Bug With a Grudge

Ants hold a grudge.

That’s the remarkable conclusion of a new study published in Current Biology that found that the laborious insects remember the smells of previously encountered nests and behave passively or aggressively depending on their past experience with those colonies.

The study came about after prior research showed that ants were more likely to bite and spray acid to kill other ants belonging to the colonies closest to them. Scientists didn’t understand why they behaved so aggressively toward their neighbors.

To figure this out, researchers at the University of Freiburg in Germany pitted nests of the black garden ant (Lasius niger) against each other for a two-part experiment.

First, the ants were exposed for one minute to either their nestmates, aggressive ants from a rival nest (nest A), or aggressive ants from another rival nest (nest B). The same rendezvous occurred once a day for five consecutive days.

Researchers said that each ant colony has a specific scent, which allows the insects to easily recognize each other. This also allows them to associate a certain smell with a certain level of aggression.

Unsurprisingly, ants were passive toward their own nestmates.

However, ants who already encountered nest A were significantly more aggressive toward them compared with ants who had never met with them. To understand why, the researchers moved to the next phase and modified the encounters.

In the second phase, ants were no longer exposed to their nest mates but only to ants from different colonies who were either aggressive or passive. They ensured the passive behavior of a certain nest by cutting off the ants’ antennae. The experiment confirmed that ants were less aggressive toward colonies they had previously encountered and perceived as passive.

The findings suggest that ants remember whether different ant colonies are friendly or dangerous based on their odor. This phenomenon is called the “nasty neighbor effect,” explained Cosmos Magazine.

This study is significant because it showed that behavior by insects such as ants isn’t just inspired by reacting to immediate stimuli but involves more complex cognitive abilities.

“We often have the idea that insects function like pre-programmed robots,” lead author Volker Nehrin said in a statement. “Our study provides new evidence that, on the contrary, ants also learn from their experiences and can hold a grudge.”