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  • A herd of wildebeests, a shoal of fish,

  • a flock of birds.

  • Many animals gather in large groups

  • that are among the most wonderful spectacles

  • in the natural world.

  • But why do these groups form?

  • The common answers include things like

  • seeking safety in numbers or hunting in packs

  • or gathering to mate or breed,

  • and all of these explanations,

  • while often true,

  • make a huge assumption about animal behavior,

  • that the animals are in control of their own actions,

  • that they are in charge of their bodies.

  • And that is often not the case.

  • This is Artemia, a brine shrimp.

  • You probably know it better as a sea monkey.

  • It's small, and it typically lives alone,

  • but it can gather in these large red swarms

  • that span for meters,

  • and these form because of a parasite.

  • These shrimp are infected with a tapeworm.

  • A tapeworm is effectively a long, living gut

  • with genitals at one end and a hooked mouth at the other.

  • As a freelance journalist, I sympathize.

  • (Laughter)

  • The tapeworm drains nutrients from Artemia's body,

  • but it also does other things.

  • It castrates them,

  • it changes their color from transparent to bright red,

  • it makes them live longer,

  • and as biologist Nicolas Rode has found,

  • it makes them swim in groups.

  • Why? Because the tapeworm, like many other parasites,

  • has a complicated life cycle

  • involving many different hosts.

  • The shrimp are just one step on its journey.

  • Its ultimate destination is this,

  • the greater flamingo.

  • Only in a flamingo can the tapeworm reproduce,

  • so to get there, it manipulates its shrimp hosts

  • into forming these conspicuous colored swarms

  • that are easier for a flamingo to spot

  • and to devour,

  • and that is the secret of the Artemia swarm.

  • They aren't sociable through their own volition,

  • but because they are being controlled.

  • It's not safety in numbers.

  • It's actually the exact opposite.

  • The tapeworm hijacks their brains and their bodies,

  • turning them into vehicles

  • for getting itself into a flamingo.

  • And here is another example

  • of a parasitic manipulation.

  • This is a suicidal cricket.

  • This cricket swallowed the larvae of a Gordian worm,

  • or horsehair worm.

  • The worm grew to adult size within it,

  • but it needs to get into water in order to mate,

  • and it does that by releasing proteins

  • that addle the cricket's brain,

  • causing it to behave erratically.

  • When the cricket nears a body of water,

  • such as this swimming pool,

  • it jumps in and drowns,

  • and the worm wriggles out

  • of its suicidal corpse.

  • Crickets are really roomy. Who knew?

  • The tapeworm and the Gordian worm are not alone.

  • They are part of an entire cavalcade

  • of mind-controlling parasites,

  • of fungi, viruses, and worms and insects and more

  • that all specialize in subverting and overriding

  • the wills of their hosts.

  • Now, I first learned about this way of life

  • through David Attenborough's "Trials of Life"

  • about 20 years ago,

  • and then later through a wonderful book called

  • "Parasite Rex" by my friend Carl Zimmer.

  • And I've been writing about these creatures ever since.

  • Few topics in biology enthrall me more.

  • It's like the parasites have subverted my own brain.

  • Because after all, they are always compelling

  • and they are delightfully macabre.

  • When you write about parasites,

  • your lexicon swells with phrases like

  • "devoured alive" and "bursts out of its body."

  • (Laughter)

  • But there's more to it than that.

  • I'm a writer, and fellow writers in the audience

  • will know that we love stories.

  • Parasites invite us to resist the allure

  • of obvious stories.

  • Their world is one of plot twists

  • and unexpected explanations.

  • Why, for example,

  • does this caterpillar

  • start violently thrashing about

  • when another insect gets close to it

  • and those white cocoons that it seems

  • to be standing guard over?

  • Is it maybe protecting its siblings?

  • No.

  • This caterpillar was attacked

  • by a parasitic wasp which laid eggs inside it.

  • The eggs hatched and the young wasps

  • devoured the caterpillar alive

  • before bursting out of its body.

  • See what I mean?

  • Now, the caterpillar didn't die.

  • Some of the wasps seemed to stay behind

  • and controlled it into defending their siblings

  • which are metamorphosing

  • into adults within those cocoons.

  • This caterpillar is a head-banging zombie bodyguard

  • defending the offspring

  • of the creature that killed it.

  • (Applause)

  • We have a lot to get through. I only have 13 minutes. (Laughter)

  • Now, some of you are probably just

  • desperately clawing for some solace

  • in the idea that these things are oddities

  • of the natural world, that they are outliers,

  • and that point of view is understandable,

  • because by their nature, parasites are quite small

  • and they spend a lot of their time

  • inside the bodies of other things.

  • They're easy to overlook,

  • but that doesn't mean that they aren't important.

  • A few years back, a man called Kevin Lafferty

  • took a group of scientists into three Californian estuaries

  • and they pretty much weighed and dissected

  • and recorded everything they could find,

  • and what they found

  • were parasites in extreme abundance.

