The behavior was reported this week in the journal Ecology and Evolution and was observed by entomologist Mark Moffett, a research associate at the Smithsonian's National Museum of Natural History. He compares the interaction to cleaner fish in the ocean that remove parasites and dead skin from larger fish, sometimes even from predators.

"This new ant species is the insect equivalent of cleaner fish in the ocean," Moffett said. "The potentially dangerous harvester ants even permit the visitors to groom between their open jaws."

A Chance Observation Leads to a Discovery

Moffett, who studies the social behavior of ants and other animals, noticed the interaction while visiting a research station in Arizona's Chiricahua Mountains. One morning, as he drank coffee, he watched worker harvester ants (Pogonomyrmex barbatus) leaving their nests to gather seeds. A few of the ants caught his attention because they appeared unusually still, which is rare for these constantly moving insects.

When he zoomed in with his camera, he realized the motionless ants were covered with tiny cone ants.

"Given the usual tendencies of ants, I first assumed that I was observing aggression," Moffett said. "But the larger ants seemed to seek the attention of the smaller ants by first visiting their nests and then allowing the small ants to lick and nibble all over them."

How the Cleaning Behavior Works

Over several days, Moffett observed at least 90 harvester ants interacting with the smaller cone ants, which belong to an undescribed species in the genus Dorymyrmex. He carefully documented the encounters with photographs.

The process followed a consistent pattern. A harvester ant would approach a cone ant nest and stand tall with her mandibles open (all worker ants are female). Within about a minute, a cone ant would emerge and climb onto the larger ant. In some cases, up to five cone ants would gather and begin grooming.

These sessions varied in length, lasting from under 15 seconds to more than five minutes. The cone ants used their tongue-like mouthparts to lick the harvester ants' bodies, even reaching inside their open jaws. The larger ants remained still and did not attack. When finished, the harvester ant would shake the smaller ants off, sometimes so forcefully that she flipped onto her back before quickly moving away.

A Rare and Unusual Interaction

Moffett says he has never seen or heard of this kind of behavior in ants or other insects. The closest comparison comes from marine ecosystems, where fish visit "cleaning stations" to have parasites removed by smaller species. Similar to the cone ants, some of those marine cleaners even work inside the mouths of larger animals.

Possible Benefits for Both Species

Researchers are still trying to understand what each species gains from the interaction. Moffett suggests the cone ants may be feeding on tiny, energy-rich particles they remove from the harvester ants' bodies, possibly fragments from the seeds the larger ants collect. Interestingly, the cone ants only showed interest in living ants and ignored dead specimens placed near their nests.

There may also be advantages for the harvester ants. While they already groom each other to remove debris, spores, and parasites, the smaller cone ants might be able to reach areas that are otherwise difficult to clean. Future studies will explore whether this behavior reduces infections or affects the microbiome of either species.

A Reminder of Nature's Hidden Surprises

Moffett believes this discovery highlights how much remains unknown about animal behavior, especially in natural environments.

"All kinds of amazing discoveries are still there to be made outside of the lab," Moffett said. "Finding new species and behaviors in nature often requires us to pay close attention to the small things -- including the ants."