Written by Joe Ballenger

@BugQuestions What’s the deal with L. humile on the West Coast (one + colony) vs. East (many warring colonies) – Why?

— Nathanael Johnson (@SavorTooth) February 12, 2015

Argentine ants tending their nest.
Picture courtesy of Robert Lord Zimlich, via BugGuide.net.
License info: CC-BY-ND-NC1.0

This is one of those questions that’s really several questions rolled into one, and I think that makes it a great question.

So, ants form colonies. There’s a queen which lays eggs, workers who do the housework and hunt. There’s also the boy ants (drones) which pretty much bro out in the colony’s basement, and occasionally go looking for nice lady ants. That’s pretty much a standard ant colony.

Ant behavior can be pretty complicated, because there’s a lot of variations on this theme. Some ants maintain a bunch of smaller colonies with larvae and pupae, but no queen. These are called ‘satellite colonies’, and are replenished with larvae and pupae from the main (or ‘parent’) colony. Carpenter ants are an example of this.

Other ants, like the Argentine ants we’ll be discussing in this post, may have colonies which have many queens. Multi-queen colonies, or polygynous colonies (as ant scientists would call them), function as single colonies despite the fact there are many different queens producing larvae. These may be sisters, or daughters, depending on who started the nest and who’s giving birth. But they’re always closely related.

…and this brings us to Argentine ants, which are really weird amongst ants. There aren’t that many Argentine ant colonies in the world, and most of the individuals people encounter only come from three or four colonies. These colonies, however, are massive and can stretch across continents. Frequently, all ants found in a country or state will belong to the same colony.

Which brings us to Nathaniel’s question. Some Argentine ants found in the same area fight. Why?

What is a supercolony?

The first thing we have to talk about is the term ‘supercolony’, in relation to ant colonies. It’s really important to talk about why ants divide themselves up into colonies, and why supercolonies are special.

On the surface, a supercolony is simply a really big ant colony with lots of queens which is spread over a large geographical area. It’s a simple definition, but figuring this out can be pretty tough.

Ant colonies, within a species, don’t really get along very well. If two unrelated colonies meet, they’ll engage in massive battles. On warm summer days, pavement ant fights are pretty commonly noticed by people. Consequently, there’s a lot of ant battle videos on YouTube.

So to figure out whether ants are from the same colony, you introduce them to one another and see if they fight.

How can a colony stretch across continents-or even oceans?

The term ‘supercolony’ is a little misleading, because it implies that all individuals from a single colony are in constant contact with one another. This isn’t necessarily the case.

Supercolonies can be split from one another, but workers from one part of the colony can be integrated into other colonies. So even within the range of a supercolony, you might have small or large segments of the colony which function more or less independently.

Obviously, portions of a supercolony which reside on opposite sides of an ocean aren’t going to be sharing resources or even communicating. However when you fly one member of the colony to meet the others, that one will be welcomed as a part of the colony.

So…what about the fighting, then?

Ants ‘talk’ to one another by tapping each other with their antennae, and this picture shows two ants having a pleasant conversation.
Picture credit: Dzlpl, via Flikr.
License info: CC-BY-SA 2.0

Ants tell who’s who through smell. Their exoskeletons are covered in chemicals, specifically wax, which can vary slightly between colonies. Those slight differences in chemical makeup can be detected by the ants, and they’re pretty quick to figure out who doesn’t belong.

On the surface, this is a pretty easy question to answer. The genetic diversity of Argentine ants from California is low, whereas the genetic diversity from the Southeast is high. So the colonies from Georgia and Florida are from a different colony than those from California. Those from the Southeastern US show aggression towards the California colony, and vice-versa.

Genetic diversity is the key here. The more diversity, the more colonies, the more fighting when these colonies meet. However, once you dig beneath the surface the answer becomes a little more murky.

For example, why is Argentine ant diversity so low in California?

The Argentine ant has spread through commerce, and was discovered outside it’s native range before we knew about South American populations. The first specimen was found in Portugal sometime around 1850, and they were first found in Argentina in 1866. By 1850, the Argentine ant had spread all over the world and had likely been established in America for quite some time before they were discovered in the early 1900s. The California population was noticed in 1905, the ones in the Southeast sometime around 1915. Nobody knows when they arrived, although their genes give us some clues.

Genetically speaking, it looks like the California population is derived from one of the Southeast populations. The genes in the California colony seem to match those from the Southeast colony, although there are other factors (like competition from fire ants) which could play a role in maintaining diversity. So even though the California population was known first, the Southeast populations were probably there well before that discovery.

Normally, insects diversify once they find a new territory. Argentine ants did not, for reasons which aren’t entirely clear to researchers. Competition is one hypothesis, but it’s clear behavior plays a role as well. Argentine ant workers appear to reject males from other colonies, which helps minimize mating between colonies.

The California population doesn’t have a lot of genetic diversity, whereas the population from the Southeastern US is extremely diverse. This means the California colonies are closely related enough to be recognized as a single colony, but those from the Southeast function as many different colonies.

It looks like the Southeastern US might be the source population of the largest Argentine ant colony in the world. When they were introduced to California, there was a very specific group which was very well adapted to the landscape. This population went through a genetic bottleneck and never regained genetic diversity, which isn’t a pattern normally seen through these sorts of invasions. The colony grew to a massive size, and was then spread throughout the world by human trade.

I should mention that this is a very simplistic answer which focuses mainly on the main California supercolony. In addition to the main California supercolony, there are four additional supercolonies in that state which are much smaller than the main supercolony. These display aggression towards one another, although there are fewer individual colonies than in the Southeast. These smaller supercolonies likely represent introductions from other sources.

The Bottom Line

Argentine ants have somehow formed a giant, global supercolony for reasons which aren’t entirely clear. One big reason is a lack of genetic diversity, which makes all the Argentine ant colonies more or less clones of one another. Since they’re so close, genetically speaking, the individuals from this supercolony don’t differentiate between one another and function as a single unit.

The Southeastern US population, however, is a little bit different. They form many different colonies, and even though they’re likely the source for the giant global supercolony, they’re still different genetically different enough from the California supercolony to be told apart by other Argentine ant colonies. They form a lot of smaller colonies, so you get fighting which normally happens in other ants.

In other words, these two populations function differently because they have different genetic structures.

There’s a lot of weird stuff going on here, because the California introduction doesn’t act like the other biological invasions which have been recorded. It’s clear there was a founder event of some sort, and genetic drift is a huge factor, but why genetic diversity wasn’t regained isn’t well understood. It’s likely that there’s a very complicated mix of environmental, genetic, and behavioral factors which play a role, and these are too complicated to go into here.