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Saturday, January 14, 2012

A parasitized ant. Image credit: David P. Hughes.

A parasitized ant. Image credit: David P. Hughes.

An ingeniously deadly fungus hijacks the bodies of ants for food and reproduction.

It’s like something out of a horror movie. A parasitic fungus infiltrates the body of a tropical carpenter ant, feeding on it and manipulating its body. The fungus forces the dying ant to the forest understory, an environment more conducive to its growth. The invasion of this fungal body-snatcher culminates with it sprouting a spore-laden fruiting body from the dead ant’s head.

An account of this deadly assault on tropical carpenter ants (Camponotus leonardi) by a parasitic fungus (Ophiocordyceps unilateralis) is described in the May 9, 2011 issue of the open access journal BMC Ecology.

Similar incidents of fungi parasitizing insects occur in other parts of the world. This particular case of zombie ants plays out in the forests of Thailand.

It’s a pretty gruesome affair, so if you find movies like The Thing and Night of the Living Dead too scary, now’s a good time to stop reading!

The paper’s lead author, Dr. David Hughes of Penn State University, described the ant-fungus interaction in a press release.

The fungus attacks the ants on two fronts. Firstly by using the ant as a walking food source, and secondly by damaging muscle and the ant’s central nervous system, resulting in zombie walking and the death bite, which place the ant in the cool damp understory. Together these provide the perfect environment for fungal growth and reproduction. This behavior of infected ants is essentially an extended phenotype of the fungus (fungal behaviour through the ant’s body) as non-infected ants never behave in this way.

A dead carpenter ant attached to a leaf in the understory of a forest in Thailand. Before killing the ant, the fungus growing in the ant changed its behavior, causing it to bite into the leaf vein. Image Credit: David Hughes, Penn State University.

Tropical carpenter ants spend most of their time high in the forest canopy. When they venture down to the jungle understory, they follow well-defined trails. It’s during this time that ants could get infected by fungal spores that land on their outer body.

The fungus can only complete its life cycle through the ant. Spores germinate, and the fungus penetrates the ant’s body. It proceeds to infect the entire animal, affecting its central nervous system. You can tell when a carpenter ant has been infected: instead of marching purposefully down a trail, an infected worker ant walks about haphazardly, displaying erratic behavior. Sporadic convulsions set in, causing the infected ant to fall from the canopy to the moist, cool, leafy forest understory, ideal conditions for the fungus to continue its growth.

A carpenter ant attached to a leaf. The ant has been dead for two to three days, and the fungus's fruiting body filled with spores sticks out of its head. Image Credit: David Hughes, Penn State University.

Infected ants on the forest understory are driven by the fungus to select leaves of saplings that are about 25 centimeters (10 inches) above the soil surface. Then, a curious thing happens when the sun shines at its highest intensity of the day, at solar noon when it reaches the highest point in the sky. The fungus commands the ant to sink its mandibles into the leaf’s main vein, on the underside of the leaf. A possible reason for this action is to attach the ant to a stable environment suitable for the fungus’s subsequent development. But this synchronization with solar noon is a mystery, and it will be the subject of follow-up research.

Scientists call this stage, when the ant bites deep into the leaf vein, the “death grip,” because the ant is now locked to the leaf, providing a secure attachment for the fungus growing inside it. At this point, the ant is close to death, usually surviving for another 6 hours following its death grip. Its head is filled with fungal cells growing between muscle fibers, as well as around the brain and postpharyngeal gland[1]. Following the death grip, the ant’s mandible muscles atrophy, leaving its jaws locked into the leaf long after it’s dead.

This image from the scientific paper shows a micrograph of an infected ant's head. It's the state of the ant's head when it bit down on the leaf, while it was still alive. Small grey blobs filling the head and mandible are the fungus. "PPG" is the postpharyngeal gland, "B" is the brain, "Mu" are muscles, and "Cu" is the cuticle (outer body of the ant). The small image in the lower left shows a close-up of what muscle would look like in a healthy ant. The small image in the lower right shows a close-up of the infected ant's muscle, right after it bit down on the leaf. The blobs between the muscle fibers are fungal cells.

About two to three days after the death grip, a fruiting body emerges from the dead ant’s head. It holds spores, which are released into the air, ready to be picked up by another tropical carpenter ant victim. Eventually, the ant’s remains fall to the ground. Scientists studying the understory where zombie ants are found attached under leaves have also found the remains of dead ants scattered on the ground – graveyards for past victims of the Ophiocordyceps fungus.

A paper in the journal BMC Ecology, published on May 9, 2011, describes the parasitism by a fungus on the tropical carpenter ant in Thailand. It’s a macabre description of how the ant is invaded by the fungus, which takes over its body, commands it to perform actions to ensure the survival and growth of the fungus, then uses its dead body to grow a stalk with spores that are ejected into the surrounding area, ready to infect other passing ants.

A tropical carpenter ant killed by a parasitic fungus in Thailand. The ant had bitten into the leaf in a "death grip," and a fungal fruiting body


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