Attack of the Zombie Slaves

There really are zombies roaming the earth. (They're just really, really small.)

Just in time for Halloween, this creepy tale of zombie slaves has all the elements of a good gross-out movie -- parasites, spores, brain control, exploding body parts -- and all of it is real.

Here's the storyline: Spores hijack the brain of an unwitting victim, force it to leave the safety of its home and travel to a spot that favors the parasite's needs.

There, the host is killed and its body is used to spread spores to more victims.

It sounds like an evil plot to take over the world, but it's one of the everyday dangers facing Camponotus leonardi, a tree-dwelling species of carpenter ant that lives high in the trees in Thailand's rainforests.

"It really does seem sinister," said David Hughes, a Harvard University organismal biologist.

One ill-fated foraging trip is all it takes for an unwary worker ant to become the zombie slave of a fungus called Ophiocordyceps unilateralis.

Researchers have long known that the fungus preys on this ant species. But it wasn't until recently that they found how it controls their minds.

Now, they know the fungus directs infected ants to tree leaves that are at the ideal height, temperature and humidity the fungus needs to grow.

That puts the fungus near the top of a class of creepy parasites that control their hosts' behavior.

Hughes is a member of an international team of researchers who studied how the fungus does its magic on the ant. Their work was published recently in the journal American Naturalist.

Hughes studies adaptive parasitic behavioral manipulation, in which parasites control their hosts' minds or decision-making abilities. (Wasn't this spelled out in a Star Trek movie?)

There are several examples of this in nature, including:

• The jewel wasp of southern Asia and Africa. This bug stings cockroaches in their brains and injects a venom that disables the roach's ability to decide where to walk.

The wasp grabs its zombie roach by an antenna and walks it to a nest where even more bad things happen. A wasp lays its egg on the roach, and its larvae eat the insect alive over several days.

• A hair worm that eats grasshoppers from the inside out. This worm controls grasshoppers and makes them jump into pools where they drown. This allows the worms to emerge and mate under water.

• A nematode called Myrmeconema neotropicum that turns ant abdomens bright red. Then this nematode forces the ant to stick its belly into the air, resembling delicious berries that birds are fond of.

Once the birds eat the ants, they spread the next generation of nematode eggs through their droppings.

Robert Dudley, a University of California, Berkeley, biologist, studies the nematode. He was among a team of researchers who discovered the parasite in 2005 in the jungles of Central America and the Amazon.

"What is obvious is the (hosts) hold their abdomens much more vertically than other ants. It's like they are advertising," Dudley said. "There is definitely some kind of neural effect. It's hijacking the brain."

Ants are often ideal targets for parasites because of their social structure, said Joan Herbers, an Ohio State University entomologist and ant expert. A centralized colony with thousands of closely packed individuals is an ideal place for a parasite to set up shop, she said.

It's creepy, for sure, but it's also natural.

"Parasites are part of what makes the world go 'round," Herbers said.

For Hughes, Orphiocordyceps is fascinating because of the control it exerts.

Think about it -- this fungus doesn't have a brain, yet it is able to make ants leave the treetops to find a leaf or twig about 10 inches from the ground.

Hughes thinks the fungus developed specialized chemicals or compounds that affect the ants' brain chemistry. He hopes to identify them in future studies.

"What compounds are necessary to change the ants' behavior in such a fine, detailed manner?" Hughes said. "It's a fabulous example of adaptation."

Discovering compounds that control ants' brains doesn't make Hughes an evil scientist. In fact, he foresees using these compounds to help exterminators keep destructive fire ants at bay.

Source here

http://www.dispatch.com/live/content/science/stories/2009/10/25/sci_antzombies.ART_ART_10-25-09_G3_IMFEJ9T.html