For those who don’t know, The Last of Us is a dark, yet beautifully crafted survival horror game that explores what the world would be like if the majority of the human population were infected by a mutated strain of Ophiocordyceps unilateralis. This video game is critically acclaimed for its storytelling, graphics, and gameplay. It is considered a masterpiece by most of its reviewers and by its players. From the little bit I have played, I would absolutely agree. There are few developers that are able to take a real world scientific event, such as an insect being manipulated by a parasitic fungus, and implement the core concept realistically in a video game. Naughty Dog, the developer of The Last of Us, does this fantastically well.
Ophiocordyceps unilateralis is in fact a real fungus. This species specifically targets ants, generally of the Camponotus genus, and takes over their brains in order to move them to a location with ideal conditions for its reproduction. One of the most interesting things about this fungus is how precisely it manipulates its host. A 2009 study in The American Naturalist found that ants infected with O. unilateralis almost always died gripping a vein on the underside of a leaf (98%) and that this location was significantly more towards the north-northwest side of the plant than any other direction. Camponotus ants were found almost exclusively at a height of 25 centimeters from the forest floor, while closely related Polyrhachis ants were seen at a mean height of 78.43 centimeters. This kind of behavioral control is known as an extended phenotype. The gene expression of a parasite within a host’s tissue causes aberrant host behavior that increases the parasite’s fitness. Keep in mind that the ant species in the study colonize the canopy, not the forest floor, so this is extremely complex behavioral manipulation. Under the fungus’s control, these infected ants are traveling very far from their initial location. Many infected ants will often die in areas where others already have because the conditions in these places are favorable for O. unilateralis. This leads to places, called graveyards, that have a high density of corpses. Any healthy ant happening to walk through such an area will likely become infected by the deadly spores.
When O. unilateralis spores come into contact with an ant, they adhere to it and start to penetrate its cuticle. After this, fungal cells start to infest the ant’s entire body and begin to alter its behavior. The infected ant then leaves the colony and starts the trek down toward the forest floor. Once a perfect spot is reached, the ant will bite the vein of a leaf and die with its jaws locked in place. This is known as the “death grip”, and keeps the ant from falling off the leaf. Any ants that end up dying within the colony due to infection will be carried far away by other workers. Within a few days, the stroma, or stalk, of the fungus will erupt from the back of its host’s head and develop a stromal plate that releases spores when mature. By this point, most of the ant’s original tissue has been replaced by fungus.
The ants in the video below are not Camponotus or Polyrhachis ants, and the fungus is not O. unilateralis so the hosts show a different behavior. However, it still illustrates the process well.
What a horror it would be if humans were susceptible to such a fate! The question is, could this actually happen? Let’s consider the ant brain for a moment. It has all the general features of a typical insect brain and is very simple compared to a vertebrate brain. However, ants have more well developed areas associated with memory and learning. These “higher” functions are controlled by the – get this – mushroom bodies (it’s funny because we’re talking about fungus taking over an ant brain here and they just happen to have structures called mushroom bodies. Mushroom bodies are much larger in worker ants than they are in the male soldiers, which are more hard wired or pre-programmed. Workers must navigate varying spacio-temporal environments using multiple cues a