HOW DO WE KNOW?

FIG. 34.20

Can a fungus influence the behavior of an ant?

BACKGROUND A curious death ritual unfolds in the rain forest of Thailand. Spores of the ascomycete fungus Ophiocordyceps infect Camponotus leonardi ants, growing hyphae inside their bodies. The fungus eventually kills the ant, and fruiting bodies emerge from the dead ant’s head to disperse spores that begin the life cycle anew. Camponotus leonardi nests and forages for food high in the forest canopy. Infected ants, however, undergo convulsions that cause them to fall to the forest floor. The infected ants wander erratically and are unable to climb more than a few meters above the ground before convulsions make them fall again. In their final act, the ants bite into the undersides of leaves and die. The fungus is then able to complete its life cycle within the humid forest understory. Is the ants’ behavior, including both the convulsions and leaf biting, induced by the fungi?

FIG. 34.20

An infected ant attached to a leaf by having bitten into it in a “death grip.”

HYPOTHESIS Parasitic fungi manipulate ant behavior to complete their own reproductive cycle.

OBSERVATIONS Infected and uninfected ants were observed for many hours and behavioral events were recorded.

RESULTS Infected ants have repeated convulsions (indicated by vertical bars in the graph), but uninfected ants show no such behavior (not shown). Transitions from erratic wandering to a “death grip,” in which the ant bites into a leaf (indicated by red triangles in the graph and shown in the photograph), occurred at about the same time of day. Dissections showed that the ants’ death grip results from jaw-muscle wasting caused by the fungus.

FIG. 34.20

Observations of numerous infected ants (vertical bars indicate convulsions; red triangles indicate the time of ant biting).

CONCLUSION The fungi change the behavior of infected ants, inducing a stereotypical behavior that facilitates completion of the fungus’s life cycle. The molecular basis of this manipulation is not yet known.

FOLLOW-UP WORK The zombie-like behavior induced by the fungus is an example of what has been called the extended phenotype: the idea that a phenotype should include the effects that an organism has on its environment—in this case, its effects on host behavior.

SOURCE Hughes, D. P., et al. 2011. “Behavioral Mechanisms and Morphological Symptoms of Zombie Ants Dying from Fungal Infections.” BMC Ecology 11:13–22.