Enemies With Benefits: How Parasitic Ants Protect Their Hosts

What might be the best-titled research paper of the year comes to us courtesy of researchers at the University of Copenhagen, an investigation entitled “Chemically armed mercenary ants protect fungus-farming societies.”

Have you got your popcorn ready?

It’s a tale of three species. We begin with a fungus-growing ant called Sericomyrmex amabilis found in the tropical rainforests of Central and South America. S. amabilis grow and maintain underground gardens of fungus that nourish the entire colony.

Mercenary ants defend nest
Mercenary Megalomyrmex ants defend a fungus patch from invaders. Photo by Anders Illum

However these peaceful fungus farmers’ efforts also attract another species of ant called Megalomyrmex symmetochus that specializes in exploiting that agricultural industry. Megalomyrex are considered “social parasites.” “We make this distinction from [regular] “parasites” because they infiltrate the colony, rather than an individual,” explained Dr. Rachelle Adams, of the Centre for Social Evolution at the University of Copenhagen, who led the study.

It works like this:  After fighting her way inside a nascent Sericomyrmex colony, a Megalomyrmex queen quickly sets up shop and begins to birth workers of her own species. Once ensconced in their new home the invaders then proceed with a series of nasty maneuvers, helping themselves to the fruits of the fungal garden and stunting the colony’s growth by feasting on the host’s brood and clipping the wings of virgin queens so that they don’t fly off and start new colonies. To add insult to injury, once her wings are clipped the host queen is left to assume the tasks of an ordinary worker.

In short, between raiding the fridge and bossing around their hosts, Megalomyrmex seem like lousy house-guests. They are parasites. But what struck Adams is that these guest ants also produce a large workforce of their own, which is strange behavior for a parasitic species that would seem better off letting the host species do all the work. Taking a closer look, the study found that “M. symmetochus guest ants produce a seemingly excess number of workers that constantly patrol the host nest,” suggesting that the relationship between the host species and its parasite is more complex and mutualistic than it at first appears.

Mercenary ant stings invader
Mercenary Megalomyrmex ant stings a Gnamptogenys raider ant. Photo by Anders Illum

It turns out that reason for this standing army is that the host species has a far worse enemy in the agro-predator ant Gnamptogenys hartmani, an aggressive species known to “usurp their gardens and nest structures with remarkable efficiency,” according to the study.

Overall, the parasitic guest ants are much better at defending the colony and its inhabitants than the more easily subdued host species, whose battlefield tactics mostly involve biting, running away, or playing dead. (None of which is very effective) In contrast the guest ant is more aggressive and moreover possesses a powerful alkaloid venom that can be dispersed in aerosol form or on contact.

Indeed the study found that the presence of even a few guest ants radically improved the survival odds of the host colony during an enemy attack because the potency of their chemical weapons allows the guest ants to efficiently dispatch large numbers of raiding Gnamptogenys ants.

“It is quite logical that the guest ants work to protect their resources. What was unexpected was how effective the guest ants were at killing the raiders,” says Adams.  “Gnamptogenys ants are known to be predators of other ant species and have an impressive recruitment strategy and powerful sting. It was incredible to see how the guest ant’s recruitment and toxic venom overwhelmed the raiders.”

Interestingly the guest ants’ toxin doesn’t merely poison the attackers but also scrambles their ability to recognize their own nestmates. “Not only is it extremely toxic and repellent, but it changes their behavior toward each other. Rather than uniting as an efficient infiltration squad they turn on each other and attack, sometimes killing their own kin,” explains Adams.

Therefore, while they benefit from their hosts fungus crops, the parasitic guests also offer them valuable protection as “protective symbionts.” In a sense “The guest ants are the better of two evils,” says Adams.  It’s a complex relationship. “The parasites are still parasites because they’re still extracting resources front the host and the host is still at some level being harmed by the parasite. But in a population that has these raiding ants, that cost is outweighed by the benefits of the protection.”

It’s a relationship known as “context dependent mutualism” and the arrangement can go on indefinitely, suggesting that the guest ants limit the amount of host brood they eat and that the two species develop a sort of sustainable balance that allows them both to survive, says Adams.

The paper concludes that much like the mercenaries used to defend cities in medieval Europe, the parasitic guest ants essentially form a “functional soldier caste” the benefit of which outweighs the strain they put on their hosts.

Mercenary Ant Stings Invader
Mercenary Megalomyrmex ant (left) deters a Gnamptogenys raider (right) by releasing airborne venom from its sting. Photo by Anders Illum

The main difference, of course, is that these mercenaries are paid in fungus.


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Meet the Author
Stefan Sirucek is a writer and journalist who reports from both sides of the Atlantic. He's written for the Huffington Post and Wall Street Journal. Follow him on Twitter at @sirstefan.