The parasitic fungi responsible for this deadly behavior are members of the genus Cordyceps, which contains more than 400 different entomopathogenic fungi. Many of them have become specialized and infect only one particular insect, but the list includes grasshoppers, weevils, and many others. In the example of the ant, many of them can be infected by a Cordyceps. The carpenter ant or the bullet ant can be the unfortunate recipient of spores of Cordyceps unilateralis. The spores attach to the exoskeleton of an ant and germinate, a necessary step before entering the body of the ant. The exoskeleton of the ant contains holes called spiracles that allow the ant to breathe; these spiracles are the port of entry of Cordyceps, which arrives in the tracheae of the ant. The fungus starts growing inside the ant’s body cavity and forms mycelia while feeding on the soft tissues of the host; however, it does not attack the vital organs of the ant, thus it keeps it alive.
When the time to sporulate has come, Cordyceps starts invading the brain of the ant, which results in a severe alteration of its behavior. The ant leaves its colony, and oddly enough, climbs up a stem. Eventually, it attaches itself to the stem by using its mandibules; there, it slowly dies as the fungus feeds on its brain. Soon, from the gaps made by the joints of the exoskeleton, the fungal fruiting bodies containing the spores start to make their way out of the ant’s head, and grow for up to three weeks. As the final step, the fruiting bodies release spore capsules which, on their way down to the ground, explode to unleash all the spores to be disseminated. One can then appreciate the strategy of the fungus; by manipulating the ant’s mind and forcing it to go to a higher altitude, the fungus ensures dissemination over a large area, where it can infect again a multitude of ants in the ground. One fungus is able to infect and decimate an entire colony through this strategy!
I tried to find in the literature an article that explained how the fungus was able to manipulate its host; as the behavioral change seems extremely well regulated. Unfortunately, nobody seems to know much at this point. Also, in an ant colony, the workers have developed a way to “detect” infected individuals and then remove them quickly from the colony. How have they learned to identify the victims? Is it something they see or something they smell?
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