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  • Which of these entities has evolved the ability

  • to manipulate an animal many times its size?

  • The answer is all of them.

  • These are all parasites,

  • organisms that live on or inside another host organism,

  • which they harm and sometimes even kill.

  • Parasite's survival depends on transmitting from one host to the next,

  • sometimes through an intermediate species.

  • Our parasites elegantly achieve this by manipulating their host's behavior,

  • sometimes through direct brain hijacking.

  • For example, this is the Gordian worm.

  • One of its hosts, this cricket.

  • The Gordian worm needs water to mate, but the cricket prefers dry land.

  • So once it's big enough to reproduce,

  • the worm produces proteins that garble the cricket's navigational system.

  • The confused cricket jumps around erratically,

  • moves closer to water,

  • and eventually leaps in, often drowning in the process.

  • The worm then wriggles out to mate

  • and its eggs get eaten by little water insects

  • that mature,

  • colonize land,

  • and are, in turn, eaten by new crickets.

  • And thus, the Gordian worm lives on.

  • And here's the rabies virus, another mind-altering parasite.

  • This virus infects mammals, often dogs,

  • and travels up the animal's nerves to its brain

  • where it causes inflammation that eventually kills the host.

  • But before it does, it often increases its host's aggressiveness

  • and ramps up the production of rabies-transmitting saliva,

  • while making it hard to swallow.

  • These factors make the host more likely to bite another animal

  • and more likely to pass the virus on when it does.

  • And now, meet Ophiocordyceps, also known as the zombie fungus.

  • Its host of choice is tropical ants that normally live in treetops.

  • After Ophiocordyceps' spores pierce the ant's exoskeleton,

  • they set off convulsions that make the ant fall from the tree.

  • The fungus changes the ant's behavior, compelling it to wander mindlessly

  • until it stumbles onto a plant leaf with the perfect fungal breeding conditions,

  • which it latches onto.

  • The ant then dies,

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

  • Within several weeks, the stalk shoots off spores,

  • which turn more ants into six-legged leaf-seeking zombies.

  • One of humanity's most deadly assailants is a behavior-altering parasite,

  • though if it's any consolation,

  • it's not our brains that are being hijacked.

  • I'm talking about Plasmodium, which causes malaria.

  • This parasite needs mosquitoes to shuttle it between hosts,

  • so it makes them bite more frequently and for longer.

  • There's also evidence that humans infected with malaria

  • are more attractive to mosquitoes,

  • which will bite them and transfer the parasite further.

  • This multi-species system is so effective,

  • that there are hundreds of millions of malaria cases every year.

  • And finally, there are cats.

  • Don't worry, there probably aren't any cats living in your body

  • and controlling your thoughts.

  • I mean, probably.

  • But there is a microorganism called Toxoplasma

  • that needs both cats and rodents to complete its life cycle.

  • When a rat gets infected by eating cat feces,

  • the parasite changes chemical levels in the rat's brain,

  • making it less cautious around the hungry felines,

  • maybe even attracted to them.

  • This makes them easy prey,

  • so these infected rodents get eaten and pass the parasite on.

  • Mind control successful.

  • There's even evidence that the parasite affects human behavior.

  • In most cases, we don't completely understand

  • how these parasites manage their feats of behavior modification.

  • But from what we do know,

  • we can tell that they have a pretty diverse toolbox.

  • Gordian worms seem to affect crickets' brains directly.

  • The malaria parasite, on the other hand,

  • blocks an enzyme that helps the mosquitoes feed,

  • forcing them to bite over and over and over again.

  • The rabies virus may cause that snarling, slobbering behavior

  • by putting the immune system into overdrive.

  • But whatever the method,

  • when you think about how effectively

  • these parasites control the behavior of their hosts,

  • you may wonder how much of human behavior is actually parasites doing the talking.

  • Since more than half of the species on Earth are parasites,

  • it could be more than we think.

Which of these entities has evolved the ability

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B2 TED-Ed parasite behavior worm cricket ant

【TED-Ed】How brain parasites change their host's behavior - Jaap de Roode

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    CUChou posted on 2015/03/16
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