It’s been a year where invasion metaphors have, well, invaded our lives. There’s been the global spread of Covid-19, for example, which has often been compared to an enemy force taking over the world country by country. Then there was the brief terror in the US over the invasion of murder hornets. But did you know that there was already an invasive species we found new facts out about this year, one which numbers in the billions and has already taken over six continents? No, not humans, but Argentine ants.
Before we dive into their vast world, a quick primer on ants, ant colonies and supercolonies. Scientists have long observed that the ant colony is the unit around which all ant lives are organized and usually consist of one or more egg-laying queens and numerous sterile females called worker ants.
Colony sizes can vary to an extraordinary degree, from a few ants living and cooperating in a twig to supercolonies that can number in the millions. Supercolonies occur when many ant colonies over a large area unite. Like the European Union, colonies within the super colony avoid aggression between each other, though unlike the European Union they do recognize some differences by avoiding mating with each other.
How far does the Ant Supercolony stretch?
Our understanding about supercolonies is quite new and rapidly expanding. Until 2000, the largest known ant supercolony was in Japan and was estimated to contain 306 million worker ants and a million queen ants living in more than 45,000 nests interconnected by underground passages. Then, in 2000 a much larger supercolony of Argentine ants (Linepithema humile) was discovered in Southern Europe. Then in 2009 scientists established that these were all parts of one megacolony so vast it spans Europe, the US and Japan.
This megacolony is certainly the largest interconnected insect group yet discovered, and is in fact the only interconnected species to rival humans in terms of world domination. It has been partly made possible by the fact that Argentine ants are quite different to other ant species, since they are a nomadic tribe which have always lived in temporary nests rather than permanent, geographically bound structures. You can think of them a little bit like the insect version of Genghis Kahn and his warrior nomads, who also spread out and left their genetic traces across so much of the planet.
And, just like the Mongol hordes, the humans in the path of these ants have learnt to fear them, given that they are a very aggressive species which will attack crops and animals, including humans. Of course, the Mongols were finally limited in their territorial ambitions by the oceans, given that they were substandard seafarers who generally had to steal other people’s boats for their limited attempts at overseas expansion.
So how did the Argentine ant megacolony do what Genghis Kahn could not, and cross vast oceans? The answer, of course, is humans unwittingly transmitted them, by boat or plane. The first Argentine ant to be found outside its native South America was one little pioneer discovered in Portugal in 1866. Now there are three particularly vast supercolonies which form the megacolony. In Europe, one vast colony of Argentine ants stretches for 3,700 miles along the Mediterranean, while the US contingent extends over 560 miles of the Californian coast. A third huge colony in Japan has been less well defined.
But what exactly does being in a megacolony mean? The answer, as researchers from Japan and Spain found a decade ago, is that Argentine ants living in Europe, Japan and California shared a strikingly similar chemical profile of hydrocarbons on their cuticles. This allows them to recognize each other and then cooperate with each other. While ants are usually extremely territorial, those living within supercolonies are tolerant of one another, even if they live tens, hundreds or thousands of miles apart.
This was absolutely true of the megacolony. The researchers pitted ants from identical species but different colonies against each other in a series of gladiatorial contests and found that they would usually be highly aggressive with each other. Geography didn’t matter, west coast Japanese ants were just as likely to attack east coast Japanese ants as ones from Spain. However, whenever ants from the main European, Californian and Japanese colonies came into contact, they acted like old friends, even rubbing antennae with one another. Essentially they acted as if they belonged to the same colony, despite the fact they had lived all of their lives thousands of miles apart and separated by vast oceans.
To be clear, this friendly behaviour isn’t simple species loyalty, and Argentine ants do not always cooperate with each other at all. It has been observed that rival colonies of Argentine ants will attack each other when they are unaffiliated, it is only within the supercolonies that a non-aggression pact holds. The key to the cooperation between the members of the megacolony appears to be a striking lack of genetic diversity between them, which makes them quite close to clones. They look and smell the same to each other, which is why they happily co-exist with one another.
In fact, they are more reliably friendly to each other than humans and far more accepting of each other. While you could take one ant from the Japanese megacolony and drop it into the American megacolony and find it instantly able to communicate and cooperate with others, the same wouldn’t be true of humans where language and cultural barriers would need to be overcome first.
With the science of supercolonies and megacolonies so new, we can expect further outposts of Argentine ants to be discovered in the future. They may not rival humans in terms of our extraordinary geographical spread, but in terms of numbers they very likely exceed us already. And, if humans continue to assist their invasion efforts through travel and commerce, that geographical advantage may also disappear. If climate change continues to accelerate, we may well be looking at the species which will take over where we leave off.
Related: 10 Fascinating Facts About Queen Ants
