'Zombie ants' lose their bodies, not their brains, to fungus

May 29, 2018, 1:23 p.m.
A zombie ant with the brain-manipulating fungus Ophiocordyceps unilateralis
Photo: Penn State/flickr

It turns out that some zombie infections don't go for the brain after all.

Ophiocordyceps unilateralis, a fungus found mostly in tropical forests, targets carpenter ants in order to multiply and spread. Commonly called the zombie ant fungus, O. unilateralis takes control of the ants and compels them to climb high into trees consuming braaaaiiiinnnsss the underside of leaves and twigs until the ant dies. After the ant's death, the fungus grows a stalk out of the ant's head (which you can see above) to develop a fruiting body that releases spores of the fungus back into the ecosystem and infect other unsuspecting ants.

Whether or not the ant latches on to a leaf or twig depends on local conditions — and scientists say that's a clue the fungus is getting "smarter." A 2018 study shows that the fungi have evolved in response to climate change. "In tropical areas, zombie ants bite onto leaves, but in temperate areas, they bite twigs or bark," Penn State associate professor of entomology and biology David P. Hughes told Phys.org.

These ants can also be found throughout the world except in Europe in areas that have distinct seasons. "In the late summer and early fall there are both leaves and twigs everywhere the ants reside," said Penn State postdoctoral scholar Raquel G. Loreto. "But in temperate areas the trees are deciduous and lose their leaves in the fall. There, the ants bite onto twigs."

How does the fungi attack the ant?

It's all pretty grisly and horrifying, and the fungus does all of this without even touching the ant's brain, according to a study published in the Proceedings of the National Academy of Sciences. Instead, the fungus takes control of the ant by invading the insect's muscle tissue. The fungus actually goes so far as to invade almost every nook and cranny of the ant's body, including the head, thorax, abdomen and legs. In doing so, O. unilateralis creates a complex web that collectively controls the ant's movements. While the fungus' cells are often found just outside the brain, it never actually enters the brain.

"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," the study's senior author and Penn State professor David Hughes said in a statement. "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."

Hughes and his team used serial block-face scanning-electron microscopy to create 3-D models of the inside of the ant. They took slices of tissue 50 nanometers in depth and captured an image of each slice using a machine that could repeat that process 2,000 times over a 24-hour period. The scientists then stacked the slices in that 3-D model to see just where the fungus invaded the ant.

While O. unilateralis doesn't take control of the brain, it could potentially alter it on a chemical level.

"We hypothesize that the fungus may be preserving the brain so the host can survive until it performs its final biting behavior — that critical moment for fungal reproduction," Hughes said. "But we need to conduct additional research to determine the brain's role and how much control the fungus exercises over it."

Editor's note: This article has been updated since it was originally published in November 2017.

Related on MNN: 11 amazing antics of ants