Question What’s Up with Zombie Ants?
Answer
Zombie ants are ants that have been infected by a fungus.
Ophiocordyceps unilateralis is a parasitic fungus that infects carpenter ants. Parasitic fungi are not uncommon in the insect world, but what makes O. unilateralis uniquely creepy, and why it’s called the “zombie fungus,” is the strange way O. unilateralis affects the behavior of an ant for its own propagation.
When a hapless ant makes contact with an O. unilateralis spore, the spore attaches itself to the ant and begins to drill into the ant’s exoskeleton with a combination of enzymes and pressure. When past the ant’s exterior armor, fungal cells feed on the ant’s insides and begin to spread throughout the unfortunate victim’s body. Once the infection progresses past a critical stage, the ant’s behavior changes. Instead of following the colony’s pheromone trails and foraging, as an ant normally would, the infected ant walks erratically. It wanders aimlessly for a time, before the fungus compels the ant to climb up a nearby twig or stem and lock on to it with its jaws. Death follows shortly after. In a few days, a stalk shoots out of the ant’s head, releasing spores to start the whole process over again.
One thing that makes this process so amazing is how precisely O. unilateralis, with no brain or nervous system of its own, can control its host.
What makes the ant change its behavior?
The exact biological mechanism that leads to the change in ant behavior is not well understood at this point. Researchers have found that the fungus seems to act on the ant’s muscles directly rather than through the brain. When researchers set out to investigate the spread of O. unilateralis throughout an infected ant, they found fungus in every part of the ant’s body except for the brain. The fungus grew around the brain but seemed to be careful to not obliterate the ant’s central control system. Researchers have found that the fungus releases a variety of chemical compounds, but exactly how these work to manipulate behavior remains a topic for further investigation.
It’s a fungus-eat-fungus world
And if all that isn’t weird enough, this parasitic fungus also has its own parasitic fungi, which infest it. Hyperparasites are parasites whose hosts are themselves parasites. Niveomyces coronatus and Torrubiellomyces zombiae are just two of the more recently discovered of several species of hyperparasites that have been found on O. unilateralis.
For the ants, is the enemy of my enemy my friend? That might be a matter of perspective. Researchers theorize that these hyperparasites keep O. unilateralis from being too lethal to the ants it infects. As an obligate parasite (meaning a parasite that needs a very specific host in order to survive), if O. unilateralis burns through its hosts too quickly, it will run out of hosts and die out. Therefore, it is possible that these hyperparasites help to keep O. unilateralis in check, allowing it to exist as a constant thorn in the side of ant-kind.
Ophiocordyceps or “Cordyceps?”
O. unilateralis has seen an increase in public interest as the inspiration for the zombifying infection in the series “Last of Us,” based on the video game of the same name. In this show, the fungus is referred to just as “Cordyceps,” which is a bit of a misnomer. Assistant Curator of Mycology at the New York Botanical Gardens João Araújo explained in the NYBG’s “Plant Talk” blog that up until 2007, the genus Cordyceps would have included Ophiocordyceps and thus O. unilateralis, but more recent analysis has differentiated Cordyceps and Ophiocordyceps into separate families, Cordycipitaceae and Ophiocordycipitaceae. It is Ophiocordycipitaceae that contains the Ophiocordyceps genus and Ophiocordyceps unilateralis. This is notable because both Cordyceps and Ophiocordyceps species are parasitic, but only O. unilateralis can infect and control its hosts.
Once thought of as a single species, researchers have learned that O. unilateralis actually includes many different species, so closely related that it can be hard to tell one from another. They continue to discover more and more individual species of O. unilateralis every year. Therefore, O. unilateralis is often described as “sensu lato,” meaning “in the broad sense.” As of 2023, there are 35 known species of Ophiocordyceps that are able to control the behavior of ants, but researchers expect there are actually many (perhaps hundreds) more.
Can it infect humans?
No. There are many factors that make transmission to humans highly unlikely. Mammalian biology is very different from that of insects, and the human body is too warm for most fungus. Which is not to say that humans don’t get fungal infections; we do. But the number of fungi that can affect humans is very small compared to the number of fungi in existence. According to the CDC, there are millions of fungal species, but only a few hundred infect humans, most commonly causing athlete’s foot and yeast infections.
O. unilateralis has evolved to parasitize ants for 45 million years, and in all that time, not only has the fungus never made a jump to infect mammals, but most species don’t seem to be able to jump even to other species of ants. For example, the O. unilateralis species Ophiocordyceps camponoti-floridani only affects the behavior of the Florida carpenter ant (Camponotus floridanus) and Ophiocordyceps kimflemingiae only affects red hazelnut carpenter ants (Camponotus castaneus). Instead, O. unilateralis and Ophiocordyceps species have grown even more specialized over time, rather than broadening their range of hosts to spread more widely.
Published: 5/23/2024. Author: Science Section, Library of Congress