1
00:00:06,449 --> 00:00:09,209
Which of these entities 
has evolved the ability

2
00:00:09,212 --> 00:00:13,485
to manipulate an animal
many times its size?

3
00:00:13,485 --> 00:00:15,888
The answer is all of them.

4
00:00:15,888 --> 00:00:17,498
These are all parasites,

5
00:00:17,498 --> 00:00:21,925
organisms that live on 
or inside another host organism,

6
00:00:21,925 --> 00:00:25,591
which they harm and sometimes even kill.

7
00:00:25,591 --> 00:00:30,447
Parasite survival depends on transmitting
from one host to the next,

8
00:00:30,447 --> 00:00:33,532
sometimes through 
an intermediate species.

9
00:00:33,532 --> 00:00:38,568
Our parasites elegantly achieve this
by manipulating their host's behavior,

10
00:00:38,568 --> 00:00:42,011
sometimes through direct brain hijacking.

11
00:00:42,011 --> 00:00:45,155
For example, this is the Gordian worm.

12
00:00:45,155 --> 00:00:47,503
One of its hosts, this cricket.

13
00:00:47,503 --> 00:00:52,033
The Gordian worm needs water to mate,
but the cricket prefers dry land.

14
00:00:52,033 --> 00:00:54,635
So once it's big enough to reproduce,

15
00:00:54,635 --> 00:00:59,676
the worm produces proteins that garble
the cricket's navigational system.

16
00:00:59,676 --> 00:01:02,875
The confused cricket 
jumps around erratically,

17
00:01:02,875 --> 00:01:04,353
moves closer to water,

18
00:01:04,353 --> 00:01:08,602
and eventually leaps in,
often drowning in the process.

19
00:01:08,602 --> 00:01:10,976
The worm then wriggles out to mate

20
00:01:10,976 --> 00:01:13,353
and its eggs get eaten 
by little water insects

21
00:01:13,353 --> 00:01:14,700
that mature,

22
00:01:14,700 --> 00:01:16,083
colonize land,

23
00:01:16,083 --> 00:01:19,166
and are in turn, eaten by new crickets.

24
00:01:19,166 --> 00:01:22,438
And thus, the Gordian worm lives on.

25
00:01:22,438 --> 00:01:26,298
And here's the rabies virus,
another mind-altering parasite.

26
00:01:26,298 --> 00:01:28,990
This virus infects mammals, often dogs,

27
00:01:28,990 --> 00:01:32,592
and travels up the animal's 
nerves to its brain

28
00:01:32,592 --> 00:01:36,258
where it causes inflammation 
that eventually kills the host.

29
00:01:36,258 --> 00:01:40,371
But before it does, it often increases
its host's aggressiveness

30
00:01:40,371 --> 00:01:45,246
and ramps up the production
of rabies-transmitting saliva,

31
00:01:45,246 --> 00:01:47,435
while making it hard to swollow.

32
00:01:47,435 --> 00:01:51,124
These factors make the host 
more likely to bite another animal

33
00:01:51,124 --> 00:01:55,332
and more likely to pass 
the virus on when it does.

34
00:01:55,332 --> 00:02:01,021
And now, meet Ophiocordyceps,
also known as the zombie fungus.

35
00:02:01,021 --> 00:02:05,841
Its host of choice is tropical ants
that normally live in tree tops.

36
00:02:05,841 --> 00:02:10,858
After Ophiocordyceps spores 
pierce the ant's exoskeleton,

37
00:02:10,858 --> 00:02:15,482
they set off convulsions 
that make the ant fall from the tree.

38
00:02:15,482 --> 00:02:20,330
The fungus changes the ant's behavior,
compelling it to wander mindlessly

39
00:02:20,330 --> 00:02:24,829
until it stumbles onto a plant leaf with
the perfect fungal breeding conditions,

40
00:02:24,829 --> 00:02:26,979
which it latches onto.

41
00:02:26,979 --> 00:02:28,432
The ant then dies,

42
00:02:28,432 --> 00:02:34,982
and the fungus parasitizes its body
to build a tall, thin stalk from its neck.

