Zombie Ants: again in the news
One of the strangest articles I’ve come across recently was this story about the odd symbiotic relationship between an ant species, and a fungus that essentially turns the ants into zombies. To summarize for those students unwilling to click the link and read that summary, the fungus Ophiocordyceps would colonize a particular species of ant while it was still alive, and modify the behavior of the ant. This enabled the fungus to use the ants to spread out over a large geographical region, a novel way for a sedentary species like a fungus to spread its genes.
Researchers at Penn State University have followed up their initial report, that again is being widely reported in the media (I heard about it first via National Public Radio.) It seems that the interaction between the ants and their parasitic fungal species is far more complicated than proposed earlier, and this new study models the spread of the fungus within the population of ants in a colony, as opposed to between individuals.
The ant species examined in this study is a relatively long-lived species, indicating a low mortality rate among individuals. The high density of individuals in an insect colony means that the risk due to infection is high, but that significant defenses against infection need to be present. The researchers found that the percentage of Ophiocordyceps infected ant corpses was significantly lower than the total number of ant corpses in graveyard sites near the ant colonies. However, many corpses were hyperparasitized by other fungal species, a situation which appeared to prevent the colonization by the “zombie” fungus Ophiocordyceps.
What the model suggests is that what the “parasite” fungus does is to prevent the spores of the “zombie” fungus from maturing, and prevents the latter species from taking over the ant colony. Previous models of the dynamics of ant hosts and their pathogens had suggested that the two players shared a complicated set of interactions; ants developed behaviors to minimize the risks of infections, which the fungus further modified the host behavior to their advantage. The current report underscores that a third party also must be included in the web of interactions, and that the other fungal species that can further parasitize the ants are significant players.