I WAS VERY HESITANT, almost dripping with nervous sweat, as I booted my computer that morning. According to numerous news reports, that was the day that the Michelangelo virus was expected to wreak havoc on infected computers around the world. I scrutinized the sequence of 'credits' that flickered across the screen as my baby did its warm-up exercises. Was that a new screenful of characters that just flashed past on the monitor, or was it one of the usual sets? I'd never really paid any attention to them before during those 30 seconds or so of booting. Was that RAM-reading any different from normal?

If you had the same experience, you can imagine the relief I felt as the day wore on with no sign of trouble. (I hope you had no problems either.) My computer continued to behave itself, misbehaving only in the usual ways. Unless tampered with, computer programs instruct a computer to behave in stereotyped, totally predictable ways. In fact, they simply cannot behave in any other way — assuming the computer hardware itself is in normal working order. If you are running a word processing program, you know it will wrap words around the end of the line 100% of the time without your having to think about it.

But there are some nasty fiends out there who spend a long time pondering how they can create a new set of instructions, commonly called a computer virus, that will cause your computer to do things you don't expect. (And don't want.) And how to infect your computer with that new set of instructions. These nasty pranksters know exactly how the virus will affect your computer once it has gained access.

Computers and animal behavior

Animal brains are similar to computers. An animal will always behave in predetermined ways. Every time I walk into the shed where I store food for our geese, they go off their honker. (Unless it's brooding season—then the female takes no notice of me as she sits mindlessly on her clutch.) Their brain tells them, though they are not conscious of it, that food is probably coming, so they start squawking noisily every time they see me head for the shed. Such predictable behavior by animals can lead us to imagine that their behavior is simple to understand! Nothing could be further from the truth. Even such everyday behavior involves highly complex interactions between brain activity, body chemistry and fluid influences in the environment. What little we know about how animal brains control the behavior of their carriers would fill many volumes. What God knows about it would fill many, many more.

Computer viruses that change the behavior of computers in predictable ways are an invention of the technological 1980s. God beat us to it by a long shot. He created living analogues of computer viruses. Not for the purpose of causing havoc in nature, but to demonstrate yet again the endlessness of His ingenuity.

What am I talking about? About little parasitic creatures that, once they have gained access into a living computer of the right kind, will change the behavior of that animal. As sci-fi as it sounds, God has created a few creatures that are able to radically alter the behavior of their victims. Janice Moore put it this way:

One of the most familiar literary devices in science fiction is alien parasites that invade a human host, forcing him to do their bidding as they multiply and spread to other hapless earthlings. Yet the notion that a parasite can alter the behavior of another organism is not mere fiction. The phenomenon is not even rare. One need only look in a field, a lake or a forest to find it (1984, p. 108).

Ever noticed a pill bug (those critters that occur in their thousands in wood piles and roll into a ball when you discomfit them) wandering around during daylight hours? Chances are its behavior has been altered by a parasite. Normally, pill bugs avoid light like children flee from shots.

The creatures that are responsible for such changes in behavior are always the larval stage of various parasites that are parasitic throughout all their life stages. (As distinct from other parasites that are free-living as adults.) The vast majority of such parasites go through highly complex life cycles, with larval stages living in one kind of host, while the adult stages live in totally different creatures. Though the basic theme is similar, the variety of ways employed by the various stages of different parasites to find their way from one host to another stretches human imagination.

Most people run screaming from the very word parasite , but for those who recognize the hand of God in nature's wonders, even repugnant freeloaders excite admiration. Those whose life's calling brings them into constant contact with these uglies of the universe are beginning to get quite excited. As one author put it, "A growing cadre of scientists now see these organisms as subtle, complex creatures, admirable in their own way and much more powerful than anyone ever imagined" (Ackerman 1997, p. 76). Once again, the power and subtlety of God's mind stands vindicated.

