decreasing the duration of the larvae immobility spells in the model lowered survival rates noticeably, increasing it had little effect – suggesting that the ant lions have taken their strategy “to the absolute extreme,” said Dr. Franks. “Which is kind of cute I guess.”
This model will allow researchers to compare the benefits of opossum play between different species and populations and help scientists understand the effectiveness of the behavior and its development, said Kennan Oyen, a postdoctoral fellow at the University of Cincinnati who was not involved in the work.
The next step is to “confirm some key assumptions made by the model in the field,” including the fact that immobility actually helps keep the ant lions from being spotted by predators, said Alejandro G. Farji-Brener, professor of ecology at the National University of Comahue in Argentina.
The ant lions remain formidable opponents to all who surround them in the food chain.
“Playing dead is a great way to waste the predator’s time,” said Dr. Franks. “It changes the game.”