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New research has revealed how infection by a parasitic fungus dramatically changes the
behavior of tropical carpenter ants, causing them to become zombie-like and to die at a
spot that has optimal reproductive conditions for the fungus.
A multinational research team including David P. Hughes, the first author of the paper and
an assistant professor of entomology and biology at Penn State University, studied ants living
high up in the rainforest canopy in Thailand. The behavior of these infected zombie ants
essentially causes their bodies to become an extension of the fungus's own behavioral
traits, as non-infected ants never behave in this way.
"The fungi, which control ant behavior, are an interesting example of adaptation by natural
selection. The life cycle of the fungus begins with a fungal spore on the forest floor. When
the ants are out foraging for food, they pass through these little 'killing fields,' as
we describe the fungal spores, and these spores attach to the ant's cuticle or its skin. And
using a combination of enzymes and mechanical pressure, the spores force their way through
the ant's cuticle, and into the body."
Using transmission-electron and light microscopes, the researchers were able to look inside the
ant in order to determine the effects of the fungus. They found that the growing fungus
filled the ant's body and head, causing muscles to atrophy and forcing muscle fibers to spread
apart. The fungus also affected the ant's central nervous system. The scientists observed
that, while normal worker ants rarely left the trail, zombie ants walked in a random
manner, unable to find their way home. The ants also suffered convulsions, which caused
them to fall to the ground.
The researchers also found that, at solar noon, when the Sun was at its strongest, the
fungus synchronized ant behavior, forcing infected ants to bite the main vein on the
underside of a leaf. The multiplying fungal cells in the ant's head caused fibers within
the muscles that open and close the ant's mandibles to become detached, causing "lock
jaw," and making an infected ant unable to release the leaf, even after death. A few
days later, the fungus grew through the ant's head producing a fruiting body called a stroma,
which released spores to be picked up by another wandering ant.
"We're looking at this phenomenon from a range of different perspectives. We're really interested
in the biodiversity of these fungi. How many do we find and whether each ant species that
we can encounter in the wild has its own specific fungal species which is changing its behavior."
Hughes's continuing research at Penn State is designed to learn how the fungus might
be used to control pest insects in homes and farms.
For ScienceCast, I'm Katrina Voss.