In addition, many scientists are coming to appreciate the beneficial role that parasites play in the great scheme of nature through their contribution in helping cull sickly individuals from animal populations. But that's another story.

Why change behavior?

The question of why such larvae might want to alter their victim's behavior can be simply answered. Think about it. Put yourself in a parasite's dilemma for a moment. How would you ever find your way from, say, a pill bug, where you spend your parasitic youth, to a starling, where you come to maturity? Easy, one may say. I wait for a starling to eat my pill bug. But it's not as easy as that. Pill bugs don't normally frolic in bird daylight. While Sam Starling is plying his trade, Pete the pill bug is cozily napping under a log.

It's all a matter of probability. Such parasites change the behavior of their intermediate host so that the probability of their gaining access to the final host is greatly increased. Even for those parasites that face a less challenging task in getting from host to host, the mathematical chances are still extremely low.

Such parasites change the behavior of their intermediate host so that the probability of their gaining access to the final host is greatly increased.

For instance, the chances of a newly laid parasite egg finding its way into, say, a rabbit, its intermediate host, are slim. Only a tiny percentage of eggs laid would ever manage this feat. Say one in a hundred achieved this. The chances of an infected rabbit then being eaten by a fox, the hypothetical parasite's final host, are also slim. Again, say one in a hundred. When you compound the chances, you can see the problem. If only one in a hundred eggs finds its way into a rabbit, and only one in a hundred rabbits ever finds its way into a fox's tummy, then the parasite egg only has one chance in ten thousand of reaching maturity. And, as is now becoming clearer, even then the parasite will not thrive unless the creature it finds its way into is already sickly.

However, infection rates are often quite high due to the enormous numbers of eggs produced. In severely affected regions, almost one hundred percent of the population of some animals can be infected by a particular parasite. And some individuals can be shockingly infected. Sheep have been found to contain up to 50,000 flukes in their liver! In all likelihood, something has gotten out of balance in such regions. God has created some fascinating strategies to increase the chances of parasites finding their way from egg to intermediate host (or hosts) to final host. Behavioral changes in an infected intermediate host is one such strategy.

How change behavior?

The question of how one creature is able to change the normal behavior of another creature and enslave it to a new mode of behavior is fascinating. As with everything God does, there is variety in the method. In some instances, the behavior is changed not by affecting the victim's instinct, but by, for instance, encysting in its muscles in such a way that the animal is no longer able to move about as normal. For example, the larvae of the nematode Tetrameres americana settle into the muscles of certain grasshoppers, making their victim less active. The grasshoppers are then far more likely to be eaten by their final hosts—chickens.

Others change the behavior of their intermediate host by invading its central nervous system. Gid, a disease that makes sheep stagger in circles and become separated from the herd, is caused by an invasion of the animal's brain or spinal column by the larva of the canine tapeworm. The loss of control by the sheep makes it easier prey for wolves and wild dogs, which are the tapeworm's final host. This is a form of changing behavior by actually damaging the central nervous system.

The other major way of changing behavior is the one that is analogous to computer viruses. By mysterious means not understood, the parasite brings about a change in behavior without doing any damage to the central nervous system. "The mechanism underlying their behavioral effects is probably biochemical; little, however, is known about it" (Moore, p. 108). The creatures that practice this form of mind control are commonly known as thorny-headed worms. Their scientific name is, as usual, a real mouthful — Acanthocephalans.

Though the number of species is not precisely known, the highest estimate puts the total at over 1,000 species. The ability to change behavior in their intermediate host may well be universal. Once again, God has presented us with a mystery that shouts loudly his faculty for ingenuity.

Evidently, the dumb little motes of life produce chemicals that home in precisely on that part of the brain that controls the behavior in question, and somehow, like a computer virus, throw the switches. The actual events that take place at the microscopic and biochemical level would undoubtedly be unbelievably complex. Isn't evolution amazing!

