Right now, a single family of ants is fighting a world war on several continents.
These are Argentine ants, Linepithema humile. They're small, brown and easy to overlook. But they've built some of the largest animal societies on Earth, and on their borders, they fight wars that have killed millions with no end in sight.
An empire without borders
Argentine ants come from South America, around northern Argentina. People have carried them, by accident, to every continent except Antarctica, and they're now one of the world's worst invasive species, wiping out native ants wherever they go.
Their secret is cooperation. In most ant species, every colony is an enemy of every other colony. But in their invaded range, Argentine ants from different nests don't fight each other. Instead, millions of nests merge into one huge "supercolony," sharing food, workers and queens. Together, they overwhelm native ants that only fight as single colonies.
These supercolonies are enormous:
- California Large stretches about 900 km along the California coast.
- European Main is thought to run around 6,000 km along the Mediterranean.
- Another huge supercolony lives on the west coast of Japan.
Why they don't fight each other
Ants recognize their nestmates by smell. And the Argentine ants that reached California were very similar genetically, probably because they all came from a small founding group. Researchers think at least 10 mated queens arrived more than a century ago, though it could have been more.
With so little genetic variety, the ants struggle to tell friend from foe. Melissa Thomas and her colleagues put it simply: the lack of genetic diversity is what lets supercolonies form. The ants treat almost everyone as family.
Wars on the borders
But not every Argentine ant belongs to the same supercolony. In southern California, at least four smaller supercolonies live next to the main one. And those borders are battlefields.
Thomas, Neil Tsutsui, David Holway and their colleagues visited nine border zones every week for six months. They found constant, intense fighting, with often huge numbers of deaths. At just one small supercolony near Lake Hodges, north of San Diego, they estimated that at least 15 million workers died over the six months.
And for all that death, the front lines barely moved. The border war was a stalemate, with neither side able to break through.
An alliance across oceans
For a long time, researchers thought each of these wars was a separate, local conflict. Then, in 2009, Eiriki Sunamura and his colleagues tried something new. They brought live ants from the main supercolonies in Europe and California to Japan and put them together with ants from Japan's main supercolony.
They didn't fight. They touched antennae and behaved like old nestmates, even though they came from different continents separated by oceans. But when they met ants from Japan's smaller supercolonies, they fought hard.
The most likely explanation is that the main supercolonies in Europe, California and Japan are one family, spread around the world by human trade. The researchers wrote that the size of this population is matched only by human society. What looked like many small border wars may be one global conflict, with a single megacolony on one side.
The cost of an empire
The Argentine ant's world war isn't just a problem for other Argentine ants. Everywhere they spread, they displace native ants, and that ripples outward. In South Africa, for example, they've pushed out native ants that spread the seeds of fynbos plants. When one ant takes over, everything that depended on the ants it replaced feels it.
No Argentine ant knows it's part of an empire. There's no general and no plan. Just a simple rule, "don't attack anyone who smells like family," repeated across trillions of tiny decisions, until it spans the globe.
For another kind of insect war, read about stingless bee wars.
Further reading
- Thomas, M. L. et al. (2006). When supercolonies collide: territorial aggression in an invasive and unicolonial social insect. Molecular Ecology.
- Thomas, M. L. et al. (2007). Contact between supercolonies elevates aggression in Argentine ants. Insectes Sociaux.
- Sunamura, E. et al. (2009). Intercontinental union of Argentine ants: behavioral relationships among introduced populations in Europe, North America, and Asia. Insectes Sociaux.
- van Wilgenburg, E., Torres, C. W. & Tsutsui, N. D. (2010). The global expansion of a single ant supercolony. Evolutionary Applications.
- Moffett, M. W. (2012). Supercolonies of billions in an invasive ant: what is a society? Behavioral Ecology.
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