The zombies in "World War Z" are just actors — but in the animal world, zombies are a fact of life. Evolution can come up with some fiendish twists: For example, there are some species that bend other creatures to their will to keep their bizarre life cycles going. Or just to feast on their delicious braaaains.
To celebrate the premiere of "World War Z," here's a top-ten list of zombies from the animal kingdom, finishing up with a particularly pernicious parasite that can pose a risk to humans:
Zombie ants: There are special kinds of zombifying fungi that infect carpenter ants in Thailand and Brazil. The fungus grows into the ant's brain and compels it to climb down and clamp onto to the low leaves that provide the fungus with its favored breeding ground. After the ant dies, the fungus sprouts from its head and shoots out spores to infect other ants. But this zombie isn't invincible: Scientists recently discovered a different kind of fungus that can castrate the zombifying fungus before it spreads.
Zombie bees: A parasitic fly known as Apocephalus borealis can inject its eggs into a honeybee's abdomen, where the fly larvae mature. The parasitized bees abandon their hives and walk in circles — but eventually they fall over. "It really painted a picture of something like a zombie," says San Francisco State University's Andrew Core. About seven days after infection, the mature larvae burst out from the dead bees' bodies, renewing the gruesome circle of life. It gets worse: The zombie bees are spreading.
Zombie birds: Great tits look so cute when they're perched on a branch. You'd never know that they have a taste for bat brains. In Hungary, the birds listen for the calls sent out by the cave-dwelling pipistrelle bats when they rouse themselves from hibernation. Then they track down the groggy bats, crack their skulls open and eat their brains. Researchers found that the trick is passed down from one generation of great tits to the next, providing an example of cultural transmission in avian species. The great tits helped inspire the title of Becky Crew's book about bizarre biology, "Zombie Birds, Astronaut Fish and Other Weird Animals."
Zombie caterpillars: You don't want to be around when a zombified caterpillar turns to goo. There's a species of baculovirus that infects gypsy moth caterpillars and sends them up into the treetops to die. When the caterpillar's body liquefies, the ooze drips down onto other caterpillars — creating more zombies. Scientists say a single gene in the virus interferes with the caterpillar's hormones, apparently triggering the uncharacteristic urge to climb during the day.
Zombie crabs: A tiny barnacle called Sacculina can latch onto male crabs and blast them with so much estrogen that they dig empty nests, made to order for the barnacle to lay its eggs inside. If the crab host happens to be a female, no problem: The barnacle merely wipes out the host's reproductive system, and then sets her digging.
Zombie crickets: Parasitic hairworms have been known to invade crickets, take over their nervous system and then force their buggy hosts to drown themselves so that the grown worm can swim out and look for a mate. Scientists aren't exactly sure how crickets pick up the parasite, but it may involve ingesting water or bugs that contain hairworm larvae.
Zombie fish: What could be ickier than having a zombie crustacean eat out your tongue — and then take its place in your mouth?That's what the beastie known as Cymothoa exigua does to spotted rose snappers. C. exigua latches onto the fish's tongue and sucks the blood out of it until it falls off. Then it attaches itself to the stub that's left behind, and changes its diet from blood to fish mucus. The fish and the parasite carry on that way for the rest of their lives. If you're freaked out by the "Alien" movies, do not look at this picture.
Zombie ladybugs: Dinocampus coccinellae, a species of parasitic wasp, lays a single egg inside the abdomen of a ladybug. The wasp larva eats its way through the ladybug's insides, then pops out and spins a cocoon. End of story for the ladybug, right? Not necessarily. The ladybug often lives on as one of the undead, partially paralyzed on top of the cocoon. There's evidence that the wasp larva provides resources to keep the ladybug alive, while the ladybug provides an extra layer of protection from bugs that might otherwise eat the larva.
Zombie spiders: Another kind of zombifying wasp targets the orb spider known as Plesiometa argyra. The wasp temporarily paralyzes the spider, lays an egg on the tip of its abdomen, and leaves it to mature. The orb spider goes about its business, weaving its usual circular webs — until, one night, the wasp larva sends a chemical signal to the spider's brain. The spider spins a weird-looking web, seemingly designed to provide shelter from rain, wind and predators. The next day, the larva kills the spider — and moves into the home it programmed the spider to build.
Zombie rats ... and humans? The most insidious zombie of the animal world may well be the tiny parasite called Toxoplasma gondii, or Toxo for short. This parasite can reproduce sexually only in cat intestines, which poses a challenge when the little critters wind up in cat poop. Here's how Toxo gets back inside the cat: When mice or rats nibble on the poop, they also gobble up some of the Toxo parasites. The parasites migrate to the brain, where they release chemicals that make the rodents lower their guard when they're around cats. Scientists think that Toxo-infected mice start to regard the smell of cat pee as something sexy rather than a cause for alarm. The cats eat the smitten rats, and the Toxo life cycle begins again.
The scary part is that Toxo can find its way into the human nervous system as well. About 30 percent of the world's population is thought to be infected. Pregnant women should be careful about avoiding infection, because the parasite can pose a risk to the unborn baby. There's also some evidence of a connection between Toxo and changes in mood or personality, and perhaps even conditions such as schizophrenia or bipolar disorder. The linkage isn't rock-solid, and most of those who are infected suffer no apparent effects. But if you need a plot device for your next zombie-movie script, Toxo is a good place to start.
More science of zombies:
- The real-life roots of zombie tales
- Zombies never die ... because we love 'em
- Did zombies roam medieval Ireland?
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with NBCNews.com's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.