43
00:02:34,982 --> 00:02:37,934
Within several weeks, 
the stalk shoots of spores,

44
00:02:37,934 --> 00:02:43,592
which turn more ants 
into six-legged leaf-seeking zombies.

45
00:02:43,592 --> 00:02:48,863
One of humanity's most deadly assaliants
is behavior-altering parasite,

46
00:02:48,863 --> 00:02:50,848
though if it's any consolation,

47
00:02:50,848 --> 00:02:54,016
it's not our brains 
that are being hijacked.

48
00:02:54,016 --> 00:02:57,968
I'm talking about Plasmodium,
which causes malaria.

49
00:02:57,968 --> 00:03:02,082
This parasite needs mosquitoes
to shuttle it between hosts,

50
00:03:02,082 --> 00:03:05,908
so it makes them bite more frequently
and for longer.

51
00:03:05,908 --> 00:03:09,020
There's also evidence 
that humans infected with malaria

52
00:03:09,020 --> 00:03:11,020
are more attractive to mosquitoes,

53
00:03:11,020 --> 00:03:14,685
which will bite them 
and transfer the parasite further.

54
00:03:14,685 --> 00:03:17,737
This multi-species system is so effective,

55
00:03:17,737 --> 00:03:22,430
that there are hundreds of millions
of malaria cases every year.

56
00:03:22,430 --> 00:03:24,742
And finally, there are cats.

57
00:03:24,742 --> 00:03:28,383
Don't worry, there probably 
aren't any cats living in your body

58
00:03:28,383 --> 00:03:29,848
and controlling your thoughts.

59
00:03:29,848 --> 00:03:31,550
I mean, probably.

60
00:03:31,550 --> 00:03:35,305
But there is a microorganism
called Toxoplasma

61
00:03:35,305 --> 00:03:39,733
that needs both cats and rodents
to complete its life cycle.

62
00:03:39,733 --> 00:03:43,082
When a rat gets infected
by eating cat feces,

63
00:03:43,082 --> 00:03:47,057
the parasite changes chemical levels
in the rat's brain,

64
00:03:47,057 --> 00:03:50,498
making it less cautious
around the hungry felines,

65
00:03:50,498 --> 00:03:52,750
maybe even attracted to them.

66
00:03:52,750 --> 00:03:54,515
This makes them easy prey,

67
00:03:54,515 --> 00:03:59,493
so these infected rodents
get eaten and pass the parasite on.

68
00:03:59,493 --> 00:04:01,774
Mind control successful.

69
00:04:01,774 --> 00:04:06,209
There's even evidence that the parasite
affects human behavior.

70
00:04:06,209 --> 00:04:08,683
In most cases, 
we don't completely understand

71
00:04:08,683 --> 00:04:12,988
how these parasites manage 
their feats of behavior modification.

72
00:04:12,988 --> 00:04:14,528
But from what we do know,

73
00:04:14,528 --> 00:04:17,005
we can tell that they have 
a pretty diverse toolbox.

74
00:04:17,005 --> 00:04:21,434
Gordian worms seem to affect
crickets' brains directly.

75
00:04:21,434 --> 00:04:23,654
The malaria parasite, on the other hand,

76
00:04:23,654 --> 00:04:26,897
blocks an enzyme 
that helps the mosquitoes feed,

77
00:04:26,897 --> 00:04:30,823
forcing them to bite over 
and over and over again.

78
00:04:30,823 --> 00:04:34,509
The rabies virus may cause 
that snarling, slobbering behavior

79
00:04:34,509 --> 00:04:37,972
by putting the immune system
into overdrive.

80
00:04:37,972 --> 00:04:39,333
But whatever the method,

81
00:04:39,333 --> 00:04:41,000
when you think about how effectively

82
00:04:41,000 --> 00:04:44,437
these parasites control 
the behavior of their hosts,

83
00:04:44,437 --> 00:04:51,374
you may wonder how much of human behavior
is actually parasites doing the talking?

84
00:04:51,374 --> 00:04:55,039
Since more than half of the species
on Earth are parasites,

85
00:04:55,039 --> 00:04:58,057
it could be more than you think.