How parasites change their host's behavior - Jaap de Roode

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 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 swollow.

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 tree tops.

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 of spores,

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

One of humanity's most deadly assaliants
is 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 you think.