By GARY MEFFE
People are fascinated by zombies. Countless movies (mostly bad) have been made about them, books expound on zombies, there are zombie-inspired songs, a zombie cocktail, and Brandon even had its very own zombie invasion last Sunday. Droves of our (mostly) normal friends and neighbors suddenly appeared as “undead” gnarly creatures, awkwardly and jerkily making their way down Park Street, threatening to turn the rest of us into their kind with a bite from their corpse-like jaws. How awful and gruesome. And how silly and absurd to think there really are zombies! Or . . . is it?
Nature invented many things long before humans came along. And there are, in fact, honest-to-goodness zombies all around us, in many and varied forms. Don’t believe me? Read on.
“Zombification” occurs when one organism—a parasite—manipulates the behavior of another organism—a host—to its own benefit. In mythical human zombies, that organism usually is a virus that re-animates corpses to go around biting living people to infect them and spread the virus, thereby keeping itself thriving. Those poor human zombies have no choice; they are mind-controlled by their viruses. Something very similar occurs throughout much of the animal kingdom, when parasites change the normal behavior of their hosts to improve their own chances of survival and reproduction. These parasites turn their hosts into “zombies,” forcing them into strange and unnatural behaviors, and thus do the bidding for their uninvited guests. Let’s look at a few examples.
There is a flatworm that inhabits salt marshes in southern California. It needs to find its way into fish-eating birds and has enlisted two other species to help it. Flatworm eggs appear in mudflats, where they are eaten by a snail. Eventually, swimming larvae of the worm emerge from the snail and infect the gills of killifish. These larvae then work their way to the brain and cause the killifish to behave erratically. The infected fish jerk, shimmy, flash (remind you of any Brandon zombies?), and thrash around at the surface, advertising their presence, where they are easy prey for fish-eating birds. The flatworms then mature in the guts of the birds, who eventually poop out eggs to be eaten by snails, completing the life cycle. Snails and killifish are the worms’ unwitting zombies.
Another flatworm has figured out another way to control and zombify another snail host. In this case, the land snail inadvertently eats eggs of the flatworm. These eggs hatch into larvae that travel to the eye stalks of the snail. Normally tiny, the eye stalks now swell to huge sizes and pulsate rhythmically. The pulsing eyestalks look remarkably like caterpillars, attracting bird predators, who eat the eye stalks, whereupon the flatworm parasite takes up residence, matures, and . . . you guessed it . . . eggs are pooped out, completing the lifecycle. Zombie snails at your service. (You can see this in a video at https://www.wired.com/2014/09/absurd-creature-of-the-week-disco-worm/).
Let’s now look at the insect world. Ichneumonid (ICK-NEW-MON-ID) wasps are famous in biological circles for their parasitic ways. They ingeniously manipulate the behavior of various hosts—turning them into zombies—in the pursuit of childcare. There are many variations on this theme, but I will demonstrate with a creature called Glyptapanteles (don’t fret over these big words; biologists have a sadistic streak and also like to show off). When ready with eggs, a female Glyptapanteles (let’s call her Glypta) finds a caterpillar and injects her eggs into the body cavity, up to 80 eggs on a good day. Our Glypta then flies off to resume her life away from what amounts to her own personal childcare facility.
The eggs develop into larvae inside the caterpillar, where they munch on bodily fluids of their host (Warning: the more squeamish among you may now wish to turn the page and move on to another article). As they feed on these goodies, they get bigger and the host swells right along with them. Curiously, the caterpillar doesn’t seem to take notice, and continues feeding the whole time, unknowingly supporting her foreign brood with a continual flow of calories. Yum!
Now it gets really interesting (Second warning to the squeamish: look away now. I mean it!). When the larvae are big and ready to greet the world, they release a chemical that paralyzes their host, whereupon they burrow through the skin (I warned you!), up to 80 youngsters pretty much all at once, bursting out. Cool, huh? But wait, there’s more!
These larvae time their emergence with a shedding of their exoskeletons, which plugs the holes they just made, thus keeping their adopted mama from bleeding out, because they are not, in fact, finished with their zombie caretaker.
A couple of the larvae mysteriously remain behind and release new and different chemicals that take over the brain and annoy the heck out of the caterpillar, who by now surely must realize something is amiss. The paralysis has worn off and next the zombie thrashes around uncontrollably. Why, you might ask? Because those emerged Glypta larvae have spun cocoons and entered the pupal stage, where they are now vulnerable. The thrashing of the adopted mom helps to keep away predators from her adopted brood. And I won’t even mention that she also spins a protective cocoon over everybody, because that would be too unbelievable.
The pupae eventually mature and hatch into brand new, shiny adult Glyptas, who now go looking for their own childcare service in another innocent caterpillar. Zombie makers on the loose!
There are well over 25,000 known species of Ichneumonid wasps in the world, more than all known bird and mammal species combined! This argues strongly for the resounding success of this zombie-causing way of life (though to be clear, I’m not recommending it for any of you. You need to take care of your own kids).
If you’re still with me, I have one final zombie treat. And it is so unbelievable you’ll think I’m making it up. But I’m not, as I’m not nearly that creative. In fact, the scientists who study it cannot quite believe it, but here goes.
In the jungles of South America lives a fungus called Ophiocordyceps unilateralis (yeah, they’re showing off again; let’s just go with Ophio). Common name: the ‘zombie fungus.’ That should be a hint of what is to come.
Let me emphasize, Ophio is a simple, non-descript blob of a fungus. Yet, it controls and manipulates ants in precise and exacting and unbelievable ways. Carpenter ants, specifically.
Ophio infects a carpenter ant through a spore that attaches to the ant’s exoskeleton and bores its way through using enzymes. It then establishes residency and begins to grow, feeding off the fluids that make up the ant’s open circulatory system. The fungus expands, and within a couple of weeks constitutes about half of the ant’s weight. Amazingly, the ant behaves normally the entire time, unnoticed by its colony mates, who would otherwise quickly escort it out of the colony and kill it. Yet, despite having this full-on fungal infection, the infected ants betray nothing of their intruders; all appears normal.
After about three weeks, the fungus makes the ant do something extraordinary: it takes over the brain (here’s the zombie part) whereupon the ant, right around noon (I’m not kidding; it is timed for mid-day) leaves its hard-working colony of buddies, climbs up nearby vegetation to a leaf 10” above the ground (I’m still not kidding), locks its mandibles on the underside of a leaf, and dies. Ten inches appears to be the precise microclimate (temperature and humidity) best suited for the Ophio fungus to grow and prosper. Not nine inches, not eleven. Ten.
Fungal hyphae (tentacles of fungi) now grow out of the mouth and around the dead ant to protect it from other fungi and bacteria. A long fungal stalk then emerges from behind the ant’s head, where it releases spores to drop on and infect unaware carpenter ants working below, thus completing the life cycle. Another successful zombification!
There are many Ophio species throughout South America and Asia, who each have their very own species of ant to zombify. And Ophio and its ilk have been at this business for quite a long time: there are fossils of this fungal fiasco from 48 million years ago. Pretty successful way of life, I’d say!
As a scientist, I cannot take seriously the notion of human zombies and just have to laugh them off. But zombies in nature are no laughing matter, especially if you are the victim of one of the thousands of mind-controlling zombifiers lurking about! So, I’d advise against eating fish thrashing at the surface or munching on caterpillar-like structures pulsing from your escargot. And for heaven’s sake, don’t lounge around for hours looking like a caterpillar laying in the grass! As for the zombie fungus . . . I think we’re pretty safe. At least here in Vermont.
