Climate crisis makes ants more aggressive

      -       english   -  deutsch
Ants studied The ant species Tetramorium alpestre lives in high alpine areas. © Ants studied The ant species Tetramorium alpestre lives in high alpine areas. © Petra Thurner, Molecular Ecology Research Group
Hostility due to heat: Effects caused by the climate crisis, such as higher temperatures and more nitrogen in the soil, lead to greater aggressiveness among ant colonies. This was shown by a team of researchers led by the Innsbruck ecologists Patrick Krapf, Birgit C. Schlick-Steiner and Florian M. Steiner of the Molecular Ecology Research Group using the example of the widespread ant Tetramorium alpestre at eight high alpine sites in Austria, Italy, France and Switzerland.

Ants play a central role in ecosystems, as their mass alone shows: according to the latest estimates, there are around 20 quadrillion ants on earth, which means more biomass than all wild mammals and birds combined. Ants dig up soil, fight pests such as bark beetles, decompose carrion, pollinate plants, and spread plant seeds. Yet studies of animals living in soil at fixed sites are sparse, especially in light of impacts from anticipated ecological change due to the climate crisis. In a new study co-financed by the Austrian Science Fund FWF, ant researcher Patrick Krapf from the Institute of Ecology at the University of Innsbruck has now analyzed the behavior of high alpine ants, focusing on aggressive behavior. The results show that in this species, aggression appears to correlate with environmental factors. "We studied eight populations of the ant Tetramorium alpestre at different altitudes along the Alpine arc for their hostile potential. To do this, we allowed workers from different colonies within the populations of one site to meet and looked at how hostile or peaceful they were with each other," explains Patrick Krapf. The colonies studied occur at altitudes between 1600 and 2300 meters and were collected in four countries. In Austria, colonies were tested from Kühtai and Hahntennjoch (both Tyrol) as well as from Mussen in Lesachtal (Carinthia), in Switzerland from Julier and Simplon Pass, in France from Col de Vars and Col du Galibier, and in Italy from a population near Colle della Maddalena.

Ants in duel

To examine behavior, the team conducted aggression tests involving two workers from different neighboring colonies at a time, in addition to several genetic and environmental analyses (see videos). Workers are the largest group in the ant colony and are responsible for foraging, nest building and brood care. "This videotaped duel is meant to simulate a clash in the wild - as occurs when ant workers forage for food," Krapf said. The total 3-minute videos were then analyzed by the team on a per-second basis, yielding an average aggression score for all pairings. "The aggressiveness of ants from warmer areas such as Italy and France was several times higher compared to cooler sites in Austria and Switzerland," the ecologist says. Nutrient enrichment in the soil - so-called eutrophication - also plays a role, as the researchers* found: "In addition to increased air temperature, we also observe a correlation between nitrogen content in the workers and in the soil and hostility. Nitrogen availability is probably also increased in soils due to ecological change caused by the climate crisis."

Aggression as a losing proposition

More fighting among ant workers may mean more food temporarily for individual colonies, and thus a short-term advantage. In the long term, however - and against the backdrop of further global warming that is considered certain - this development should be seen as detrimental, according to Krapf: "That ants show aggressive behavior toward other colonies when foraging is normal. But if these fighting activities increase, it costs the workers a lot of energy and time. This could have a negative effect on the development of the entire ant colony, because the number of ants then decreases and, for example, less food is available." The fact that higher temperatures lead to more aggression has already been proven in other studies for humans, ungulates and voles, for example. Nevertheless, there is still a lot of research to be done here, Patrick Krapf is convinced - and therefore proposes further studies with the research group: "Since ants are very important ecosystem service providers, a better understanding of the consequences of global change is of great importance."
High mountain ant Tetramorium alpestre: These videos, for example, document peaceful behavior from Austrian colonies and aggressive behavior between two workers from Italian colonies.

Peaceful ants Austrian colony (mp4):
Hostile ants - Italian colony (mp4):
Credit: Patrick Krapf

Patrick Krapf, Wolfgang Arthofer, Manfred Ayasse, Florian M. Steiner, Birgit C. Schlick-Steiner, Global change may make hostile - Higher ambient temperature and nitrogen availability increase ant aggression, Science of The Total Environment, Volume 861, 2023, ( ) For this study, fragments of ant colonies were collected in the field and carefully maintained in the laboratory. The ants were provided with suitable nesting facilities, food and water, and were kept in a species-appropriate manner until their natural death.