How to pronounce "toxo"

A herd of wildebeests, a shoal of fish,

a flock of birds.

Many animals gather in large groups

that are among the most wonderful spectacles

in the natural world.

But why do these groups form?

The common answers include things like

seeking safety in numbers or hunting in packs

or gathering to mate or breed,

and all of these explanations,

while often true,

make a huge assumption about animal behavior,

that the animals are in control of their own actions,

that they are in charge of their bodies.

And that is often not the case.

This is Artemia, a brine shrimp.

You probably know it better as a sea monkey.

It's small, and it typically lives alone,

but it can gather in these large red swarms

that span for meters,

and these form because of a parasite.

These shrimp are infected with a tapeworm.

A tapeworm is effectively a long, living gut

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

As a freelance journalist, I sympathize.

(Laughter)

The tapeworm drains nutrients from Artemia's body,

but it also does other things.

It castrates them,

it changes their color from transparent to bright red,

it makes them live longer,

and as biologist Nicolas Rode has found,

it makes them swim in groups.

Why? Because the tapeworm, like many other parasites,

has a complicated life cycle

involving many different hosts.

The shrimp are just one step on its journey.

Its ultimate destination is this,

the greater flamingo.

Only in a flamingo can the tapeworm reproduce,

so to get there, it manipulates its shrimp hosts

into forming these conspicuous colored swarms

that are easier for a flamingo to spot

and to devour,

and that is the secret of the Artemia swarm.

They aren't sociable through their own volition,

but because they are being controlled.

It's not safety in numbers.

It's actually the exact opposite.

The tapeworm hijacks their brains and their bodies,

turning them into vehicles

for getting itself into a flamingo.

And here is another example

of a parasitic manipulation.

This is a suicidal cricket.

This cricket swallowed the larvae of a Gordian worm,

or horsehair worm.

The worm grew to adult size within it,

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

and it does that by releasing proteins

that addle the cricket's brain,

causing it to behave erratically.

When the cricket nears a body of water,

such as this swimming pool,

it jumps in and drowns,

and the worm wriggles out

of its suicidal corpse.

Crickets are really roomy. Who knew?

The tapeworm and the Gordian worm are not alone.

They are part of an entire cavalcade

of mind-controlling parasites,

of fungi, viruses, and worms and insects and more

that all specialize in subverting and overriding

the wills of their hosts.

Now, I first learned about this way of life

through David Attenborough's "Trials of Life"

about 20 years ago,

and then later through a wonderful book called

"Parasite Rex" by my friend Carl Zimmer.

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

Few topics in biology enthrall me more.

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

Because after all, they are always compelling

and they are delightfully macabre.

When you write about parasites,

your lexicon swells with phrases like

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

(Laughter)

But there's more to it than that.

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

will know that we love stories.

Parasites invite us to resist the allure

of obvious stories.

Their world is one of plot twists

and unexpected explanations.

Why, for example,

does this caterpillar

start violently thrashing about

when another insect gets close to it

and those white cocoons that it seems

to be standing guard over?

Is it maybe protecting its siblings?

No.

This caterpillar was attacked

by a parasitic wasp which laid eggs inside it.

The eggs hatched and the young wasps

devoured the caterpillar alive

before bursting out of its body.

See what I mean?

Now, the caterpillar didn't die.

Some of the wasps seemed to stay behind

and controlled it into defending their siblings

which are metamorphosing

into adults within those cocoons.

This caterpillar is a head-banging zombie bodyguard

defending the offspring

of the creature that killed it.

(Applause)

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

Now, some of you are probably just

desperately clawing for some solace

in the idea that these things are oddities

of the natural world, that they are outliers,

and that point of view is understandable,

because by their nature, parasites are quite small

and they spend a lot of their time

inside the bodies of other things.

They're easy to overlook,

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

A few years back, a man called Kevin Lafferty

took a group of scientists into three Californian estuaries

and they pretty much weighed and dissected

and recorded everything they could find,

and what they found

were parasites in extreme abundance.

Especially common were trematodes,

tiny worms that specialize in castrating their hosts

like this unfortunate snail.

Now, a single trematode is tiny, microscopic,

but collectively they weighed as much

as all the fish in the estuaries

and three to nine times more than all the birds.

And remember the Gordian worm that I showed you,

the cricket thing?

One Japanese scientist called Takuya Sato

found that in one stream,

these things drive so many crickets

and grasshoppers into the water

that the drowned insects

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

Manipulation is not an oddity.

It is a critical and common part

of the world around us,

and scientists have now found

hundreds of examples of such manipulators,

and more excitingly, they're starting to understand

exactly how these creatures control their hosts.

And this is one of my favorite examples.

