Garden wars: Ants remember their enemies, hold grudges

FREIBURG, Germany — Memory shapes behavior across the animal kingdom, from elephants recognizing long-lost herd members to dogs remembering where they buried their favorite toys. Now, research reveals that even tiny ant brains can form lasting memories of hostile encounters, leading them to hold what amounts to insect grudges against rival colonies.

Scientists at the University of Freiburg have discovered that ants possess a sophisticated learning ability that helps them recognize and respond more aggressively to ants from colonies that previously attacked them. This study, published in Current Biology, proves even neighborhood feuds are occurring in tiny colonies underground.

The study focused on the black garden ant (Lasius niger), a common species found throughout Europe. These ants live in colonies that frequently interact with neighboring groups as they forage for food and defend their territory. Each nest maintains its own unique chemical signature or scent, which ants use to distinguish friends from foes. When encountering rivals, they may open their mandibles threateningly, bite aggressively, or even spray acid to kill their competitors.

In a series of elegant experiments, the research team exposed individual ants to brief encounters with ants from other colonies. These meetings lasted just one minute each day over five days. The scientists carefully tracked the ants’ aggressive behaviors, such as opening their mandibles threateningly or attempting to bite their opponents.

As it turns out, the ants became increasingly aggressive toward individuals from colonies they had previously fought with, while showing less hostility toward unfamiliar ants from colonies they had never encountered. This pattern suggests that ants can form specific memories of their adversaries and adjust their behavior accordingly.

Even three brief encounters within a 45-minute period were sufficient for ants to develop enhanced recognition of their opponents, though the effect was stronger when the learning occurred over several days.

Most intriguingly, the research revealed what scientists call a “nasty neighbor effect.” Ants showed the highest levels of aggression toward colonies located within their foraging range (about 5 meters), compared to colonies situated farther away. This heightened hostility toward nearby rivals makes evolutionary sense, as neighboring colonies compete directly for resources and represent the most immediate threat.

The learning process appears to be driven by associative learning, similar to how Pavlov’s dogs learned to associate a bell with food. In this case, ants associate the chemical signatures of rival colonies with the aggressive encounters they experience. When researchers prevented aggressive interactions by disabling the attacking abilities of some ants, the learning effect disappeared, confirming that the physical confrontation serves as a crucial trigger for memory formation.

This mechanism helps explain why older worker ants, particularly foragers who spend more time outside the nest, tend to be more aggressive than their younger nestmates. Their increased exposure to rival colonies provides more opportunities to learn and remember potential threats, making them more effective colony defenders.

“We often have the idea that insects function like pre-programmed robots,” says Dr. Volker Nehring, research associate in the Evolutionary Biology and Animal Ecology group at the University of Freiburg, in a statement. “Our study provides new evidence that, on the contrary, ants also learn from their experiences and can hold a grudge.”

Looking ahead, the research team plans to investigate whether and to what extent ants adapt their olfactory receptors based on these experiences, potentially uncovering the physiological basis for this remarkable learning ability.

In the end, it seems these tiny insects have more in common with elephants than we realized – they too never forget, especially when it comes to their enemies.