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Ants Are Resentful and Can Even Take Revenge

Black garden ant (Lasius niger) on a toothpick
Ants (here the black garden ant Lasius niger) are social insects that can remember who has behaved aggressively towards them when living together with conspecifics, as a study shows Photo: Getty Images

January 20, 2025, 8:42 am | Read time: 6 minutes

Ants are social insects and live in large communities. There are sometimes disputes – especially between rival colonies. A study has now shown for the first time that ants learn from bad experiences with their neighbors and are resentful. Some even take revenge on their fellow ants.

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Those who live together in a social group must know who is a friend and who is a foe. Ants recognize their nestmates by smell and know exactly who belongs to the clan and who does not. However, researchers have now discovered that the small insects even remember which strangers they have had bad experiences with. For example, if they have been attacked by them. The next time they encounter them, they react with increased aggression, showing ants can be resentful. One might even describe ants as holding grudges. The results were published in the journal Current Biology.

Do Ants Recognize Where Other Ants Come From?

It is already well-known and well-researched that ants fight each other and raid other ants’ nests. However, one thing has so far puzzled scientists: Ants behave particularly aggressively towards individuals that come from neighboring colonies that are particularly close to their own. Conflicts probably occur more frequently between these colonies. But how do the ants know which colonies other ants come from and how far away they are?

In order to shed light on ant rivalry, a team of scientists from the Evolutionary Biology and Animal Ecology working group at the University of Freiburg investigated how black garden ants (Lasius niger) behave when confronted with different conspecifics from foreign nests. Would the ants make a distinction between individuals, or do they generally attack anyone who is “foreign” and does not belong to their own colony?

The ‘Unpleasant Neighbor Effect’

The researchers conducted the experiment in two phases. In the first phase, which was called the “unpleasant neighbor effect,” the ants were to gain various experiences with conspecifics. One group encountered ants from their own nest, the second group encountered ants from a rival nest A, and the third group encountered ants from a rival nest B. Nest A was closer to the test ants’ colony than nest B. Nest A was closer to the colony of the test ants. It can, therefore, be assumed that the animals had already had more negative experiences with these individuals than with the ants from nest B, which was further away.

In the second test phase, how the ants from the different groups behaved when they encountered the competitors from nest A was investigated. Would the animals remember that the rivals had behaved aggressively towards them and react accordingly?

Ants Are Resentful

In fact, the test ants remembered negative experiences they had had during encounters with rivals in the first phase. They behaved more aggressively when they encountered ants from a nest that they had previously experienced as aggressive than ants from nests that were unknown to them. However, they did not recognize individual ants but generally reacted more aggressively to ants that came from the nearby nest A and with whose individuals they had already had bad experiences. In contrast, animals that encountered members of a nest from which they had previously only met passive ants were less aggressive. This indicates that ants can hold grudges.

The results also suggest that ants can recognize which conspecifics come from which nests by their scent. The researchers suspect that this happens via certain hydrocarbons. Previous studies have already shown that ants can associate hydrocarbons with a food reward. In this case, the effect is the other way around: the ants associate the recognition cues of rivals with the “bad experience” when they attack them. This is why they are more aggressive when confronted with competitors from nests they already know.

Ants Take Revenge on Unfamiliar Conspecifics

“With our experiments, we show that the specific experience of individuals contributes to behavioral variation,” write the researchers in their discussion of the results. These show that ants are more aggressive towards conspecifics from other nests if they have already had bad experiences with other individuals from the same colony.

So, if an ant is attacked by a competitor from the neighboring colony, it remembers its scent. In this way, it later recognizes whether other ants it encounters are from the same colony and behaves aggressively towards them – even if they may not be attacking at that moment. Thus, ants are not only capable of holding grudges but also exhibit retaliatory behavior towards members of foreign nests.

Effect Was Not Very Robust

From a biological perspective, it is logical that ants would exhibit such grudge-holding behavior. Defending their own nest is a highly complex behavior. Recognizing which conspecifics pose a potential threat to one’s own colony is an advantage. However, the effect that ants are more aggressive towards conspecifics from known neighboring colonies than towards unknown ones was not very robust in all experiments, as the researchers write. This can also be seen in the graphs of the results, which generally show a large scatter. This means that there were relatively many individuals who behaved completely differently.

On the one hand, this may be due to differences in the experimental design, as the researchers themselves note. However, the caste of the individual ants is also likely to have played a role. Similar to other social insects, ants live in colonies with a division of labor. Three castes each take on different tasks: While the queen lays eggs and the males fertilize them, the workers care for the offspring and take care of food and nest building. 1

Individual Variations

Since foragers spend more time outside the nest, they generally have more experience with other members of their species. This enables them to better recognize intruders later on and thus attack them earlier, the researchers write. In the experiment, this could have led to older workers, especially foragers, being more aggressive than workers within the nest.

“The variation in experiences among individuals results in diverse nest defense behaviors, indicating a greater degree of individuality in social insects than can be accounted for by age polyethism or size polymorphism alone,” the researchers conclude.

More on the topic

Are Bees Also Resentful?

“We often have the idea that insects function like pre-programmed robots,” the journal Physorg quotes study leader Dr. Volker Nehring. “Our study provides new evidence that, contrary to this notion, ants are capable of learning from their experiences and exhibiting grudge-like behavior.” Next, Nehring and his team want to investigate whether and to what extent ants adapt their olfactory receptors to their experiences and thus also reflect what they have learned at this level.

The researchers also suspect similar effects in other social insects. This means that not only ants but also honeybees or termites could be resentful. After all, they also live together in large colonies and are attacked by neighboring colonies. It is, therefore, just as important for them to distinguish between who belongs to their own nest and who comes from a colony that has attacked their own colony in the past.

This article is a machine translation of the original German version of PETBOOK and has been reviewed for accuracy and quality by a native speaker. For feedback, please contact us at info@petbook.de.

Topics insects News from science and research

Sources

  1. uni-jena.de, "Division of labor in ants for over 100 million years" (accessed on 09.01.2025) ↩︎
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