0:00:00.645,0:00:03.196
A herd of wildebeests, a shoal of fish,

0:00:03.196,0:00:04.825
a flock of birds.

0:00:04.825,0:00:06.893
Many animals gather in large groups

0:00:06.893,0:00:08.879
that are among the most wonderful spectacles

0:00:08.879,0:00:10.597
in the natural world.

0:00:10.597,0:00:13.321
But why do these groups form?

0:00:13.321,0:00:14.958
The common answers include things like

0:00:14.958,0:00:17.924
seeking safety in numbers or hunting in packs

0:00:17.924,0:00:20.420
or gathering to mate or breed,

0:00:20.420,0:00:22.088
and all of these explanations,

0:00:22.088,0:00:23.325
while often true,

0:00:23.325,0:00:26.368
make a huge assumption about animal behavior,

0:00:26.368,0:00:29.545
that the animals are in control of their own actions,

0:00:29.545,0:00:31.796
that they are in charge of their bodies.

0:00:31.796,0:00:35.223
And that is often not the case.

0:00:35.223,0:00:38.147
This is Artemia, a brine shrimp.

0:00:38.147,0:00:40.911
You probably know it better as a sea monkey.

0:00:40.911,0:00:42.906
It's small, and it typically lives alone,

0:00:42.906,0:00:45.983
but it can gather in these large red swarms

0:00:45.983,0:00:48.230
that span for meters,

0:00:48.230,0:00:51.320
and these form because of a parasite.

0:00:51.320,0:00:54.006
These shrimp are infected with a tapeworm.

0:00:54.006,0:00:56.257
A tapeworm is effectively a long, living gut

0:00:56.257,0:00:59.145
with genitals at one end and[br]a hooked mouth at the other.

0:00:59.145,0:01:01.680
As a freelance journalist, I sympathize.

0:01:01.680,0:01:03.835
(Laughter)

0:01:03.835,0:01:06.743
The tapeworm drains nutrients from Artemia's body,

0:01:06.743,0:01:08.820
but it also does other things.

0:01:08.820,0:01:10.440
It castrates them,

0:01:10.440,0:01:15.158
it changes their color from transparent to bright red,

0:01:15.158,0:01:16.674
it makes them live longer,

0:01:16.674,0:01:19.031
and as biologist Nicolas Rode has found,

0:01:19.031,0:01:22.390
it makes them swim in groups.

0:01:22.390,0:01:25.842
Why? Because the tapeworm,[br]like many other parasites,

0:01:25.842,0:01:27.423
has a complicated life cycle

0:01:27.423,0:01:29.687
involving many different hosts.

0:01:29.687,0:01:32.091
The shrimp are just one step on its journey.

0:01:32.091,0:01:34.703
Its ultimate destination is this,

0:01:34.703,0:01:36.650
the greater flamingo.

0:01:36.650,0:01:39.675
Only in a flamingo can the tapeworm reproduce,

0:01:39.675,0:01:42.964
so to get there, it manipulates its shrimp hosts

0:01:42.964,0:01:46.055
into forming these conspicuous colored swarms

0:01:46.055,0:01:48.772
that are easier for a flamingo to spot

0:01:48.772,0:01:50.331
and to devour,

0:01:50.331,0:01:53.263
and that is the secret of the Artemia swarm.

0:01:53.263,0:01:55.426
They aren't sociable through their own volition,

0:01:55.426,0:01:57.502
but because they are being controlled.

0:01:57.502,0:01:59.196
It's not safety in numbers.

0:01:59.196,0:02:01.055
It's actually the exact opposite.

0:02:01.055,0:02:04.096
The tapeworm hijacks their brains and their bodies,

0:02:04.096,0:02:05.492
turning them into vehicles

0:02:05.492,0:02:09.459
for getting itself into a flamingo.

0:02:09.459,0:02:10.889
And here is another example

0:02:10.889,0:02:13.415
of a parasitic manipulation.

