The natural world is full of intricate relationships, but few are as compelling as those between a parasite and its host. In some cases, a parasite influences the behavior of the animal it infects, creating a scenario reminiscent of a fictional zombie where an organism’s actions are no longer its own. The case of an infected ant is a well-documented example of this powerful influence.
The Zombie-Ant Fungus
The parasite behind this behavior is a fungus called Ophiocordyceps unilateralis. This is not a single entity but a complex of fungal species, each adapted to a specific ant species. These fungi are most commonly found in the warm, humid conditions of tropical rainforests, which provide the perfect environment for their growth and transmission.
Ophiocordyceps is distinct from more familiar fungi, like mushrooms, in its life cycle and interaction with its host. It has evolved a sophisticated method of reproduction by co-opting an ant’s body and behavior. The specialization of each fungal species to a particular ant highlights a long co-evolutionary history.
The Infection Process
The infection begins when a microscopic fungal spore lands on an ant’s exoskeleton. The spore germinates and penetrates the shell, growing into the ant’s body. Once inside, the fungus proliferates, spreading a network of cells throughout the ant’s tissues. These fungal cells communicate and coordinate, forming a singular entity inside the host.
This internal growth leads to a takeover of the ant’s muscles. The fungus surrounds the muscle fibers, allowing it to direct the ant’s movements. The fungus does not destroy the ant’s brain; instead, it controls the body directly, leaving the brain intact but powerless. This manipulation compels the ant to abandon its colony and normal foraging routes.
Driven by the fungus, the ant exhibits unusual behaviors. It will climb a plant to a specific height, around 25 centimeters, where the temperature and humidity are ideal for the fungus. The ant is then forced to perform a final action: biting down hard onto a leaf vein or twig. This “death grip” is so powerful that the ant remains attached even after it dies.
After the ant’s death, the fungus enters its reproductive stage. A stalk grows from the back of the deceased ant’s head, and this structure eventually releases new spores. These spores rain down onto the forest floor below, where they can infect other foraging ants, starting the cycle anew. The entire process, from infection to death, can take four to ten days.
Other Ant Infections
The Ophiocordyceps fungus is not the only parasite that can manipulate ants. Another example is the lancet fluke, a tiny flatworm named Dicrocoelium dendriticum. This parasite has a complex life cycle involving snails, ants, and grazing animals like cattle or deer. An ant becomes infected by eating the slime ball of an infected snail.
Once inside the ant, one fluke migrates to its brain and influences its behavior. At dusk, when temperatures cool, the infected ant climbs to the top of a blade of grass and locks its jaws. This position makes it more likely to be eaten by a grazing animal, allowing the fluke to reach its final host to mature and reproduce. The fluke’s eggs are then passed in the grazer’s dung, which snails may eat, continuing the cycle.
The Colony’s Response and Ecological Role
Ant colonies are not defenseless against these parasitic threats. They have developed collective behaviors, known as social immunity, to protect the group. Healthy worker ants can detect when a member of the colony is infected and will carry the individual far away from the nest. This prevents the parasite from completing its life cycle close to the colony and spreading to others.
This parasitic relationship, while detrimental to the individual ant, serves a purpose in the broader ecosystem. By targeting specific ant species, fungi like Ophiocordyceps help control their populations. This prevents any single ant species from becoming too dominant on the forest floor, which helps maintain biodiversity.