From Life to Undeath: Zombie Ants

High in a tree, a zombie scurries upwards. Its mind is not its own; it climbs towards its inevitable death. However, this zombie is not the typical lumbering, gray human portrayed in movies—it’s a mind-controlled ant. 

These ants are inhabited by Ophiocordyceps unilateralis, a fungus commonly known as cordyceps that is predominantly found in Thailand, Brazil, Central America, and Africa. The tropical conditions of these regions are ideal because moist, warm conditions improve fungi growth. This fungus can infect organisms from 10 different insect orders but displays a unique control over ants. Cordyceps’ ability to directly manipulate an ant’s behavior and movement has led scientists to probe deeper into this uncommon parasite-host relationship.

Like all fungi, cordyceps reproduces by expelling spores (cells containing the parent fungus’s genetic data) into the air. Released spores disperse over large areas, carried by the wind or animals, which increases their chances of finding suitable conditions without competition from other fungi. After an ant comes into contact with cordyceps spores, it produces branching filaments called infective hyphae that penetrate the ant’s rigid, protective exterior. This establishes an infection, allowing the fungus to infiltrate the ant and begin the next phase of manipulation. 

After entering the ant, the cordyceps feeds on it from within, siphoning the ant’s nutrients until it makes up almost half of its body mass. After a few days, it begins to interfere with the ant’s behavior. For example, the cordyceps activates genes that disrupt the release of neurotransmitters like serotonin, norepinephrine, and dopamine. These chemicals carry messages from nerve cells in the brain to body cells. By disrupting them, the fungus can cause hallucinations, loss of coordination, and muscle spasms. In addition, the fungus activates a set of proteins known as lipocalins, which affect immune system regulation. Pathogens usually secrete chemicals that consume iron, a nutrient vital for DNA synthesis and oxygen transportation in both the host and pathogen. Lipocalins usually destroy this chemical, but a shortage can lead to major iron loss in the host, leaving it depleted of nutrients while the pathogen strengthens. 

The cordyceps’s ability to interrupt regular bodily functions allows the fungal cells to grow easily without being attacked by the ant’s immune system. For instance, neutrophils, a type of white blood cell, usually play a key role in destroying invading fungus spores. Neutrophils produce and overload the invading spores with reactive oxygen, damaging the spores’ proteins. Impaired proteins lead to reduced cell functionality, preventing the spores from effectively attacking the host. Since iron depletion leads to reduced oxygen transportation, the neutrophil’s protective oxygen response is halted. This allows the spores to easily overpower the impaired immune system. Additionally, scientists noticed that two genes known to regulate circadian rhythm and daily behavioral patterns are hyperactive in infected ants. These disturbances contribute to the ant’s erratic behavior and desire to leave its nest colony permanently. Then, the ant loses control over the majority of its behavioral and physical systems, becoming a zombie ant.

At noon, the ant exits its colony and searches for an elevated point in the forest. Ants in a colony depend on each other for survival; ants hunt, fight, and even scout possible nest sites as a group. Each member has a specialized job, from caretaker to food-gatherer to nest-defender; together, they form a well-coordinated system. Thus, it’s highly unusual for an ant to leave its colony unaccompanied, illustrating another way the cordyceps alters the ant’s behavior.

The ant waits until it detects a location with high moisture and temperature. Then, it crawls onto the underside of a nearby leaf to limit sun exposure because darkness results in optimal fungal growth. The cordyceps activates genes in the zombie-ant that deteriorate its jaw muscles, forcing it to bite onto the leaf. This action, known as a “death grip,” stops the corpse from falling downwards, ensuring a larger spore dispersal range. Shortly after, the cordyceps consumes and kills the ant. It uses the ant’s corpse as a protective shell to grow in but erupts from it after a few days. The fungus’s main stalk, which holds new reproductive cells, bursts out of the carcass’s head and drops the spores in an approximately 10-square-foot area. The spores then fall downwards, where new ants eventually come into contact with them—the parasitic cycle resets.  

This cycle is highly effective due to its ability to manipulate the brain indirectly. Researchers from Pennsylvania State University used 3D visualization models to determine that the fungus forms a tubular pattern inside and around the ant’s muscles, leaving the brain intact. The cordyceps controls the host’s behaviors by releasing bioactive compounds into its muscle fibers to disrupt the nervous system. The chemicals carry messages to motor neurons, nerve cells located at the end of each muscle fiber. Motor neurons send the signal to other neurons in the spinal cord, which eventually connects to the brain. By injecting compounds into the muscle fibers, cordyceps can alter the host’s behavior without unintentionally damaging other parts of the brain. 

Scientists have also formed hypotheses on why the fungi’s manipulative behavior exists in the first place. Some believe that it originated when the fungus transitioned from attacking solitary insects (like beetles) to sociable ants. Ants developed a behavior known as social immunity: those who display symptoms of illness are kicked out of the colony to prevent the spread of infection. Cordyceps’s evolution decreased the likelihood of ants detecting its invasions, allowing the fungus to spread more easily to other members of the colony.

The cordyceps’s medley of successful but terrifying survival adaptations can raise concerns about human infections. However, the average human body temperature of 37 degrees Celsius is significantly higher than the fungus’s optimal temperature of 20 to 30 degrees. Beyond its range of tolerance, the fungus’s proteins and complex molecules would denature, and it would die before it could manipulate human behavior. However, scientists are concerned that the threat of fungal infections could rise as temperatures increase with global warming. Climate change could slowly expose fungi to warmer conditions and fuel their adaptation to heat. This doesn’t mean that cordyceps will learn to infect humans in the near future, but it could lead to an increase in general fungal infections. The cordyceps fungus serves as both an eerie absurdity and a glimpse into the looming hazards of a changing world.