By Jessica Crooker
Of the many ghosts, ghouls, and goblins that have long haunted our collective consciousness, few have captured the imagination like brain-eating zombies. Mindless dead corpses animated to carry out strange behaviors beyond their own control, zombies can be found puncturing their way into almost every genre, making appearances as varied as Sean of the Dead and Game of Thrones. The zombie apocalypse has yet to hit humanity, but for some insect species, zombie parasites are a terrifying reality.
Picture this: an ant is walking erratically. It stumbles around, occasionally falling and convulsing on the ground. Based on normal behavioral pattern, this ant shouldn’t even be out of the nest. When morning comes, the ant slowly climbs up a nearby stalk. Once high in the foliage, it turns and sinks its teeth deep into the stem. The ant’s legs lose their grip and the ant hangs there by its mouth, its legs twitching violently, as it slowly dies.
This ‘manipulated biting’ is just one of the behaviors exhibited in Carpenter ants infected with Ophiocordyceps unilateralis s.l., a fungal parasite whose symptoms are often compared to a zombie virus. Infected Carpenter ants exhibit entirely new behaviors that the ant has no control over, including: leaving the nest at atypical times of day; stumbling and walking without direction; convulsing; climbing high into the foliage; and finally, ‘manipulated biting,’ where the ants bite into a stem and hang from their mouths for hours before dying. While hanging, the fungus grows and uses the ant’s resources, protruding an antenna through the ant’s head to take advantage of the hanging position to better spread its spores (Mangold, Ishler, Loreto, Hazen, & Hughes, 2019).
While there are other parasites known to impact host behavior, few parasites have effects as dramatic as manipulated biting. The behavior occurs at coordinated times of day, happening in all infected ants during the first few hours of the morning, and leads to a brutal death. To understand how the fungus manipulates ant behavior to such a dramatic extent, de Bekker et al. (2015) investigated the gene expression in Carpenter ants engaged in manipulated biting. The ants were infected with the fungus and then observed throughout the course of the disease until the biting stage was reached. At that stage both alive and dead ants were collected, and the heads were assessed to see what RNA molecules were present. This allowed for the researchers to learn what genes were being expressed and what proteins were present that may have contributed to the biting behavior.
The brain of the ant isn’t colonized by the parasite; instead, de Bekker et al. (2015) found the biggest difference between infected and uninfected ants was gene expression; both within the ant’s own genome and with expression of fungal genes in the infected ant. The fungal genes expressed, coded for proteins that have powerful physiological effects in ants, including impacting muscle control. The fungus also changed the ants’ own gene expression, down-regulating genes related to the immune and stress responses, as well as upregulating genes important for parasite-host relations, inducing cell death, and impairing the ant’s ability to communicate with other ants and the rest of its environment.
Infected Carpenter ants may not be true zombies, but they’re frightening in much the same way, along with some unique, terrifying aspects. The ants lose all control of their actions, still moving while the parasite degrades their body. They struggle for life hanging from their mouths, unable to save themselves. These ants are infected with something terrifying, but there is an upside. Better understanding how this fungus works has the potential to lead to advances in medicine and drug design due to its brain-manipulating power (de Bekker et al., 2015). While frightening, there’s no need for humans to fear the zombie-ants…yet.
de Bekker, C., Ohm, R. A., Loreto, R. G., Sebastian, A., Albert, I., Merrow, M., … Hughes, D. P. (2015). Gene expression during zombie ant biting behavior reflects the complexity underlying fungal parasitic behavioral manipulation. BMC genomics, 16(1), 620. doi:10.1186/s12864-015-1812-x