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A herd of wildebeests, a shoal of fish,

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a flock of birds.

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Many animals gather in large groups

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that are among the most wonderful spectacles

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in the natural world.

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But why do these groups form?

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The common answers include things like

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seeking safety in numbers or hunting in packs

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or gathering to mate or breed,

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and all of these explanations,

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while often true,

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make a huge assumption about animal behavior,

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that the animals are in control of their own actions,

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that they are in charge of their bodies.

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And that is often not the case.

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This is Artemia, a brine shrimp.

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You probably know it better as a sea monkey.

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It's small, and it typically lives alone,

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but it can gather in these large red swarms

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that span for meters,

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and these form because of a parasite.

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These shrimp are infected with a tapeworm.

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A tapeworm is effectively a long, living gut

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

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As a freelance journalist, I sympathize.

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(Laughter)

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The tapeworm drains nutrients from Artemia's body,

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but it also does other things.

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It castrates them,

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it changes their color from transparent to bright red,

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it makes them live longer,

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and as biologist Nicolas Rode has found,

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it makes them swim in groups.

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Why? Because the tapeworm,
like many other parasites,

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has a complicated life cycle

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involving many different hosts.

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The shrimp are just one step on its journey.

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Its ultimate destination is this,

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the greater flamingo.

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Only in a flamingo can the tapeworm reproduce,

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so to get there, it manipulates its shrimp hosts

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into forming these conspicuous colored swarms

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that are easier for a flamingo to spot

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and to devour,

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and that is the secret of the Artemia swarm.

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They aren't sociable through their own volition,

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but because they are being controlled.

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It's not safety in numbers.

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It's actually the exact opposite.

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The tapeworm hijacks their brains and their bodies,

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turning them into vehicles

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for getting itself into a flamingo.

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And here is another example

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of a parasitic manipulation.

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This is a suicidal cricket.

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This cricket swallowed the 
larvae of a Gordian worm,

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or horsehair worm.

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The worm grew to adult size within it,

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

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and it does that by releasing proteins

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that addle the cricket's brain,

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causing it to behave erratically.

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When the cricket nears a body of water,

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such as this swimming pool,

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it jumps in and drowns,

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and the worm wriggles out

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of its suicidal corpse.

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Crickets are really roomy. Who knew?

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The tapeworm and the Gordian worm are not alone.

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They are part of an entire cavalcade

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of mind-controlling parasites,

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of fungi, viruses, and worms and insects and more

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that all specialize in subverting and overriding

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the wills of their hosts.

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Now, I first learned about this way of life

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through David Attenborough's "Trials of Life"

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about 20 years ago,

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and then later through a wonderful book called

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"Parasite Rex" by my friend Carl Zimmer.

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

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Few topics in biology enthrall me more.

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

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Because after all, they are always compelling

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and they are delightfully macabre.

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When you write about parasites,

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your lexicon swells with phrases like

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

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(Laughter)

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But there's more to it than that.

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

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will know that we love stories.

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Parasites invite us to resist the allure

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of obvious stories.

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Their world is one of plot twists

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and unexpected explanations.

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Why, for example,

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does this caterpillar

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start violently thrashing about

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when another insect gets close to it

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and those white cocoons that it seems

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to be standing guard over?

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Is it maybe protecting its siblings?

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No.

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This caterpillar was attacked

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by a parasitic wasp which laid eggs inside it.

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The eggs hatched and the young wasps

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devoured the caterpillar alive

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before bursting out of its body.

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See what I mean?

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Now, the caterpillar didn't die.

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Some of the wasps seemed to stay behind

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and controlled it into defending their siblings

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which are metamorphosing

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into adults within those cocoons.

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This caterpillar is a head-banging 
zombie bodyguard

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defending the offspring

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of the creature that killed it.

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(Applause)

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

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Now, some of you are probably just

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desperately clawing for some solace

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in the idea that these things are oddities

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of the natural world, that they are outliers,

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and that point of view is understandable,

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because by their nature, parasites are quite small

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and they spend a lot of their time

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inside the bodies of other things.

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They're easy to overlook,

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

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A few years back, a man called Kevin Lafferty

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took a group of scientists
into three Californian estuaries

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and they pretty much weighed and dissected

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and recorded everything they could find,

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and what they found

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were parasites in extreme abundance.

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Especially common were trematodes,

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tiny worms that specialize in castrating their hosts

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like this unfortunate snail.

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Now, a single trematode is tiny, microscopic,

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but collectively they weighed as much

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as all the fish in the estuaries

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and three to nine times more than all the birds.

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And remember the Gordian worm that I showed you,

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the cricket thing?

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One Japanese scientist called Takuya Sato

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found that in one stream,

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these things drive so many crickets

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and grasshoppers into the water

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that the drowned insects

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

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Manipulation is not an oddity.

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It is a critical and common part

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of the world around us,

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and scientists have now found

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hundreds of examples of such manipulators,

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and more excitingly, they're starting to understand

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exactly how these creatures control their hosts.

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And this is one of my favorite examples.

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This is Ampulex compressa,

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the emerald cockroach wasp,

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and it is a truth universally acknowledged

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that an emerald cockroach wasp in possession

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of some fertilized eggs

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must be in want of a cockroach.

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When she finds one,

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she stabs it with a stinger

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that is also a sense organ.

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This discovery came out three weeks ago.

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She stabs it with a stinger that is a sense organ

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equipped with small sensory bumps

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that allow her to feel the distinctive texture

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of a roach's brain.

