0:00:06.449,0:00:09.209
Which of these entities [br]has evolved the ability

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to manipulate an animal[br]many times its size?

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The answer is all of them.

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These are all parasites,

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organisms that live on [br]or inside another host organism,

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which they harm and sometimes even kill.

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Parasite survival depends on transmitting[br]from one host to the next,

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sometimes through [br]an intermediate species.

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Our parasites elegantly achieve this[br]by manipulating their host's behavior,

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sometimes through direct brain hijacking.

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For example, this is the Gordian worm.

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One of its hosts, this cricket.

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The Gordian worm needs water to mate,[br]but the cricket prefers dry land.

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So once it's big enough to reproduce,

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the worm produces proteins that garble[br]the cricket's navigational system.

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The confused cricket [br]jumps around erratically,

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moves closer to water,

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and eventually leaps in,[br]often drowning in the process.

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The worm then wriggles out to mate

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and its eggs get eaten [br]by little water insects

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that mature,

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colonize land,

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and are in turn, eaten by new crickets.

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And thus, the Gordian worm lives on.

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And here's the rabies virus,[br]another mind-altering parasite.

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This virus infects mammals, often dogs,

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and travels up the animal's [br]nerves to its brain

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where it causes inflammation [br]that eventually kills the host.

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But before it does, it often increases[br]its host's aggressiveness

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and ramps up the production[br]of rabies-transmitting saliva,

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while making it hard to swollow.

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These factors make the host [br]more likely to bite another animal

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and more likely to pass [br]the virus on when it does.

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And now, meet Ophiocordyceps,[br]also known as the zombie fungus.

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Its host of choice is tropical ants[br]that normally live in tree tops.

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After Ophiocordyceps spores [br]pierce the ant's exoskeleton,

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they set off convulsions [br]that make the ant fall from the tree.

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The fungus changes the ant's behavior,[br]compelling it to wander mindlessly

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until it stumbles onto a plant leaf with[br]the perfect fungal breeding conditions,

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which it latches onto.

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The ant then dies,

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and the fungus parasitizes its body[br]to build a tall, thin stalk from its neck.

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Within several weeks, [br]the stalk shoots of spores,

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which turn more ants [br]into six-legged leaf-seeking zombies.

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One of humanity's most deadly assaliants[br]is behavior-altering parasite,

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though if it's any consolation,

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it's not our brains [br]that are being hijacked.

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I'm talking about Plasmodium,[br]which causes malaria.

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This parasite needs mosquitoes[br]to shuttle it between hosts,

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so it makes them bite more frequently[br]and for longer.

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There's also evidence [br]that humans infected with malaria

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are more attractive to mosquitoes,

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which will bite them [br]and transfer the parasite further.

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This multi-species system is so effective,

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that there are hundreds of millions[br]of malaria cases every year.

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And finally, there are cats.

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Don't worry, there probably [br]aren't any cats living in your body

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and controlling your thoughts.

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I mean, probably.

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But there is a microorganism[br]called Toxoplasma

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that needs both cats and rodents[br]to complete its life cycle.

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When a rat gets infected[br]by eating cat feces,

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the parasite changes chemical levels[br]in the rat's brain,

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making it less cautious[br]around the hungry felines,

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maybe even attracted to them.

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This makes them easy prey,

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so these infected rodents[br]get eaten and pass the parasite on.

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Mind control successful.

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There's even evidence that the parasite[br]affects human behavior.

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In most cases, [br]we don't completely understand

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how these parasites manage [br]their feats of behavior modification.

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But from what we do know,

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we can tell that they have [br]a pretty diverse toolbox.

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Gordian worms seem to affect[br]crickets' brains directly.

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The malaria parasite, on the other hand,

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blocks an enzyme [br]that helps the mosquitoes feed,

0:04:26.897,0:04:30.823
forcing them to bite over [br]and over and over again.

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The rabies virus may cause [br]that snarling, slobbering behavior

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by putting the immune system[br]into overdrive.

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But whatever the method,

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when you think about how effectively

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these parasites control [br]the behavior of their hosts,

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you may wonder how much of human behavior[br]is actually parasites doing the talking?

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Since more than half of the species[br]on Earth are parasites,

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it could be more than you think.