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Humans know the surprising prick
of a needle,

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the searing pain of a stubbed toe,

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and the throbbing of a toothache.

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We can identify many types of pain
and have multiple ways of treating it.

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But what about other species?

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How do the animals all around us
experience pain?

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It's important that we find out.

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We keep animals as pets,

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they enrich our environment,

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we farm many species for food,

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and we use them in experiments
to advance science and human health.

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Animals are clearly important to us,

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so it's equally important that we avoid
causing them unnecessary pain.

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For animals that are similar to us,
like mammals,

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it's often obvious when they're hurting.

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But there's a lot that isn't obvious,

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like whether pain relievers that work
on us also help them.

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And the more different 
an animal is from us,

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the harder it is to understand 
their experience.

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How do you tell whether 
a shrimp is in pain?

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Snake?

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A snail?

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In vertebrates, including humans,

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pain can be split 
into two distinct processes.

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In first, nerves and the skin sense
something harmful

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and communicate that information
to the spinal cord.

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There, motor neurons activate movements

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that make us rapidly 
jerk away from the threat.

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This is the physical recognition of harm
called nociception,

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and nearly all animals,

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even those with very simple 
nervous systems,

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experience it.

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Without this ability, animals would be
unable to avoid harm

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and their survival would be threatened.

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The second part is the conscious
recognition of harm.

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In humans, this occurs when the sensory
neurons in our skin

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make a second round of connections
via the spinal cord to the brain.

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There, millions of neurons in multiple
regions create the sensations of pain.

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For us, this is a very complex experience
associated with emotions like fear,

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panic,

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and stress,

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which we can communicate to others.

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But it's harder to know exactly
how animals experience

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this part of the process

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because most them can't show us
what they feel.

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However, we get clues from observing
how animals behave.

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Wild, hurt animals are known
to nurse their wounds,

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make noises to show their distress,

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and become reclusive.

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In the lab, scientists have discovered
that animals like chickens and rats

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will self-administer pain-reducing
drugs if they're hurting.

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Animals also avoid situations where
they've been hurt before,

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which suggests awareness of threats.

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We've reached the point that research
has made us so sure

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that vertebrates recognize pain

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that it's illegal in many countries
to needlessly harm these animals.

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But what about other types of animals
like invertebrates?

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These animals aren't legally protected,

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partly because their behaviors
are harder to read.

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We can make good guesses
about some of them,

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like oysters,

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worms,

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and jellyfish.

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These are examples of animals
that either lack a brain

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or have a very simple one.

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So an oyster may recoil when squirted
with lemon juice, for instance,

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because of nociception.

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But with such a simple nervous system,

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it's unlikely to experience 
the conscious part of pain.

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Other invertebrate animals 
are more complicated, though,

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like the octopus,

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which has a sophisticated brain

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and is thought to be one of the most 
intelligent invertebrate animals.

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Yet, in many countries, people continue
the practice of eating live octopus.

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We also boil live crawfish, shrimp, 
and crabs

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even though we don't really know
how they're effected either.

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This poses an ethical problem

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because we may be causing these animals
unnecessary suffering.

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Scientific experimentation,
though controversial, gives us some clues.

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Tests on hermit crabs show that they'll
leave an undesirable shell

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if they're zapped with electricity

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but stay if it's a good shell.

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And octopi that may originally curl up
an injured arm to protect it

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will risk using it to catch prey.

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That suggests that these animals make
value judgements around sensory input

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instead of just reacting 
reflexively to harm.

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Meanwhile, crabs have been known
to repeatedly rub a spot on their bodies

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where they've received an electric shock.

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And even seas lugs flinch

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when they know they're about 
to receive a noxious stimulus.

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That means they have some memory
of physical sensations.

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We still have a lot to learn about
animal pain.

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As our knowledge grows,

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it may one day allow us to live in a world
where we don't cause pain needlessly.