Ants and sheep and other such

Let's consider some amazing examples. First we will give some cases of parasites that actually invade the central nervous system to carry out their plots. Then we'll take a look at the cleverest of all alien invaders, the thorny-headed worms.

Ever heard of sheep eating ants? And being infected with flukes as a result? Probably not. Well, once upon a time a sick sheep passed some pernicious eggs. Soon thereafter a snail crawled over the offended spot and, in the course of fulfilling its duty to its stomach, swallowed a number of the eggs. Alack and alas, the eggs were not nutritious little balls of food, as the snail had thought, but packets of potential parasites.

The nasty little eggs sprung open upon reaching the snail's gut, and gave birth to a brood of miracidia — an intermediate stage of the critter. In no time at all, the tiny shock troops bored through the gut wall and, by means not fully understood, forced their way into the snail's digestive gland. Here, over the next three months, the wee intruders went through a series of magical changes. Their grandchildren then made their way to the breathing pore of the snail, where the snail packaged them up in tiny mucus slime balls containing about 500 larvae and then, when the temperature fell, "sneezed" them out. All very strange, but what happened next was stranger still.

In no time at all, the tiny shock troops bored through the gut wall…

Certain ants, upon discovering these slime balls, carried them willingly but unwittingly to their home nests. Here, worker ants pounced on them and gorged themselves literally silly. Soon, two remarkable things occurred that altered the normal behavior of ants—changes that greatly increased the chances of the ant being gobbled by a sheep. And you don't need me to tell you that sheep don't normally fancy ants on the menu.

First, by some mysterious, biochemical, brain-altering mechanism, the ants were seduced into climbing a blade of grass. Once they reached their Everest, there they stayed put. Definitely not normal ant behavior.

Second, one of the numerous little parasites inside the ant encysted in the sub-esophageal ganglion (how's that for a mouthful?), the part of the nervous system that controls the insect's mouth parts. When the temperature fell, the little ants perched on the tip of the grass were then somehow induced by the cyst to lock onto the blade of grass by their 'jaws'. There they remained, as if in a drunken torpor, during the cool periods of both day and night. Safely-grazing sheep, (or rabbits, deer or other grazing mammal) in naive stupidity, nibbled the deadly tips. The cycle was thus complete.

Other curious examples

Then there is the case of the straying marine snails. Jennifer Ackerman was puzzled to discover mud-dwelling marine snails on the US Atlantic coast stranded on top of sandbars some distance from the water. What she found out amazed her. The snails were parasitized by fluke larvae that had to find their way from the snail into a beach flea and thence into a bird in order to complete the life cycle. Problem is, beach fleas live higher up the beach from where the mud snails live. Let her complete the picture:

The larvae gobble up the mud snail's glands and then somehow prod the snail to leave its customary habitat and move to the higher ground traveled by beach fleas. There they emerge from the snail's body and enter the body of a flea. When a shorebird eats the flea, parasite and all, the larvae mature in the bird's gut and produce eggs, which are released with the bird's faces onto the flats. The cycle then begins anew (p. 90).

How convenient that the infected snails crawled up-beach. If the parasite didn't know exactly what it was doing, it may induce the snail to crawl around in circles, head for Davy Jones's locker, or even try to burrow to China or fly to the moon. How could blind evolution ever get it so right? And exactly what goes on in the snail's brain (if you'll pardon the exaggeration) is just about anybody's guess. The larval parasite must be a precision driver to steer the snail in precisely the right direction. How could such a tiny mote of nothingness become so clever as to take over another creature's mind? We can't do it! No luck is involved here. It's clearly a case of intelligent planning.

In yet another bizarre case, flukes that inhabit the guts of some European birds have an amazing trick up their sleeve to increase the chances of the larval stage being eaten by the birds. The larval stages infest certain snails, which normally hide in leaf litter as soon as dawn approaches. Infected snails continue to wander around much longer. Birds don't have to be early to chance upon these late snails. But there is more. The parasites migrate in the early hours of the morning into the snail's antennae, squeezing their way in and enlarging the antenna. The stretched wall of the antenna becomes so thin that the brilliantly-colored parasite can easily be seen inside. Then it begins to pulsate, creating a kind of barber's-pole effect. Such infected snails become highly conspicuous. The birds then complete the story by pecking off the poor snail's infected tentacles and swallowing them.