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So like a person blindly rooting about in a bag,

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she finds the brain, and she injects it with venom

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into two very specific clusters of neurons.

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Israeli scientists Frederic Libersat and Ram Gal

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found that the venom is a
very specific chemical weapon.

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

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The roach could walk away

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or fly or run if it chose to,

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but it doesn't choose to,

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because the venom nixes its motivation to walk,

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and only that.

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

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in the roach's operating system,

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allowing her to lead her helpless victim

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back to her lair by its antennae

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like a person walking a dog.

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And once there, she lays an egg on it,

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egg hatches, devoured alive, bursts out of body,

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yadda yadda yadda, you know the drill.

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(Laughter) (Applause)

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Now I would argue that, once stung,

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the cockroach isn't a roach anymore.

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It's more of an extension of the wasp,

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

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These hosts won't get to survive or reproduce.

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They have as much control over their own fates

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as my car.

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Once the parasites get in,

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the hosts don't get a say.

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Now humans, of course,

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are no stranger to manipulation.

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We take drugs to shift the chemistries of our brains

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and to change our moods,

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and what are arguments or advertising or big ideas

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

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But our attempts at doing this

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are crude and blundering compared

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to the fine-grained specificity of the parasites.

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Don Draper only wishes he was as elegant

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and precise as the emerald cockroach wasp.

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Now, I think this is part of what makes parasites

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so sinister and so compelling.

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We place such a premium on our free will

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and our independence

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that the prospect of losing those qualities

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to forces unseen

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informs many of our deepest societal fears.

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Orwellian dystopias and shadowy cabals

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and mind-controlling supervillains --

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these are tropes that fill our darkest fiction,

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but in nature, they happen all the time.

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Which leads me to an obvious

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and disquieting question:

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Are there dark, sinister parasites

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that are influencing our behavior

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without us knowing about it,

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besides the NSA?

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If there are any —

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(Laughter) (Applause)

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

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(Laughter)

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

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This is Toxoplasma gondii, or Toxo, for short,

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because the terrifying creature

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always deserves a cute nickname.

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Toxo infects mammals,

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a wide variety of mammals,

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but it can only sexually reproduce in a cat.

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And scientists like Joanne Webster have shown that

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if Toxo gets into a rat or a mouse,

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it turns the rodent into a cat-seeking missile.

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If the infected rat smells the delightful odor

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of cat piss,

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it runs towards the source of the smell

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rather than the more sensible direction of away.

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

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

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(Laughter) (Applause)

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You're very charitable, generous people.

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Hi, Elizabeth, I loved your talk.

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How does the parasite control its host

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in this way?

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We don't really know.

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We know that Toxo releases an enzyme

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that makes dopamine, a substance involved

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in reward and motivation.

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

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including those involved in sexual arousal.

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But how those puzzle pieces fit together

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is not immediately clear.

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What is clear is that this thing

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is a single cell.

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This has no nervous system.

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It has no consciousness.

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It doesn't even have a body.

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But it's manipulating a mammal?

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We are mammals.

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We are more intelligent than a mere rat, to be sure,

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but our brains have the same basic structure,

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the same types of cells,

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the same chemicals running through them,

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and the same parasites.

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Estimates vary a lot, but some figures suggest

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that one in three people around the world

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have Toxo in their brains.

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Now typically, this doesn't lead to any overt illness.

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The parasite holds up in a dormant state

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for a long period of time.

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But there's some evidence that those people

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00:11:11,439 --> 00:11:14,473
who are carriers score slightly differently

283
00:11:14,473 --> 00:11:17,202
on personality questionnaires than other people,

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00:11:17,202 --> 00:11:19,686
that they have a slightly 
higher risk of car accidents,

285
00:11:19,686 --> 00:11:22,289
and there's some evidence
that people with schizophrenia

286
00:11:22,289 --> 00:11:24,835
are more likely to be infected.

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Now, I think this evidence is still inconclusive,

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and even among Toxo researchers,

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opinion is divided as to whether the parasite

290
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is truly influencing our behavior.

291
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But given the widespread
nature of such manipulations,

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it would be completely implausible

293
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for humans to be the only species

294
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that weren't similarly affected.

295
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And I think that this capacity to constantly

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subvert our way of thinking about the world

297
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makes parasites amazing.

298
00:11:51,660 --> 00:11:54,869
They're constantly inviting us to
look at the natural world sideways,

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and to ask if the behaviors we're seeing,

300
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whether they're simple and obvious

301
00:11:58,527 --> 00:12:00,060
or baffling and puzzling,

302
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are not the results of individuals

303
00:12:02,146 --> 00:12:03,999
acting through their own accord

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but because they are being bent

305
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to the control of something else.

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And while that idea may be disquieting,

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and while parasites' habits may be very grisly,

308
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I think that ability to surprise us

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00:12:15,233 --> 00:12:18,408
makes them as wonderful and as charismatic

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00:12:18,408 --> 00:12:21,772
as any panda or butterfly or dolphin.

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At the end of "On the Origin of Species,"

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Charles Darwin writes about the grandeur of life,

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and of endless forms most beautiful

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00:12:29,633 --> 00:12:31,065
and most wonderful,

315
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and I like to think he could easily have been talking

316
00:12:33,644 --> 00:12:36,824
about a tapeworm that makes shrimp sociable

317
00:12:36,824 --> 00:12:40,073
or a wasp that takes cockroaches for walks.

318
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But perhaps, that's just a parasite talking.

319
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Thank you.

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(Applause)