0:02:13.415,0:02:16.391
This is a suicidal cricket.

0:02:16.391,0:02:19.695
This cricket swallowed the [br]larvae of a Gordian worm,

0:02:19.695,0:02:21.588
or horsehair worm.

0:02:21.588,0:02:23.744
The worm grew to adult size within it,

0:02:23.744,0:02:26.782
but it needs to get into water in order to mate,

0:02:26.782,0:02:29.000
and it does that by releasing proteins

0:02:29.000,0:02:31.188
that addle the cricket's brain,

0:02:31.188,0:02:33.352
causing it to behave erratically.

0:02:33.352,0:02:35.307
When the cricket nears a body of water,

0:02:35.307,0:02:36.926
such as this swimming pool,

0:02:36.926,0:02:39.183
it jumps in and drowns,

0:02:39.183,0:02:40.908
and the worm wriggles out

0:02:40.908,0:02:44.764
of its suicidal corpse.

0:02:44.764,0:02:48.367
Crickets are really roomy. Who knew?

0:02:48.367,0:02:51.582
The tapeworm and the Gordian worm are not alone.

0:02:51.582,0:02:53.302
They are part of an entire cavalcade

0:02:53.302,0:02:55.379
of mind-controlling parasites,

0:02:55.379,0:02:59.328
of fungi, viruses, and worms and insects and more

0:02:59.328,0:03:01.901
that all specialize in subverting and overriding

0:03:01.901,0:03:04.110
the wills of their hosts.

0:03:04.110,0:03:05.847
Now, I first learned about this way of life

0:03:05.847,0:03:08.340
through David Attenborough's "Trials of Life"

0:03:08.340,0:03:10.062
about 20 years ago,

0:03:10.062,0:03:11.764
and then later through a wonderful book called

0:03:11.764,0:03:14.438
"Parasite Rex" by my friend Carl Zimmer.

0:03:14.438,0:03:16.581
And I've been writing about[br]these creatures ever since.

0:03:16.581,0:03:19.392
Few topics in biology enthrall me more.

0:03:19.392,0:03:23.098
It's like the parasites have subverted my own brain.

0:03:23.098,0:03:25.709
Because after all, they are always compelling

0:03:25.709,0:03:26.997
and they are delightfully macabre.

0:03:26.997,0:03:28.389
When you write about parasites,

0:03:28.389,0:03:30.473
your lexicon swells with phrases like

0:03:30.473,0:03:33.519
"devoured alive" and "bursts out of its body."

0:03:33.519,0:03:34.756
(Laughter)

0:03:34.756,0:03:36.296
But there's more to it than that.

0:03:36.296,0:03:38.398
I'm a writer, and fellow writers in the audience

0:03:38.398,0:03:41.107
will know that we love stories.

0:03:41.107,0:03:43.500
Parasites invite us to resist the allure

0:03:43.500,0:03:45.350
of obvious stories.

0:03:45.350,0:03:47.660
Their world is one of plot twists

0:03:47.660,0:03:51.267
and unexpected explanations.

0:03:51.267,0:03:52.763
Why, for example,

0:03:52.763,0:03:54.941
does this caterpillar

0:03:54.941,0:03:56.915
start violently thrashing about

0:03:56.915,0:03:58.583
when another insect gets close to it

0:03:58.583,0:04:00.840
and those white cocoons that it seems

0:04:00.840,0:04:02.278
to be standing guard over?

0:04:02.278,0:04:05.374
Is it maybe protecting its siblings?

0:04:05.374,0:04:06.823
No.

0:04:06.823,0:04:08.280
This caterpillar was attacked

0:04:08.280,0:04:11.499
by a parasitic wasp which laid eggs inside it.

0:04:11.499,0:04:12.933
The eggs hatched and the young wasps

0:04:12.933,0:04:14.247
devoured the caterpillar alive

0:04:14.247,0:04:15.918
before bursting out of its body.

