Ed Yong begins by showing us beautiful images of animals gathering in large groups. And the reasons for them are fascinating and many. But Yong, an award-winning science writer, points out that most explanations “make an assumption about animal behavior — that they are in charge of their actions.”
But many animals gather in groups, in fact, because they are infected by parasites.
For example, the brine shrimp artemia salina — or sea monkeys — are often found in large red swarms that span for meters. Do they do it for safety or mating? No, they do it because they’re infected by tapeworms. The parasite changes their color, makes them live longer, and makes them congregate in groups. Those large, visible swarms make them easy prey for the flamingo, the final host for the tapeworm. “That is the secret of the artemia swarm,” says Yong. “It’s not safety in numbers, it’s the exact opposite.”
Another examples is a suicidal cricket. The horsehair worm larva, which infects the cricket, needs to get to water. It releases a protein that causes the cricket to run to water and jump in, drowning the cricket and releasing a surprisingly large and wiggly worm.
Those, says Yong, are only a couple examples of many parasites that override their host’s will. There are fungi, viruses, worms and more. Yong himself first learned about them from David Attenborough’s, “Trials of Life” program, and Carl Zimmer’s book Parasite Rex. He says, “It’s like the parasites have subverted my own brain.” In part that’s because “they are always compelling. When you study them your lexicon swells with phrases like ‘devoured alive’ and ‘bursts out of the body.'”
Another reason Yong loves parasites is that he loves stories, and parasites resist the allure of the obvious stories. For example, he shows a caterpillar that flails around to seemingly protect its young, in white cocoons nearby. In fact, it was infected by a wasp, which laid its eggs in the caterpillar’s body, and they hatched and burst out of the body and formed the cocoons. But some of the wasps stayed behind and controlled the caterpillar to guard their siblings. It is “a head-banging zombie bodyguard defending the offspring of the creature that killed it.”
And now Yong notes some might be hoping for some solace in the fact that such zombifying parasites are rare. In fact, they are not. They are small and easy to overlook, but parasites are everywhere. A team of scientists lead by Kevin Lafferty recently counted the parasites in one area of an estuary. They found that they collectively weighed as much as all the fish and three times as much as all the birds in the area. As he says, “Manipulation is not an oddity, it is a critical and common part of the world around us.” Scientists have found hundreds of examples, and are starting to understand how they control their hosts.
Take the emerald cockroach wasp. It stings a cockroach with a stinger that has sense organs to find specific parts of the brain and injects those parts with venom. It’s a very specific venom — it turns off the roach’s motivation to walk and only that. The wasp can then lead the roach back to its lair, “lays its eggs, burst out of the body, yadda yadda yadda.” Yong would argue that once infected, the cockroach is no longer independent, it’s more of an extension of the wasp — “These hosts won’t get to survive or reproduce, they have as much control over their future as my car.”
And now we get to the really disturbing implication. Humans are no stranger to manipulation. We have ads and speeches. But Yong points out these attempts are crude compared to nature. “Don Draper only wishes he was as elegant and precise as the emerald cockroach wasp.” To Yong, this is what makes parasites so compelling and so repulsive. We place such a huge value on our independence. The idea of losing it, particularly to mysterious forces, is terrifying.
So, is our own free will being eroded by a parasite? One possibility is toxoplama gondii, or toxo for short (“You should always give the terrifying parasite a cute nickname”). This single-celled parasite can infect many mammals, but can only reproduce in a cat. So if toxo gets into a rodent, it becomes a “cat-seeking missile,” running to the nearest cat to get eaten. The parasite can then develop and mate in the cat. It’s a classic tale, says Yong, of “eat, prey, love.”
Well, we are mammals, and in fact around one-third of humans are toxo carriers. It generally has no adverse effects, but there is some evidence that those with toxo display slight differences in personality questionnaires. The evidence is weak, and there is divided opinion among toxo researchers as to whether that’s a real effect. But, says, Yong, even if it isn’t from toxo, “Given the widespread nature of such manipulations, it would be completely implausible if humans were the only creature not under the same thrall.”
Yong is particularly taken by parasites’ “capacity to subvert our thinking about the world… They invite us to look at the world sideways. And this makes them as wonderful and charismatic and wonderful as any panda, butterfly, or dolphin.”