What Parasites Can Teach Us About Society

Who knew that the workings of a tapeworm could provide some very relevant implications about human nature and social control? Like many parasites, Schistocephalus solidus has a complex life cycle: it reproduces in the guts of waterbirds, from whose droppings its eggs are deposited; they hatch and the larva infect small crustaceans, which are eaten by stickleback fish, which are then eaten by the waterbirds, and…you get the picture.

So far, so typical of parasites. But as The Atlantic reports, the transition from one lifeform to another is facilitated by a pretty insidious form of mind control, which works far beyond the immediately infected animals.

The tapeworm doesn’t passively go along for this convoluted ride. When it enters sticklebacks, it somehow changes their behavior so they swim toward warmer water, where the worm can grow more quickly; at maximum size, it can make up half of a stickleback’s weight. The tapeworm also emboldens its hosts. They’re more likely to venture outside the safety of a shoal. They’re less likely to flee from predators. And consequently, they’re more likely to be eaten by birds.

Nicolle Demandt and Benedikt Saus from the University of Munster developed a simple way of assessing the tapeworm’s control over its host. They would put groups of sticklebacks in a tank, lure them to the surface with floating patches of food, and then attack them with an artificial “bird”—a fake beak on a bent stick that could be jabbed into the water with a handle. (“We built it from Lego,” says Jörn Peter Scharsack, who led the study. “It’s very simple but very efficient.”)

After the attacks, groups of uninfected fish would flee to the bottom of the tank, to hide among the plants there. By contrast, infected sticklebacks stuck to the danger zone. “These guys, they don’t care,” says Scharsack. “If you try to scare them they hardly respond.”

Here’s where it gets interesting: social animals like sticklebacks live in shoals, where they swim together in a coordinated manner, enjoying safety in numbers. However, this also means that they are very receptive to the actions of their shoal-mates (no pun intended). One small change in direction could trigger a large collective movement by the rest of the group. Hence, the handful of fish under the control of a parasite will in turn end up influencing non infected members, too.

Demandt and Saus demonstrated this by repeating their experiment with mixed groups of infected and uninfected individuals. They showed that if the infected were in the majority, the uninfected ones followed them, staying in the danger zone instead of fleeing. Such indirect control has never been documented before, and might benefit the parasite. If a larger shoal of devil-may-care fish stays at the water’s surface, predators might be more likely to find and attack them, again increasing the chances that they’ll swallow an infected individual.

And what of the sticklebacks? Sticking with the group isn’t necessarily the wrong decision, given that it provides safety in numbers. But in this case, the tapeworm might be converting safety in numbers into danger for all.

“Over the last half a century, parasitologists have been enamored with the idea that some parasites can alter their host’s behavior to serve their own interests,” says Julia Buck, an ecologist at the University of California at Santa Barbara. There are wasps that walk cockroaches, worms that turn crickets suicidal, fungi that zombify ants, and more. “This study demonstrates another way that parasites can alter host behavior: without infecting their hosts.”

Pretty creepy stuff. And that is where the implications for humanity come in. While we are not vulnerable to mind-controlling parasites, we do display an analogous tendency to be inadvertently influenced by the actions of a handful of individuals; this is hardly a novel finding either.

In classic experiments from the 1950s, Solomon Asch showed that volunteers could often be persuaded to give what were clearly wrong answers to simple questions if others around them—actually paid actors—answered wrongly too. In many cases, the volunteers were certain that the actors were wrong, but went along with their decisions nonetheless. A bad idea, just like a parasitic tapeworm, can also influence the minds of those who aren’t directly touched by it.

Definitely some food for thought in a world where one tweet or Facebook can whip up a mob of otherwise disparate strangers, and where savvy users command a lot of attention and influence across the globe.

What are your thoughts?