0:04:15.918,0:04:18.193
See what I mean?

0:04:18.193,0:04:21.031
Now, the caterpillar didn't die.

0:04:21.031,0:04:23.616
Some of the wasps seemed to stay behind

0:04:23.616,0:04:26.953
and controlled it into defending their siblings

0:04:26.953,0:04:28.587
which are metamorphosing

0:04:28.587,0:04:30.607
into adults within those cocoons.

0:04:30.607,0:04:34.146
This caterpillar is a head-banging [br]zombie bodyguard

0:04:34.146,0:04:35.447
defending the offspring

0:04:35.447,0:04:37.669
of the creature that killed it.

0:04:37.669,0:04:41.535
(Applause)

0:04:41.535,0:04:45.830
We have a lot to get through. [br]I only have 13 minutes. (Laughter)

0:04:45.830,0:04:47.641
Now, some of you are probably just

0:04:47.641,0:04:49.859
desperately clawing for some solace

0:04:49.859,0:04:52.119
in the idea that these things are oddities

0:04:52.119,0:04:54.050
of the natural world, that they are outliers,

0:04:54.050,0:04:55.870
and that point of view is understandable,

0:04:55.870,0:04:58.248
because by their nature, parasites are quite small

0:04:58.248,0:04:59.588
and they spend a lot of their time

0:04:59.588,0:05:02.542
inside the bodies of other things.

0:05:02.542,0:05:04.313
They're easy to overlook,

0:05:04.313,0:05:06.898
but that doesn't mean that they aren't important.

0:05:06.898,0:05:09.103
A few years back, a man called Kevin Lafferty

0:05:09.103,0:05:12.472
took a group of scientists[br]into three Californian estuaries

0:05:12.472,0:05:14.220
and they pretty much weighed and dissected

0:05:14.220,0:05:16.215
and recorded everything they could find,

0:05:16.215,0:05:17.443
and what they found

0:05:17.443,0:05:20.659
were parasites in extreme abundance.

0:05:20.659,0:05:22.703
Especially common were trematodes,

0:05:22.703,0:05:25.353
tiny worms that specialize in castrating their hosts

0:05:25.353,0:05:27.899
like this unfortunate snail.

0:05:27.899,0:05:30.788
Now, a single trematode is tiny, microscopic,

0:05:30.788,0:05:32.587
but collectively they weighed as much

0:05:32.587,0:05:34.773
as all the fish in the estuaries

0:05:34.773,0:05:38.480
and three to nine times more than all the birds.

0:05:38.480,0:05:40.474
And remember the Gordian worm that I showed you,

0:05:40.474,0:05:41.862
the cricket thing?

0:05:41.862,0:05:44.415
One Japanese scientist called Takuya Sato

0:05:44.415,0:05:46.111
found that in one stream,

0:05:46.111,0:05:48.082
these things drive so many crickets

0:05:48.082,0:05:49.381
and grasshoppers into the water

0:05:49.381,0:05:50.690
that the drowned insects

0:05:50.690,0:05:54.994
make up some 60 percent of the diet of local trout.

0:05:54.994,0:05:57.928
Manipulation is not an oddity.

0:05:57.928,0:06:00.048
It is a critical and common part

0:06:00.048,0:06:01.939
of the world around us,

0:06:01.939,0:06:03.460
and scientists have now found

0:06:03.460,0:06:05.942
hundreds of examples of such manipulators,

0:06:05.942,0:06:08.379
and more excitingly, they're starting to understand

0:06:08.379,0:06:12.437
exactly how these creatures control their hosts.

0:06:12.437,0:06:14.961
And this is one of my favorite examples.

