What's Up With This Skull-Collecting Ant's Bizarre Headhunting Behavior?

A collection of ant skulls by the skull-collecting ant. Ant Lab/YouTube

For all of its beauty, nature also has a morbid streak. Case in point, there exists a species of ant in Florida that collects the skulls of other ants that they've killed, and decorates their nests with them, reports Phys.org.

"Add 'skull-collecting ant' to the list of strange creatures in Florida," said Adrian Smith, a scientist at the North Carolina Museum of Natural Sciences and North Carolina State University, who has conducted new research into the bizarre habits of this headhunter of the insect world.

Formica archboldi was a species first identified in 1958, and right away researchers began to notice that its nests were frequently littered with dismembered body parts of other ants, most commonly the skulls of trap-jaw ants. Because trap-jaw ants have a reputation for being particularly fearsome predators in their own right, researchers didn't automatically assume the F. archboldi were stashing kill trophies. The working assumption for a long time was that these ants just had a curious habit of inheriting old trap-jaw ant nesting sites.

Satisfied with that explanation, or perhaps too creeped out to consider alternative theories, new research on the bizarre species stalled. It wasn't until Smith and his team came across the original 60-year-old research papers that described the ant that F. archboldi, and its macabre behavior, finally got a second look.

"Odds were that these ant heads weren't in Formica nests by chance and that there was some interesting biology behind this natural history note," said Smith.

And it didn't take long before observations revealed the gruesome truth behind the ant's curious cohabitation with dead foes' skulls. F. archboldi is actually a specialized predator with adaptations that make it particularly adept at preying on trap-jaw ants. Smith and his team first noticed that F. archboldi is capable of chemically mimicking the signatures of trap-jaw ants, which allows them to get close to their prey without notice. Once they've closed in, F. archboldi then fire streams of formic acid at their opponents, which almost instantly paralyzes the hapless victims.

After the kill is complete, the trap-jaw ants are carried back to the F. archboldi nests and dismembered. The exoskeletons are then put on display like trophies.

Researchers still aren't entirely clear about the purpose of the trophy displays. Perhaps these ants simply lack tidiness. Whatever the reason, they've more than earned their description as "skull-collecting ants."

"Now Formica archboldi is the most chemically diverse ant species we know of. 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," said Smith.

A video depicting the behavior of this species can be viewed at the top of the article.