This is Ampulex compressa,

the emerald cockroach wasp,

and it is a truth universally acknowledged

that an emerald cockroach wasp in possession

of some fertilized eggs

must be in want of a cockroach.

When she finds one,

she stabs it with a stinger

that is also a sense organ.

This discovery came out three weeks ago.

She stabs it with a stinger that is a sense organ

equipped with small sensory bumps

that allow her to feel the distinctive texture

of a roach's brain.

So like a person blindly rooting about in a bag,

she finds the brain, and she injects it with venom

into two very specific clusters of neurons.

Israeli scientists Frederic Libersat and Ram Gal

found that the venom is a very specific chemical weapon.

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

The roach could walk away

or fly or run if it chose to,

but it doesn't choose to,

because the venom nixes its motivation to walk,

and only that.

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

in the roach's operating system,

allowing her to lead her helpless victim

back to her lair by its antennae

like a person walking a dog.

And once there, she lays an egg on it,

egg hatches, devoured alive, bursts out of body,

yadda yadda yadda, you know the drill.

(Laughter) (Applause)

Now I would argue that, once stung,

the cockroach isn't a roach anymore.

It's more of an extension of the wasp,

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

These hosts won't get to survive or reproduce.

They have as much control over their own fates

as my car.

Once the parasites get in,

the hosts don't get a say.

Now humans, of course,

are no stranger to manipulation.

We take drugs to shift the chemistries of our brains

and to change our moods,

and what are arguments or advertising or big ideas

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

But our attempts at doing this

are crude and blundering compared

to the fine-grained specificity of the parasites.

Don Draper only wishes he was as elegant

and precise as the emerald cockroach wasp.

Now, I think this is part of what makes parasites

so sinister and so compelling.

We place such a premium on our free will

and our independence

that the prospect of losing those qualities

to forces unseen

informs many of our deepest societal fears.

Orwellian dystopias and shadowy cabals

and mind-controlling supervillains --

these are tropes that fill our darkest fiction,

but in nature, they happen all the time.

Which leads me to an obvious

and disquieting question:

Are there dark, sinister parasites

that are influencing our behavior

without us knowing about it,

besides the NSA?

If there are any —

(Laughter) (Applause)

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

(Laughter)

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

This is Toxoplasma gondii, or Toxo, for short,

because the terrifying creature

always deserves a cute nickname.

Toxo infects mammals,

a wide variety of mammals,

but it can only sexually reproduce in a cat.

And scientists like Joanne Webster have shown that

if Toxo gets into a rat or a mouse,

it turns the rodent into a cat-seeking missile.

If the infected rat smells the delightful odor

of cat piss,

it runs towards the source of the smell

rather than the more sensible direction of away.

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

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

(Laughter) (Applause)

You're very charitable, generous people.

Hi, Elizabeth, I loved your talk.

How does the parasite control its host

in this way?

We don't really know.

We know that Toxo releases an enzyme

that makes dopamine, a substance involved

in reward and motivation.

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

including those involved in sexual arousal.

But how those puzzle pieces fit together

is not immediately clear.

What is clear is that this thing

is a single cell.

This has no nervous system.

It has no consciousness.

It doesn't even have a body.

But it's manipulating a mammal?

We are mammals.

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

but our brains have the same basic structure,

the same types of cells,

the same chemicals running through them,

and the same parasites.

Estimates vary a lot, but some figures suggest

that one in three people around the world

have Toxo in their brains.

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

The parasite holds up in a dormant state

for a long period of time.

But there's some evidence that those people

who are carriers score slightly differently

on personality questionnaires than other people,

that they have a slightly higher risk of car accidents,

and there's some evidence that people with schizophrenia

are more likely to be infected.

Now, I think this evidence is still inconclusive,

and even among Toxo researchers,

opinion is divided as to whether the parasite

is truly influencing our behavior.

But given the widespread nature of such manipulations,

it would be completely implausible

for humans to be the only species

that weren't similarly affected.

And I think that this capacity to constantly

subvert our way of thinking about the world

makes parasites amazing.

They're constantly inviting us to look at the natural world sideways,

and to ask if the behaviors we're seeing,

whether they're simple and obvious

or baffling and puzzling,

are not the results of individuals

acting through their own accord

but because they are being bent

to the control of something else.

And while that idea may be disquieting,

and while parasites' habits may be very grisly,

I think that ability to surprise us

makes them as wonderful and as charismatic

as any panda or butterfly or dolphin.

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

Charles Darwin writes about the grandeur of life,

and of endless forms most beautiful

and most wonderful,

and I like to think he could easily have been talking

about a tapeworm that makes shrimp sociable

or a wasp that takes cockroaches for walks.

But perhaps, that's just a parasite talking.

Thank you.

(Applause)