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>> DAVID ATTENBOROUGH: Incredibly, 80 percent of all insects

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live in jungles.

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Few are more successful
than the ants.

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There can be eight million
individuals in a single hectare.

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But jungle ants don't
have it all their own way.

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These bullet ants are showing
some worrying symptoms.

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Spores from a parasitic
fungus called cordyceps have

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infiltrated their
bodies and their minds.

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Its infected brain
directs this ant upwards,

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then utterly disorientated, it
grips a stem with its mandibles.

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Those afflicted, if they're
discovered by the workers,

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are quickly taken away and
dumped far away from the colony.

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It seems extreme, but
this is the reason why.

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Like something out of science
fiction,

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the fruiting body of the cordyceps erupts
from the ant's head.

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[EERIE MUSIC]

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It can take three weeks to grow.

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And when finished, the deadly
spores will burst from its tip.

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Then, any ant in the
vicinity will be

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in serious risk of death.

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The fungus is so virulent,

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it can wipe out whole
colonies of ants.

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And it's not just ants that
fall victim to this killer.

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There are literally thousands

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of different types
of cordyceps fungi.

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And remarkably, each
specializes on just one species.

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[ Music ]

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But these attacks do
have a positive effect

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on the jungle's diversity, since parasites

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like these stop any one group of
animal getting the upper hand.

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The more numerous a species becomes,

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the more likely it'll be attacked by its nemesis,

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a cordyceps fungus.