Ants in Florida Collect the Skulls of Other Ants to Decorate Their Nests

Scientists are taking a closer look at a species of ant in Florida that decorates its nest with the skulls of other ants it has killed. As if that wasn't macabre enough, they have now discovered it kills its enemies by mimicking them, then spraying them with acid.

The Florida ant Formica archboldi has been the subject of study for more than 60 years. Their habitat is restricted to the Southeastern U.S., mainly found in Florida and parts of Alabama and Georgia.

Upon its discovery, experts soon noticed its nests were full of the decapitated heads of trap-jaw ants. Species from this carnivorous genus (Odontomachus) are known to be fearsome predators—so at first researchers thought maybe Florida ants moved into former trap-jaw nesting sites.

Alternative theories suggested the Florida ants were some sort of specialized predator that goes around hunting trap-jaw ants.

In a study published in the journal Insectes Sociaux, Adrian Smith, from North Carolina State University, has now analyzed what happens when the Florida ants attack jaw-trap ants. He found Florida ants chemically mimic the trap jaw ant by making a layer of wax that covers the surface of the ant the same as the wax that covers its prey.

Time-lapse footage then showed what happened next. The Florida ant sprayed the trap-jaw ant with formic acid to immobilize it. These freshly killed creatures are then dragged into the nest and dismembered.

"This behavior leads to the presence of trap-jaw ant head cases in their colony, like what has been reported for this species in field notes over the last 60 years," the study said. It is thought the Florida ants eat the trap-jaw ants—the body parts left over are normally hollowed out.

skull collecting ant
Florida's skull-collecting ant, Formica archboldi, next to trap-jaw ant body parts. Adrian Smith

Smith told Newsweek: "Some ant species have refuse piles in their nests, while some will discard their trash outside of the nest. It's possible that F. archboldi is a species that leaves its trash in its nest. So, the ant head cases might be left over from after they are done getting what they want from them. Kind of like leaving a pile of chicken bones lying around after eating wings."

Smith did not establish a link between the chemical mimicry and the Florida ant's predatory behavior, but said chemical mimicry is a tactic associated with social parasites—a species that invades and takes advantage of other species. "Usually, these species use chemical mimicry to avoid detection and aggression from their target species," he said. "The weird thing about this instance of chemical mimicry is that F archboldi is a non-parasitic species. The experiments I performed were unable to correlate chemical mimicry with any reduced levels of aggression.

"However, it is possible that this instance of chemical mimicry is still useful for avoiding aggression from trap-jaws. It's possible that my experiments didn't hit on the exact right behavioral context. Another possibility for the chemical mimicry is that F archboldi are disguising themselves as trap-jaws to avoid being parasitism themselves."

He said more research will be needed to understand the evolutionary relationship between Florida and trap-jaw ants—and what advantage the former gains from this behavior. "Now F. archboldi is the most chemically diverse ant species we know of," he said 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."

Until then, Smith said we can "add 'skull-collecting ant' to the list of strange creatures in Florida."

This story has been updated to include quotes from Adrian Smith.