In a macabre discovery, scientists have found that a parasite, which creates armies of zombie ants, does so by hijacking their bodies—not their brains as was previously thought.
The fungal parasite Ophiocordyceps unilateralis—also known as zombie ant fungus—was first discovered by Alfred Russel Wallace in the 19th century. The parasite invades the bodies of carpenter ant workers in order to manipulate their behavior. It forces them to abandon their normal activities, climb onto vegetation then bite down onto the undersides of leaves or twigs, where they die.
The parasite then grows through the ant body and produces a stalk from the ant’s head. From this position it can discharge infectious spores to the ground below to infect even more ants.
How the parasite is able to do this was not known—although previous research suggested it takes control of the ant’s brain in some way.
This, however, is not the case.
In a study published in the journal Proceedings of the National Academy of Scientists, researchers led by Maridel Fredericksen, of Pennsylvania State University, have shown that the parasite hijacks the body rather than the brain, invading the muscle fibers to create a 3D network that allows it to control the ant’s movements.
"To better understand how such microbial parasites control animal behavior, we looked at cell-level interactions between the parasite and its carpenter-ant host at a crucial moment in the parasite's lifecycle—when the manipulated host fixes itself permanently to vegetation by its mandibles," Fredericksen said in a statement.
Scientists infected ants with either the zombie ant parasite or a general fungal pathogen in order to look at the differences that take place after it has entered the body. They then used microscope technology to create 3D images showing the distribution, abundance and interactions of the parasite in the host bodies.
Over 24 hours, they were able to get an “unprecedented view” of how the parasite moves through the body and takes control. They found that the parasite invaded almost every part of the ant’s body, including the legs, abdomen and head. However, it left the brain intact.
"We found that a high percentage of the cells in a host were fungal cells," said senior author David Hughes, also of Penn State. "In essence, these manipulated animals were a fungus in ants' clothing.
"Normally in animals, behavior is controlled by the brain sending signals to the muscles, but our results suggest that the parasite is controlling host behavior peripherally. Almost like a puppeteer pulls the strings to make a marionette move, the fungus controls the ant's muscles to manipulate the host's legs and mandibles."
Why the parasite keeps the brain intact is unclear, but the researchers suggest it may preserve it so it can survive until its final suicidal act of biting down on the vegetation. “We need to conduct additional research to determine the brain's role and how much control the fungus exercises over it,” Hughes added.