If you haven’t had bad neighbors, you’ve certainly heard of them. Whether they didn’t invite you to a cookout, or can’t seem to stay on their side of the fence, the people next door can always find a way to get on your nerves. If your neighbors were a colony of immigrant pavement ants (Tetramorium immigrans), however, you might just be inclined to move.
In the bustling cities south of the Italian peninsula, the battle of property lines and town barbecues plays out on a much smaller, and much more violent, scale: between two colonies of ants. These ants live in concrete cracks a mere five centimeters apart. The two species are of different sizes and forage in different areas – one looks for food on the ground, while the other climbs trees. The larger ants (Camponotus barbaricus), however, often harass and kill their smaller neighbors.
This may read as a classic underdog story, but one shouldn’t be so sure in the world of insects. The immigrant pavement ant is actually an invasive species, and it is highly successful in urban areas such as Palermo, Sicily, where this recent study took place.
How are these little ants so successful? It might have something to do with a remarkable behavior reported among a handful of ant species: soil-dropping. Soil-dropping involves an entire colony’s worth of worker ants surrounding another colony’s nest and dropping small pieces of dirt over the entrance. This makes it more difficult for resident ants to leave their nests and look for food. The entrances don’t even have to be covered all of the way – it’s an intimidation tactic. Think of it like an angry neighbor who keeps piling wood in front of your door. It might not be physically impossible for you to leave, but you might just want to stay inside until they’re done before making a trip to the grocery store. Perhaps more intimidating is the fact that the ants were not just dropping dirt on their unsuspecting neighbors – they were doing so in organized waves.
The immigrant ants’ tactic can be quite successful. Each of the larger ants can kill tens of immigrant pavement ants by itself. After just over a minute of being covered by soil, the entire colony of larger ants stayed inside for several hours. Most of the time, however, a few aggressive interactions are enough to force another colony to relocate. In fact, this was seen with the very same colony of immigrant pavement ants – they drove away a different species of ants without even picking up a piece of soil.
Still, if soil-dropping is so rare, then why was it observed here? Simply put, these two groups of ants got in each other’s way a lot, and they weren’t nice about it. It also certainly didn’t help that they lived so close to one another.
The fact that these ants drop soil suggests that many more ants are capable of doing so. And it’s certainly not unbelievable, because soil-dropping is incredibly difficult to observe; it’s very hard to make ants drop dirt on each other in a lab, and even harder to make it happen in nature. It’s no surprise that, so far, only seven ant species have been seen doing so. However, they are not all closely related, meaning that many more types of ants may begin soil-dropping when threatened. This behavior is not limited to unneighborly competition; ants have also been observed dropping dirt, small pieces of stone, and vegetation on predatory wasps.
Because scientists aren’t sure exactly how much this happens, or under what circumstances, we can’t be sure if soil-dropping gives ants a competitive edge in urban environments. That said, fighting dirty certainly does secure prime real estate.
References
1. Schifani E, Castracani C, Giannetti D, Spotti FA, Mori A, Grasso DA. Tool use in pavement battles between ants: first report of Tetramorium immigrans (Hymenoptera, Formicidae) using soil-dropping as an interference strategy. Insectes Sociaux [Internet]. 2022 Aug 27 [cited 2022 Oct 12]; Available from: https://link.springer.com/10.1007/s00040-022-00876-2