0:06:14.961,0:06:17.418
This is Ampulex compressa,

0:06:17.418,0:06:19.642
the emerald cockroach wasp,

0:06:19.642,0:06:21.936
and it is a truth universally acknowledged

0:06:21.936,0:06:23.579
that an emerald cockroach wasp in possession

0:06:23.579,0:06:25.375
of some fertilized eggs

0:06:25.375,0:06:27.841
must be in want of a cockroach.

0:06:27.841,0:06:29.041
When she finds one,

0:06:29.041,0:06:31.252
she stabs it with a stinger

0:06:31.252,0:06:32.701
that is also a sense organ.

0:06:32.701,0:06:35.372
This discovery came out three weeks ago.

0:06:35.372,0:06:37.677
She stabs it with a stinger that is a sense organ

0:06:37.677,0:06:39.471
equipped with small sensory bumps

0:06:39.471,0:06:42.026
that allow her to feel the distinctive texture

0:06:42.026,0:06:44.133
of a roach's brain.

0:06:44.133,0:06:47.174
So like a person blindly rooting about in a bag,

0:06:47.174,0:06:49.651
she finds the brain, and she injects it with venom

0:06:49.651,0:06:53.852
into two very specific clusters of neurons.

0:06:53.852,0:06:56.935
Israeli scientists Frederic Libersat and Ram Gal

0:06:56.935,0:07:00.660
found that the venom is a[br]very specific chemical weapon.

0:07:00.660,0:07:03.204
It doesn't kill the roach, nor does it sedate it.

0:07:03.204,0:07:05.331
The roach could walk away

0:07:05.331,0:07:08.356
or fly or run if it chose to,

0:07:08.356,0:07:10.231
but it doesn't choose to,

0:07:10.231,0:07:13.611
because the venom nixes its motivation to walk,

0:07:13.611,0:07:14.725
and only that.

0:07:14.725,0:07:18.431
The wasp basically un-checks[br]the escape-from-danger box

0:07:18.431,0:07:20.271
in the roach's operating system,

0:07:20.271,0:07:23.504
allowing her to lead her helpless victim

0:07:23.504,0:07:25.615
back to her lair by its antennae

0:07:25.615,0:07:28.254
like a person walking a dog.

0:07:28.254,0:07:29.931
And once there, she lays an egg on it,

0:07:29.931,0:07:32.207
egg hatches, devoured alive, bursts out of body,

0:07:32.207,0:07:34.396
yadda yadda yadda, you know the drill.

0:07:34.396,0:07:37.626
(Laughter) (Applause)

0:07:37.626,0:07:40.506
Now I would argue that, once stung,

0:07:40.506,0:07:42.511
the cockroach isn't a roach anymore.

0:07:42.511,0:07:44.861
It's more of an extension of the wasp,

0:07:44.861,0:07:47.704
just like the cricket was an[br]extension of the Gordian worm.

0:07:47.719,0:07:50.519
These hosts won't get to survive or reproduce.

0:07:50.519,0:07:52.318
They have as much control over their own fates

0:07:52.318,0:07:54.102
as my car.

0:07:54.102,0:07:55.602
Once the parasites get in,

0:07:55.602,0:07:58.167
the hosts don't get a say.

0:07:58.167,0:07:59.481
Now humans, of course,

0:07:59.481,0:08:01.513
are no stranger to manipulation.

0:08:01.513,0:08:03.873
We take drugs to shift the chemistries of our brains

0:08:03.873,0:08:05.004
and to change our moods,

0:08:05.004,0:08:09.024
and what are arguments or advertising or big ideas

0:08:09.024,0:08:12.727
if not an attempt to influence someone else's mind?

0:08:12.727,0:08:14.254
But our attempts at doing this

0:08:14.254,0:08:16.032
are crude and blundering compared

0:08:16.032,0:08:19.127
to the fine-grained specificity of the parasites.

0:08:19.127,0:08:21.519
Don Draper only wishes he was as elegant

0:08:21.519,0:08:26.479
and precise as the emerald cockroach wasp.