  • Especially common were trematodes,

  • tiny worms that specialize in castrating their hosts

  • like this unfortunate snail.

  • Now, a single trematode is tiny, microscopic,

  • but collectively they weighed as much

  • as all the fish in the estuaries

  • and three to nine times more than all the birds.

  • And remember the Gordian worm that I showed you,

  • the cricket thing?

  • One Japanese scientist called Takuya Sato

  • found that in one stream,

  • these things drive so many crickets

  • and grasshoppers into the water

  • that the drowned insects

  • make up some 60 percent of the diet of local trout.

  • Manipulation is not an oddity.

  • It is a critical and common part

  • of the world around us,

  • and scientists have now found

  • hundreds of examples of such manipulators,

  • and more excitingly, they're starting to understand

  • exactly how these creatures control their hosts.

  • And this is one of my favorite examples.

  • This is Ampulex compressa,

  • the emerald cockroach wasp,

  • and it is a truth universally acknowledged

  • that an emerald cockroach wasp in possession

  • of some fertilized eggs

  • must be in want of a cockroach.

  • When she finds one,

  • she stabs it with a stinger

  • that is also a sense organ.

  • This discovery came out three weeks ago.

  • She stabs it with a stinger that is a sense organ

  • equipped with small sensory bumps

  • that allow her to feel the distinctive texture

  • of a roach's brain.

  • So like a person blindly rooting about in a bag,

  • she finds the brain, and she injects it with venom

  • into two very specific clusters of neurons.

  • Israeli scientists Frederic Libersat and Ram Gal

  • found that the venom is a very specific chemical weapon.

  • It doesn't kill the roach, nor does it sedate it.

  • The roach could walk away

  • or fly or run if it chose to,

  • but it doesn't choose to,

  • because the venom nixes its motivation to walk,

  • and only that.

  • The wasp basically un-checks the escape-from-danger box

  • in the roach's operating system,

  • allowing her to lead her helpless victim

  • back to her lair by its antennae

  • like a person walking a dog.

  • And once there, she lays an egg on it,

  • egg hatches, devoured alive, bursts out of body,

  • yadda yadda yadda, you know the drill.

  • (Laughter) (Applause)

  • Now I would argue that, once stung,

  • the cockroach isn't a roach anymore.

  • It's more of an extension of the wasp,

  • just like the cricket was an extension of the Gordian worm.

  • These hosts won't get to survive or reproduce.

  • They have as much control over their own fates

  • as my car.

  • Once the parasites get in,

  • the hosts don't get a say.

  • Now humans, of course,

  • are no stranger to manipulation.

  • We take drugs to shift the chemistries of our brains

  • and to change our moods,

  • and what are arguments or advertising or big ideas

  • if not an attempt to influence someone else's mind?

  • But our attempts at doing this

  • are crude and blundering compared

  • to the fine-grained specificity of the parasites.

  • Don Draper only wishes he was as elegant

  • and precise as the emerald cockroach wasp.

  • Now, I think this is part of what makes parasites

  • so sinister and so compelling.

  • We place such a premium on our free will

  • and our independence

  • that the prospect of losing those qualities

  • to forces unseen

  • informs many of our deepest societal fears.

  • Orwellian dystopias and shadowy cabals

  • and mind-controlling supervillains --

  • these are tropes that fill our darkest fiction,

  • but in nature, they happen all the time.

  • Which leads me to an obvious

  • and disquieting question:

  • Are there dark, sinister parasites

  • that are influencing our behavior

  • without us knowing about it,

  • besides the NSA?

  • If there are any

  • (Laughter) (Applause)

  • I've got a red dot on my forehead now, don't I?

  • (Laughter)

  • If there are any, this is a good candidate for them.

  • This is Toxoplasma gondii, or Toxo, for short,

  • because the terrifying creature

  • always deserves a cute nickname.

  • Toxo infects mammals,

  • a wide variety of mammals,

  • but it can only sexually reproduce in a cat.

  • And scientists like Joanne Webster have shown that

  • if Toxo gets into a rat or a mouse,

  • it turns the rodent into a cat-seeking missile.

  • If the infected rat smells the delightful odor

  • of cat piss,

  • it runs towards the source of the smell

  • rather than the more sensible direction of away.

  • The cat eats the rat. Toxo gets to have sex.

  • It's a classic tale of Eat, Prey, Love.

  • (Laughter) (Applause)

  • You're very charitable, generous people.

  • Hi, Elizabeth, I loved your talk.

  • How does the parasite control its host

  • in this way?

  • We don't really know.

  • We know that Toxo releases an enzyme

  • that makes dopamine, a substance involved

  • in reward and motivation.

  • We know it targets certain parts of a rodent's brain,

  • including those involved in sexual arousal.

  • But how those puzzle pieces fit together

  • is not immediately clear.

  • What is clear is that this thing