A zombie crawls through the forest. When it reaches a good spot, it freezes in place. A stalk slowly grows from its head. The stalk then spews out spores that spread, turning others into zombies.
This is no Halloween story about the zombie apocalypse. It's all true. The zombie isn’t a human, though. It’s an ant. And the stalk that emerges from its head is a fungus. Its spores infect other ants, which lets the zombie cycle begin anew.
In order to grow and spread, this fungus must hijack an ant’s brain. However weird this might seem, it isn’t all that unusual. The natural world is full of zombies under mind control. Zombie spiders and cockroaches babysit developing wasp larvae — until the babies devour them. Zombie fish flip around and dart toward the surface of the water, seeming to beg for birds to eat them. Zombie crickets, beetles and praying mantises drown themselves in water. Zombie rats are drawn to the smell of the pee of cats that may devour them.
All of these “zombies” have one thing in common: parasites. A parasite lives inside or on another creature, known as its host. A parasite may be a fungus, a worm or another tiny creature. All parasites eventually weaken or sicken their hosts. Sometimes, the parasite kills or even eats its host. But death of the host isn’t the freakiest goal. A parasite might get its host to die in a certain place, or be eaten by a certain creature. In order to accomplish these tricks, some parasites have evolved the ability to hack into the host’s brain and influence its behavior in very specific ways.
How do parasites turn insects and other animals into the walking almost-dead? Every parasite has its own method, but the process usually involves altering chemicals within the victim’s brain. Researchers are working hard to identify which chemicals are involved and how they end up so bizarrely altering their host’s behavior.
Brains, brains! Ant brains!
A fungus doesn’t have a brain. And worms and single-celled critters obviously aren’t very smart. Yet somehow they still control the brains of larger, and smarter, animals.
“It blows my mind,” says Kelly Weinersmith. She is a biologist who studies parasites at Rice University in Houston, Texas. She is particularly interested in “zombie” creatures. True zombies, she points out, aren’t exactly like the type you find in horror stories. “In no way are these animals coming back from the dead,” she says. Most real zombies are doomed to die — and some have very little control over their actions.
The horsehair worm, for instance, needs to emerge in water. To make this happen, it forces its insect host to leap into a lake or swimming pool. Often, the host drowns.
Toxoplasma gondii (TOX-oh-PLAZ-ma GON-dee-eye) is a single-celled creature that can only complete its life cycle inside a cat. But first, this parasite must live for a time in a different animal, such as a rat. To ensure this part-time host gets eaten by a cat, the parasite turns rats into cat-loving zombies.
In Thailand, a species of fungus — Ophiocordyceps — can force an ant to climb almost exactly 20 centimeters (about 8 inches) up a plant, to face north and then to bite down on a leaf. And it makes the ant do this when the sun is at its highest point in the sky. This provides ideal conditions for the fungus to grow and release its spores.
For More Details Tutoring Solution Video