Zombie ants, the ghostly slaves of a mind-controlling fungus seen creeping around places like South America for years, have now been spotted in the United States. But don't panic---they've probably been here all along, and we only just now noticed.
Scientists at Penn State have for the first time shown that a fungus here in the U.S. invades the brains of ants, manipulates them into a very specific spot in the forest, and kills them before raining down spores on their comrades.
Scientists had discovered the same fungus---now known as Ophiocordyceps unilateralis sensu lato (*sensu lato *meaning “in the broad sense,” meaning they’re working on a name)---in the U.S. back in the late 1800s, but had no idea it was capable of manipulating ants. Then in 2009, Penn State's David Hughes stumbled across the Flickr feed of a woman named Kim Fleming, who had found and photographed ants infected with the mind-controlling fungus in her backyard in South Carolina. He and his team have been working with her to describe the remarkable zombification ever since.
Now, it was thought that each species of mind-controlling Ophiocordyceps fungi specialized in attacking just a single species of ant. This new species, however, is capable of infecting two different species of ant in the southern U.S.
The researchers, led by Charissa de Bekker, took these two ant species into the lab, along with two others the fungus was not known to attack in nature, and injected them all with the fungus. As expected, the fungus was able to manipulate the first two species as it would in the wild. But instead of assuming control of the other two species, it just outright killed them. The researchers then set up a second experiment to determine what chemicals the fungus is releasing in these hosts, by surgically removing ant brains and submerging them in fluid to keep the tissue alive. To this soup they added the fungus.
Strangely, the fungus didn’t immediately grow directly on the brains, and de Bekker thinks the same is happening in living ants infected in the wild. “The fungal cells appear to grow beside the brain and manipulate it by secreting compounds and not by growing into or on the tissue itself,” she said. It only directly attacks the brain tissue later on, “when the ant dies and the fungus starts consuming everything to create the biomass needed to grow the fruiting body.”
But back to why she was making ant-brain-fungus soup in the first place: de Bekker found that the fungus releases different chemicals depending on the species of ant. The hosts that it is capable of mind-controlling are inundated with entirely different compounds than those it isn’t capable of controlling. It's as if the fungus knows whether it's next to its target species or not, and then reacts accordingly, she says.
And that’s pretty damn impressive for an organism without a brain. Which raises the question: Could it be that Ophiocordyceps fungi regularly invade species they aren’t capable of mind-controlling? Probably not, said de Bekker, because they didn’t evolve one of the animal kingdom’s most insanely complex acts of zombification just to be jerks to ants. They need it to survive.
Here’s Ophiocordyceps’ life cycle in full: A fungus spore lands on the cuticle of an ant, fusing to its body and building up an incredible amount pressure (equal to that in the tire of a 747 jet) to blow itself through the exoskeleton. Infiltrating the ant’s brain, it directs the host out of the colony, where workers would surely notice their comrade’s weird behavior and drag it into a graveyard well away from home base. Directing the ant up onto the underside of a leaf at a specific height and always facing a specific direction, the fungus orders the ant to bite down on the vein. At this point it kills the ant and erupts as a stalk out of the back of its head, raining spores onto its comrades below.
The fungus goes through all of this trouble to best position itself to infect more ants and further its own species. Simply killing its host, like it did to the two other ant species in de Bekker’s lab, doesn’t get it anywhere, because ants are obsessive about dragging their dead and dying into their aforementioned graveyards, where the spores have little chance of spreading.
Interestingly, this North American species of Ophiocordyceps controls its victims a bit differently than its South American counterparts. “What is different about the temperate system is that these ants don't bite the leaves, they bite twigs,” said de Bekker. “Which is actually very interesting, because of course in a temperate system the trees lose their leaves over the winter, and sometimes dead ants have to overwinter into the next season for the fungus to completely grow out and make the spores and finish its life cycle.”
There is still much to be learned here, though. While in this experiment de Bekker isolated two different compounds---GBA and sphingosine in case you were interested---that seem to play a part in zombification (and perhaps tellingly also play a part in human neurological disorders), these are still just candidates. “What we do see is that there's a whole array of compounds that seem to be working together,” she said. “And this is probably also why it is so difficult to figure out what are the key compounds because it is likely that we need a mixture of different chemicals working in concert to get something as complex as this.”
So don’t get excited for a magical mind-control serum to come out of that lab anytime soon. I’m looking at you, CIA.
Reference:
De Bekker, C, et al. (2014) Species-specific ant brain manipulation by a specialized fungal parasite. BMC Evolutionary Biology. 14:166 doi:10.1186/s12862-014-0166-3
