Zombie roaches and other parasite tales | Ed Yong (re-release)

Host Elise Hu opens by describing October as a season of pumpkins, fake cobwebs, costumes, and traditions surrounding the dead and the mysterious. She introduces the episode as part of TED Talks Daily’s Halloween playlist of five “haunting” talks, including subjects such as grieving crows and zombie roaches. The featured talk is from science writer Ed Yong, recorded at TED 2014 and originally published in March 2014. Hu explains that Yong studies parasites that hijack the minds and bodies of their hosts. She previews examples including a worm that makes crickets leap into water and a wasp that leads a cockroach around like a dog on a leash. The central question is whether anything comparable could happen to humans. Hu recommends watching the talk on TED.com because Yong’s presentation includes striking and sometimes stomach-turning photographs. After the introduction, the episode runs ads for TikTok, Walmart Business, and Apple Card. The TikTok spot connects physical surroundings with thinking; Walmart Business promotes simpler procurement and operations; and Apple Card advertises 2% Daily Cash back on purchases made with Apple Pay.

Yong begins by challenging familiar explanations for animal groups. Herds, shoals, and flocks are often understood as animals seeking safety, hunting, or mating, but that reasoning assumes animals control their own behavior. His example is Artemia, or brine shrimp—better known as sea monkeys—which normally live alone but can form red swarms several meters across. The swarms are caused by tapeworms. The parasite drains nutrients, castrates the shrimp, turns them from transparent to bright red, extends their lives, and makes them swim in groups. These changes serve the tapeworm’s life cycle: it must eventually reach a greater flamingo, where it can reproduce, so it makes the shrimp more conspicuous and likely to be eaten. The swarm is therefore not “safety in numbers,” but a way of turning shrimp into vehicles for reaching flamingos. Yong then describes a Gordian, or horsehair, worm that grows inside a cricket and releases proteins that disrupt the cricket’s brain. When the cricket approaches water, it jumps in and drowns, allowing the adult worm to emerge and mate. He also describes a caterpillar whose body is being used by a parasitic wasp: after the wasp larvae consume it and burst out, some remain behind to control the dying caterpillar as a head-banging guard for the wasp pupae.

Yong argues that mind-controlling parasites are not obscure curiosities. He says that he became fascinated with them after seeing David Attenborough’s Trials of Life and reading his friend Carl Zimmer’s book Parasite Rex. Their stories overturn obvious interpretations of animal behavior: a caterpillar that appears to protect siblings may actually be defending the offspring of the wasp that killed it. He cites research led by Kevin Lafferty in three Californian estuaries, where scientists weighed, dissected, and recorded nearly everything they could find. Trematodes—tiny worms that specialize in castrating hosts—were so abundant that their combined weight equaled that of all the fish in the estuaries and was three to nine times the weight of all the birds. In another example, Japanese scientist Takuya Soto found that drowned crickets and grasshoppers driven into one stream by Gordian worms made up about 60% of local trout diets. Yong then explains the emerald cockroach wasp, Ampulex compressa. The female stabs a cockroach with a stinger that doubles as a sensory organ, allowing her to locate the roach’s brain by feeling its texture. She injects venom into two precise clusters of neurons. The venom neither kills nor paralyzes the roach; instead, it removes the motivation to walk away. The wasp can then lead the roach by its antennae to her lair, lay an egg on it, and leave the larva to devour it alive.

Yong says that after the wasp’s sting, the cockroach is effectively no longer an independent roach but an extension of the wasp, much as the cricket becomes an extension of the Gordian worm. The host can still physically walk, fly, or run, but it no longer chooses to escape. Yong compares this exact manipulation with human attempts to influence behavior through drugs, arguments, advertising, and big ideas. Those methods, he jokes, are crude compared with the wasp’s fine-grained chemistry; “Don Draper only wishes he was as elegant and precise as the emerald cockroach wasp.” This loss of autonomy is what makes parasites especially sinister to humans. Yong links the idea to fears represented by Orwellian dystopias, shadowy cabals, and mind-controlling supervillains. He then asks whether unseen parasites could be influencing human behavior, adding the joke that the NSA might be one possibility before presenting a biological candidate: Toxoplasma gondii, or “Toxo.” Toxo can infect many mammals but reproduces sexually only in cats. Researchers including Joanne Webster have found that infected rats and mice become attracted to cat urine rather than fleeing from it. The cat eats the rodent, and the parasite reaches the host in which it can reproduce. Yong calls this a classic cycle of “eat, pray, love,” while emphasizing that the parasite’s manipulation is far more specific than ordinary human persuasion.

Yong explains that scientists do not yet know exactly how Toxo changes a rodent’s behavior. The parasite releases an enzyme that affects dopamine, a chemical involved in reward and motivation, and targets brain regions connected with sexual arousal. The remarkable point is that Toxo is a single cell with no nervous system, consciousness, or even a conventional body, yet it can manipulate a mammal. Because humans are mammals with similar brain structures, cell types, and brain chemicals, Yong asks whether we might also be affected. He cites estimates suggesting that about one in three people worldwide carry Toxo in their brains. In most cases it remains dormant and causes no obvious illness. However, some studies have reported that carriers score slightly differently on personality questionnaires, have a somewhat higher risk of car accidents, and that people with schizophrenia are more likely to be infected. Yong carefully qualifies these claims. He says the evidence is still inconclusive, and even researchers who study Toxo disagree about whether it truly changes human behavior. Still, he considers it implausible that humans would be the only species untouched by the kinds of manipulation that occur throughout nature. Parasites therefore encourage people to question whether behavior that appears voluntary may sometimes result from an unseen biological influence.

Yong closes by returning to his larger lesson: parasites force us to look at ordinary behavior from an unexpected angle. A behavior may seem simple and self-directed, or puzzling and irrational, yet in either case it could result from an organism being bent to the control of another. Their life cycles are grisly—hosts are castrated, driven to drown, devoured from within, or turned into tools—but their ability to produce such unexpected outcomes makes them fascinating. He compares parasites with more conventionally admired animals such as pandas, butterflies, and dolphins, arguing that parasites deserve similar recognition for their complexity and charisma. He invokes Charles Darwin’s closing description in On the Origin of Species of the “grandeur” of life and its “endless forms, most beautiful and most wonderful.” Yong suggests Darwin could just as easily have been describing a tapeworm that makes shrimp sociable or a wasp that walks a cockroach, then undercuts the conclusion with the line, “But perhaps that’s just a parasite talking.” Elise Hu then identifies the talk as Ed Yong’s TED 2014 presentation and provides the episode’s production credits and TED curation link reference. Additional post-roll ads follow for Capital One’s Venture X Business card and Onnit’s AlphaBrain before the episode ends.