These Ants Immobilize Prey With Acid Then Drag Them Back to Nest for Dismemberment

Decapitated heads, dismembered limbs litter the floor of Formica archboldi nests

The skull-collecting ants use chemical mimicry, a behavior usually observed amongst parasitic species, to entrap prey Adrian Smith

Floridian Formica archboldi ants have eclectic interior decorating tastes, to say the least: Whereas most ant species are content to cozy up in sand- or soil-filled mounds, F. archboldi prefer to litter their underground nests with the dismembered limbs and decapitated heads of hapless prey.

This behavioral tic has baffled scientists since the species’ discovery in 1958, but as Hannah Osborne reports for Newsweek, a new study published in Insectes Sociaux reveals exactly how the deceptively deadly F. archboldi—which is typically not known for preying on other ants—targets a specific species of trap-jaw ant, or Odontomachus.

Researchers led by Adrian Smith of North Carolina State University and the North Carolina Museum of Natural Sciences have found that the key to these skull-collecting ants’ success is formic acid. F. archboldi spray their trap-jaw prey with the immobilizing chemical, then drag their kills back to the nest for dismemberment.

But trap-jaw ants are far from easy prey, Gemma Tarlach writes for Discover. Thanks to a set of spring-loaded mandibles capable of striking enemies more than 41 times per second, the trap-jaw ant is actually the more likely predator of the two species. In fact, Cosmos’ Nick Carne notes, scientists have previously posited that F. archboldi is either a highly specialized predator or a moocher of sorts, simply moving into abandoned trap-jaw nesting sites.

To better understand the relationship between F. archboldi and the trap-jaw ant, Smith and his team created a miniature test arena and pitted either an F. archboldi or Formica pallidefulva ant—a related species that has no known connection with Odontomachus—against a trap-jaw. Over the course of 10 trials, F. pallidefulva partially immobilized the trap-jaw just one time. Comparatively, F. archboldi bested the trap-jaw 10 out of 10 times. Seven out of 10 contests resulted in the trap-jaw’s complete immobilization.

Florida's Skull-Collecting Ant

The process of spraying victims with formic acid is known as chemical mimicry, according to Inverse’s Sarah Sloat. Trap-jaws are capable of producing the same formic acid as F. archboldi, but the latter happen to be more effective sprayers. Typically, chemical mimicry occurs amongst parasitic species that invade and overtake their prey. But, Smith tells Sloat, there is no evidence that F. archboldi is parasitic. Instead, the researchers suggest the ants’ deployment of formic acid is a defense mechanism designed to provide camouflage and ward off stronger predators.

In addition to observing interactions between Formica and trap-jaw ants, the team recorded high-speed footage of attacks and time-lapse footage of attack aftermaths.

“You could see the Formica ants pull in a trap-jaw ant from where they get their food and bring it into the nest,” Smith says in an interview with The Verge’s Rachel Becker. “And they’d start licking it, biting it, moving it around on the ground like they would with food. And then all of a sudden, 18 hours later, you’d see the head start to pop off of the trap-jaw ant. They would pull it apart, and start to dismember it.”

The new report offers insights on how these skull-gathering creatures trap their prey, but the exact reasoning behind the process remains unclear. As Smith tells Newsweek, he thinks the F. archboldi feed on the trap-jaws and leave behind their hollow head casings in a manner similar to humans casting off chicken bones after eating a pile of wings. Still, this explanation doesn’t fully account for the ant’s use of chemical mimicry, nor the long evolutionary history hinted at by the unusual predator-prey relationship.

Formica archboldi is the most chemically diverse ant species we know of,” Smith says in a statement. “Before this work, it was just a species with a weird head-collecting habit. Now we have what might be a model species for understanding the evolution of chemical diversification and mimicry.”

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