Non-aggressive Behaviour Facilitates Ant Invasion

Under certain circumstances, ant colonies can team up to form huge supercolonies extending across thousands of kilometres and displace native species, if they refrain from aggressive behaviour. A research group from Innsbruck is now investigating how this situation can arise.

The enemy arrived by boat: ever since the Argentine ant was introduced into southern Europe in the first half of the 20th century, it has been spreading rapidly, displacing the native species. The recipe for success of this invasive species is the formation of so-called "supercolonies". These are interrelated networks of ant colonies with countless queens, whose individuals cooperate and behave like a single colony. The largest such colony extends for five thousand kilometres along the Italian coast to northern Spain.

As yet, researchers are puzzling over the reasons for this extraordinary phenomenon. Recent observations do not support theories that the merger is based on ants within the colony being related. As part of a project funded by the Austrian Science Fund FWF, a team led by the two ecologists Florian Steiner and Birgit Schlick-Steiner is now investigating an ant species native to the Alps, which also has the ability to form large colonies with multiple queens, although their colonies are considerably smaller than those of the Argentine ant. The researchers combine field research with methods borrowed from genetics.

Evolutionary origin unclear

"Supercolonies are problematic from the perspective of evolutionary theory, because it cannot explain them," notes principle investigator Florian Steiner. "There would have to be a mechanism that leads to the genetic characteristics of particularly cooperative individuals being found particularly often in the next generation. But that's not the case with supercolonies."

The slow pace of evolution makes it difficult to study the phenomenon in detail. "It is not easy to find the mechanisms that lead to the emergence of supercoloniality, if all you can observe is the end point of the development," says Steiner. The Argentine ant has obviously reached this end point, as Birgit Schlick-Steiner explains: "Individual colonies do not show any hostile behaviour towards each other. When animals from the coast of Italy are combined with animals from the coast of Portugal, there is no aggression. This is surprising, because ants normally react aggressively to each other, even if their nests are only a few metres apart." It would therefore be very useful to observe an ant species that is in the middle of its evolutionary development towards the formation of supercolonies. And such species do indeed seem to exist.

Alpine ant species as a model

One ant species showing characteristics of particular interest in this respect is Tetramorium alpestre, which is native to the Alps. "This ant is native to our region, lives in the mountains at an altitude of 2000 metres above sea level and is not invasive anywhere as far as we can tell," reports Florian Steiner. "It has a wide range of behaviours and social structures, from small colonies with one queen that are aggressive towards each other to colonies with several queens that have the characteristics of a supercolony." Steiner stresses that here we are dealing with individuals that belong to the same species.

The researchers started by collecting individuals over the entire Alpine range. Team member Patrick Krapf spent four years collecting data for his thesis to investigate which factors can decide on behavioural differences. Florian Steiner reports, however, that this approach to the problem has not led to a breakthrough. Neither location factors nor kinship relations between the ants seem to make a difference. "With this new project, we're trying to take a closer look."

Behavioural experiments with individual ants

The researchers conducted behavioural experiments with several thousand ants captured in the mountains. They staged encounters between the animals and documented them by video. Would they show aggression? Would they be non-aggressive? "We selected 72 of these animals that were particularly representative," notes Steiner. "These individuals were shock-frozen, and we used the head for RNA extraction, the rest of the body for DNA extraction."

While information about inherited behaviour can be obtained from DNA, an analysis of RNA, the so-called transcriptome, provides information about which parts of the genetic material are genuinely active, thus providing insights into the animal's previous experience. The aim is to determine more precisely the factors that determine behaviour.

This is a new approach, as Steiner reports: "There are video recordings of each individual, which can be used to evaluate their behaviour towards another ant individual. We also conducted chemical analyses in the respective nest." The analysis thus covers the entire chain from in-the-field biology, in the Alps at 2000 meters, to semiochemistry and specific genetics issues.

"We hope to make progress with this approach, where we know each ant's exact origin, exact behaviour and, on top of that, its genome and transcriptome. The analysis of the genetic test is currently being carried out by the bioinformatician Martin Schilling; initial results do not point to any apparent DNA commonalities between individuals that behave similarly. - The team is thus eagerly awaiting the evaluation of the RNA data. The project will run until 2022.

Excluding interference noise

Tetramorium alpestre is not a typical model organism for ant research, but it does have some very favourable properties for research. "There are a few other species that are known to have such a wide range of behaviour, but not confined to such a small geographical area," says Steiner. "In this way, we can rule out any influence of environmental factors on the results." In addition, the behavioural experiments were carried out using a blind method: not even the researchers themselves knew whether they were investigating related animals or individuals from different colonies.

Better understanding of invasive organisms

Ultimately, Steiner intends to gain a better understanding of the spread of bioinvaders such as the Argentine ant. Ants which form supercolonies are a particular threat to other ecosystems. In the USA, for example, the red imported fire ant, which originates from South America, has formed supercolonies and puts pressure on the native ant species. "Alien species are being introduced somewhere else countless times a day all over the world," explains Steiner, "but few species are so well suited to the new habitat that they can gain a foothold there and actually displace the native fauna as biological invaders." Supercolonies, however, pose a particular threat - and are still shrouded in mystery.

(Scilog/FWF)

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