0:08:26.479,0:08:30.219
Now, I think this is part of what makes parasites

0:08:30.219,0:08:33.005
so sinister and so compelling.

0:08:33.005,0:08:35.627
We place such a premium on our free will

0:08:35.627,0:08:36.888
and our independence

0:08:36.888,0:08:38.469
that the prospect of losing those qualities

0:08:38.469,0:08:39.747
to forces unseen

0:08:39.747,0:08:42.915
informs many of our deepest societal fears.

0:08:42.915,0:08:45.368
Orwellian dystopias and shadowy cabals

0:08:45.368,0:08:47.222
and mind-controlling supervillains --

0:08:47.222,0:08:50.250
these are tropes that fill our darkest fiction,

0:08:50.250,0:08:55.048
but in nature, they happen all the time.

0:08:55.048,0:08:57.680
Which leads me to an obvious

0:08:57.680,0:08:59.980
and disquieting question:

0:08:59.980,0:09:01.703
Are there dark, sinister parasites

0:09:01.703,0:09:03.659
that are influencing our behavior

0:09:03.659,0:09:05.234
without us knowing about it,

0:09:05.234,0:09:08.053
besides the NSA?

0:09:08.053,0:09:09.360
If there are any —

0:09:09.360,0:09:13.326
(Laughter) (Applause)

0:09:13.326,0:09:15.648
I've got a red dot on my forehead now, don't I?

0:09:15.648,0:09:18.175
(Laughter)

0:09:18.175,0:09:21.129
If there are any, this is a good candidate for them.

0:09:21.129,0:09:24.407
This is Toxoplasma gondii, or Toxo, for short,

0:09:24.407,0:09:25.996
because the terrifying creature

0:09:25.996,0:09:28.206
always deserves a cute nickname.

0:09:28.206,0:09:30.488
Toxo infects mammals,

0:09:30.488,0:09:31.966
a wide variety of mammals,

0:09:31.966,0:09:35.047
but it can only sexually reproduce in a cat.

0:09:35.047,0:09:37.808
And scientists like Joanne Webster have shown that

0:09:37.808,0:09:40.001
if Toxo gets into a rat or a mouse,

0:09:40.001,0:09:43.473
it turns the rodent into a cat-seeking missile.

0:09:43.473,0:09:46.549
If the infected rat smells the delightful odor

0:09:46.549,0:09:47.773
of cat piss,

0:09:47.773,0:09:50.493
it runs towards the source of the smell

0:09:50.493,0:09:54.412
rather than the more sensible direction of away.

0:09:54.412,0:09:56.947
The cat eats the rat. Toxo gets to have sex.

0:09:56.947,0:09:59.819
It's a classic tale of Eat, Prey, Love.

0:09:59.819,0:10:04.487
(Laughter) (Applause)

0:10:07.512,0:10:10.714
You're very charitable, generous people.

0:10:10.714,0:10:13.299
Hi, Elizabeth, I loved your talk.

0:10:13.299,0:10:16.667
How does the parasite control its host

0:10:16.667,0:10:17.871
in this way?

0:10:17.871,0:10:19.197
We don't really know.

0:10:19.197,0:10:20.885
We know that Toxo releases an enzyme

0:10:20.885,0:10:22.877
that makes dopamine, a substance involved

0:10:22.877,0:10:24.690
in reward and motivation.

0:10:24.690,0:10:27.121
We know it targets certain parts of a rodent's brain,

0:10:27.121,0:10:29.586
including those involved in sexual arousal.

0:10:29.586,0:10:31.787
But how those puzzle pieces fit together

0:10:31.787,0:10:33.631
is not immediately clear.

0:10:33.631,0:10:35.927
What is clear is that this thing

0:10:35.927,0:10:37.319
is a single cell.

0:10:37.319,0:10:38.689
This has no nervous system.

0:10:38.689,0:10:39.821
It has no consciousness.

0:10:39.821,0:10:41.122
It doesn't even have a body.

