Neighborhood disputes are the order of the day for ants: the social insects react particularly aggressively to individuals from nearby nests. A study now shows that this behavior is probably based on associative learning. Accordingly, the ants remember when they have already come into conflict with members of their own species from a foreign nest – and react more aggressively the next time they encounter them.
The black garden ant (Lasius niger) typically reacts aggressively to members of its species that do not belong to its own colony. If a forager encounters a competitor from a foreign nest while foraging, the two insects bite each other with their mandibles or spray acid to drive away or kill the foreign ant. How hard the ants act depends, among other things, on which nest the other person comes from. They react particularly aggressively to individuals whose odor signature is very different from their own – but also to individuals from closely adjacent nests.
Increasing aggression
Experiments by a research team led by Mélanie Bey from the University of Freiburg now provide an explanation for this “bad neighbor effect”. According to this, ants remember who they have already come into conflict with and react more aggressively in subsequent encounters. For their study, Bey and her team put individual ants from different nests together for one minute on several consecutive days and observed the interactions between the animals.
“When an ant was repeatedly confronted with a conspecific from another nest, the aggression increased day by day,” report the researchers. If, after this training phase lasting several days, a test ant again encountered an ant from the same unfamiliar nest, it behaved more aggressively than if it instead encountered an ant from another nest whose members it had not previously had any experience with. Apparently, the ants learned from the repeated conflict-filled encounters to associate the smell of their known competitors negatively and to react with increased aggression.
Hostile and neutral encounters
But what if the first contacts were not hostile? To test this, Bey and her team removed the antennae of some ants before putting them together with a test ant from another nest. Lacking their most important sensory organs, the ants manipulated in this way behaved neutrally. The test ant was able to learn that it does not have to fear attacks from members of this strange nest.
And indeed: when the test ant was put together after several of these neutral encounters with other, non-manipulated individuals from the same foreign nest, it behaved significantly less aggressively than towards individuals from unknown foreign nests. “This shows that the aggression an ant experiences changes the evaluation of the identifying features,” the researchers conclude. According to Bey’s colleague Volker Nehring, this result underlines that ants act more individually than expected: “We often have the idea that insects work like pre-programmed machines. Our study provides new evidence that, on the contrary, ants also learn from their experiences and can hold grudges.”
Strong individual differences
However, the researchers point out that their results show a wide spread. “The effect of ants being more aggressive toward members of colonies they know than toward strangers was not very robust,” they write. Some ants showed a high level of aggressiveness the first time they encountered a strange member of their species, while others reacted comparatively little aggressively even after repeated encounters.
From the researchers’ perspective, this could be due to the different previous experiences of the individual test ants. For example, within an ant colony there are certain – usually younger – individuals who carry out their work primarily inside the ant burrow, while others go out looking for food and probably come into conflict more often with members of foreign colonies. “The behavior of social insect workers is very variable, even in important tasks such as nest defense,” explain the researchers. “With our experiments, we show that individuals’ specific experiences contribute to this behavioral variation.”
Source: Mélanie Bey (Albert-Ludwigs-Universität Freiburg im Breisgau) et al., Current Biology, doi: 10.1016/j.cub.2024.11.054