Humans have a long history of bolstering their armies by paying mercenaries to fight on their behalf. Now, Rachelle Adams from the University of Copenhagen has found that some ants do the same.

Some ants defend their colonies with special soldiers, which are bigger and wield more formidable weapons. But others lack a proper army. Sericomyrmex ants, for example, are fungus-farmers. They bring bits of vegetation back to their nests and use these to nourish a fungus, which they then eat. They are poorly defended, and a sitting target for raiders and pirates. That’s why they rely on other ants to fight for them.

Another ant called Megalomyrmex symmetochus forms its own colonies, complete with queen and workers, inside those of Sericomyrmex colonies. This guest is present in over 80 percent of the farmers’ nests and, at first glance, it looks like a parasite. It eats the fungus that the farmers so assiduously grow, without contributing any labour of its own. Worse still, it eats some of the farmers’ larvae and clips the wings of their young queens, so they contribute to the gardening rather than flying off to start their own colonies.

But M.symmetochus doesn’t just freeload off its hosts. In some cases, it can be their salvation.

View Images Megalomyrmex mercenary (top right) defends a fungus garden from a Gnamptogenys raider (bottom). Credit: Anders Illum

Sericomyrmex nests are often attacked by a third ant called Gnamptogenys hartmani—a sort of six-legged pirate. It raids the colonies of farming species, drives them out, usurps their nests and gardens, and eats any remaining larvae.

The farmers can do very little against these raiders, since they lack specialised soldiers and have mostly lost their stings. Their powerful jaws can deliver a strong bite, but at such close quarters, they risk getting stung and bitten themselves. When attacked, they’re much more likely to feign death or flee. That is, unless there’s a M.symmetochus colony living in their nests.

Back in 2011, Adams let some raiders loose upon a colony of Sericomyrmex farmers, which was already being parasitized by M.symmetochus. “To our surprise, the hosts hid and the parasites rose to the top of the garden to confront and kill the invaders,” she says.

Unlike their hosts, the parasitic ants are far from defenceless. They raise their stings and release a powerful venom directly into the air—an airborne chemical weapon that kills the raiders and befuddles any survivors. “Rather than uniting as an efficient infiltration squad they turn on each other and attack, sometimes killing their own kin,” says Adams.

M.symmetochus behaves like a colony of mercenaries. They can cause problems for the farmers during peace-time, but they provide an invaluable defence when an invading force arrives.

By pitting the three types of ants against each other, first in one-to-one battles and then in more realistic groups, Adams’ team showed that the mercenaries are much better at subduing the raiders than the farmers. It takes just two of them to overpower a single raider, while the same task requires at least eight farmers. On average, a pair of raiders can kill 70 percent of farmers in an unprotected nest, but just 10 percent of them if there are six mercenaries around.

And the raiders seem to know it. When the team gave them a choice between two nests, only one of which was defended by mercenaries, they were more likely to attack the unprotected nest. In the experiment, a wire mesh prevented the raiders from actually touching the colonies, so they were probably put off by the smell of the mercenaries’ chemical weapons. Just by living in the same colonies, the mercenaries protect the farmers by cloaking them in a defensive miasma.

The thing that clinches this incredible relationship is probably Megalomyrmex’s life cycle. They form a life-long bond with a single colony of farmers, and won’t leave to seek out a different host. They exploit their hosts but they don’t overexploit them, and if raiders attack, they mount a defence rather than abandoning the farmers to their fates.

Next, the team wants to study how this alliance varies over time and space. For example, it should be easier to find the mercenaries in the farmers’ nests in places where the raiders are more common. Similarly, a different farmer called Trachymyrmex zeteki should be relatively unbothered by raiders, since it also hosts a parasitic mercenary but in fewer than 6 percent of its nests.

For now, they very astutely compare the mercenary ants to a human disease called sickle cell anaemia. It’s an inherited genetic disease that warps the shape of red blood cells and causes a gradual building illness. The mutated gene behind the disease is common in Africa because two copies might cause sickle-cell anaemia, but one copy protects a person against malaria. Similarly, M.symmetochus is like a mild chronic disease that exerts a toll upon its host, while protecting it from an acute and more destructive infection—the Gnamptogenys raiders.

Reference: Adams, Liberti, Illum, Jones, Nasha & Boomsma. 2013. Chemically armed mercenary ants protect fungus-farming societies. PNAS http://dx.doi.org/10.1073/pnas.1311654110