Ed Yong: Suicidal wasps, zombie roaches and other parasite tales

302,411 views ・ 2014-03-26

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00:12
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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who are carriers score slightly differently
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on personality questionnaires than other people,
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that they have a slightly higher risk of car accidents,
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and there's some evidence that people with schizophrenia
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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
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is truly influencing our behavior.
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But given the widespread nature of such manipulations,
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it would be completely implausible
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for humans to be the only species
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that weren't similarly affected.
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And I think that this capacity to constantly
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subvert our way of thinking about the world
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makes parasites amazing.
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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,
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whether they're simple and obvious
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or baffling and puzzling,
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are not the results of individuals
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acting through their own accord
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but because they are being bent
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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,
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I think that ability to surprise us
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makes them as wonderful and as charismatic
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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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and most wonderful,
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and I like to think he could easily have been talking
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about a tapeworm that makes shrimp sociable
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or a wasp that takes cockroaches for walks.
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But perhaps, that's just a parasite talking.
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Thank you.
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(Applause)
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