0:10:41.122,0:10:43.746
But it's manipulating a mammal?

0:10:43.746,0:10:45.172
We are mammals.

0:10:45.172,0:10:47.606
We are more intelligent than a mere rat, to be sure,

0:10:47.606,0:10:50.130
but our brains have the same basic structure,

0:10:50.130,0:10:51.523
the same types of cells,

0:10:51.523,0:10:53.111
the same chemicals running through them,

0:10:53.111,0:10:55.430
and the same parasites.

0:10:55.430,0:10:57.778
Estimates vary a lot, but some figures suggest

0:10:57.778,0:11:00.138
that one in three people around the world

0:11:00.138,0:11:01.851
have Toxo in their brains.

0:11:01.851,0:11:05.339
Now typically, this doesn't lead to any overt illness.

0:11:05.339,0:11:07.217
The parasite holds up in a dormant state

0:11:07.217,0:11:09.400
for a long period of time.

0:11:09.400,0:11:11.439
But there's some evidence that those people

0:11:11.439,0:11:14.473
who are carriers score slightly differently

0:11:14.473,0:11:17.202
on personality questionnaires than other people,

0:11:17.202,0:11:19.686
that they have a slightly [br]higher risk of car accidents,

0:11:19.686,0:11:22.289
and there's some evidence[br]that people with schizophrenia

0:11:22.289,0:11:24.835
are more likely to be infected.

0:11:24.835,0:11:27.018
Now, I think this evidence is still inconclusive,

0:11:27.018,0:11:28.929
and even among Toxo researchers,

0:11:28.929,0:11:31.369
opinion is divided as to whether the parasite

0:11:31.369,0:11:33.754
is truly influencing our behavior.

0:11:33.754,0:11:36.413
But given the widespread[br]nature of such manipulations,

0:11:36.413,0:11:38.258
it would be completely implausible

0:11:38.258,0:11:39.907
for humans to be the only species

0:11:39.907,0:11:42.853
that weren't similarly affected.

0:11:42.853,0:11:47.246
And I think that this capacity to constantly

0:11:47.246,0:11:50.113
subvert our way of thinking about the world

0:11:50.113,0:11:51.660
makes parasites amazing.

0:11:51.660,0:11:54.869
They're constantly inviting us to[br]look at the natural world sideways,

0:11:54.869,0:11:57.040
and to ask if the behaviors we're seeing,

0:11:57.040,0:11:58.527
whether they're simple and obvious

0:11:58.527,0:12:00.060
or baffling and puzzling,

0:12:00.060,0:12:02.146
are not the results of individuals

0:12:02.146,0:12:03.999
acting through their own accord

0:12:03.999,0:12:05.808
but because they are being bent

0:12:05.808,0:12:08.478
to the control of something else.

0:12:08.478,0:12:10.518
And while that idea may be disquieting,

0:12:10.518,0:12:13.441
and while parasites' habits may be very grisly,

0:12:13.441,0:12:15.233
I think that ability to surprise us

0:12:15.233,0:12:18.408
makes them as wonderful and as charismatic

0:12:18.408,0:12:21.772
as any panda or butterfly or dolphin.

0:12:21.772,0:12:23.991
At the end of "On the Origin of Species,"

0:12:23.991,0:12:26.980
Charles Darwin writes about the grandeur of life,

0:12:26.980,0:12:29.633
and of endless forms most beautiful

0:12:29.633,0:12:31.065
and most wonderful,

0:12:31.065,0:12:33.644
and I like to think he could easily have been talking

0:12:33.644,0:12:36.824
about a tapeworm that makes shrimp sociable

0:12:36.824,0:12:40.073
or a wasp that takes cockroaches for walks.

0:12:40.073,0:12:43.222
But perhaps, that's just a parasite talking.

0:12:43.222,0:12:44.953
Thank you.

0:12:44.953,0:12:48.953
(Applause)