As another example of the power to change its host's behavior, consider the single-celled creature known as Toxoplasma gondyi. This mote of life enjoys the hospitality of a rat before reaching its final host — a cat. Normally speaking, self-respecting rats avoid cats like the plague. Their sensitive sense of smell can pick up cat urine very effectively, initiating a fleeing response. Toxoplasma cleverly reverses this normal rat distaste for cat urine — an infected rat seems to develop a real predilection for the stuff, and you can guess the rest.

Thorny-headed worms

Acanthocephalans, or thorny-headed worms, comprise a phylum all their own of very approximately 1,200 species worldwide. Though nobody knows for sure, specialists believe that the ability to change the behavior of intermediate hosts is universal within the phylum. They present quite a puzzle to evolutionists (what doesn't?) "largely because no other organism looks very much like an acanthocephalan".

The larvae of thorny-headed worms are actually able to control their host's behavior without violently knocking down the door of their central nervous system. How they accomplish their sleight of brain is simply unknown, except that it must be biochemically-driven.

We'll give a brief generalized account of their life history and note a few examples. Those species that have been studied all live as adults in the small intestine of vertebrates, particularly birds and fishes. (Those of you reading this who are humans may be grateful to know we are not numbered amongst their friendly hosts.) There the female releases eggs that are excreted by the bird or fish, which are then eaten by the intermediate host — the one whose behavior may be changed. Usually the intermediate host is an insect or crustacean. The egg hatches in the intestines, and the larva burrows through the intestinal wall and enters the creature. Somewhere inside its new host it develops into a stage that can then infest its vertebrate host—if only it can get there. To achieve that, it pumps its drugs into the intermediate host's system to make it behave in a way that will greatly increase the chances of the final host finding it and snapping it up.

We've already mentioned one kind that infects songbirds such as starlings as its final host. The starling excretes the eggs in its droppings. Pill bugs then eat the droppings. The eggs hatch, and the little invaders proceed to impose their will on the poor little pill bug's activities.

Another kind of thorny-headed worm has mallards as its final host. The intermediate host is a small amphipod, a kind of crustacean. The amphipods normally live at the bottom of a pond, even in the mud. They are programmed to move away from light, thus always staying at the bottom. Mallards don't dive very deep, so they don't normally eat these amphipods. An infected amphipod, however, switches normal behavior and heads for the surface whenever it is disturbed. There they skim along the surface, sometimes clinging to vegetation or other floating objects. Here they are far more likely to be eaten by a surface swimming mallard. Investigation has interestingly shown that the changes in amphipod behavior don't occur until the larva has reached the stage where it can infect mallards. If the amphipod were eaten at an earlier stage, the immature parasite would not survive.

The larva interferes with three crucial amphipod reactions. Sane amphipods prefer darkness, but mad ones seek light. Sane amphipods let gravity do its trick, but mad ones rebel against it. Sane amphipods duck when they detect a duck; insane ones carry on regardless. Thus, mad amphipods are much more likely to get eaten.

Cockroaches infested with another species of thorny-headed worm are also attracted to light. Furthermore, they become hyperactive. Hyperactive cockroaches that move toward light are less likely to remain hidden, and so are probably more likely to be caught by rats, which are the definitive (final) host.

This carefully-coordinated, two-pronged strategy cannot possibly have arisen through Darwinian natural selection. What will God think of next?

References and notes

Ackerman, Jennifer October 1997, Parasites: Looking for a Free Lunch, National Geographic

Moore, Janice May 1984, Parasites That Change the Behaviour of Their Host, Scientific American

Further reading

Other printed material