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To human eyes, the world at night
is a formless canvas of grey.
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Many nocturnal animals, on the other hand,
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experience a rich and varied world
bursting with details, shapes, and colors.
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What is it, then, that separates moths
from men?
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Moths and many other nocturnal animals
see at night
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because their eyes are adapted
to compensate for the lack of light.
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All eyes, whether nocturnal or not,
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depend on photoreceptors in the retina
to detect light particles,
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known as photons.
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Photoreceptors then report information
about these photons to other cells
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in the retina and brain.
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The brain sifts through that information
and uses it to build up an image
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of the environment the eye perceives.
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The brighter the light is,
the more photons hit the eye.
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On a sunny day,
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upwards of 100 million times
more photons are available to the eye
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than on a cloudy, moonless night.
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Photons aren't just less numerous
in darkness,
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but they also hit the eye
in a less reliable way.
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This means the information
that photoreceptors collect
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will vary over time,
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as will the quality of the image.
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In darkness, trying to detect the sparse
scattering of randomly arriving photons
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is too difficult for the eyes
of most daytime animals.
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But for night creatures,
it's just a matter of adaptation.
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One of these adaptations is size.
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Take the tarsier, whose eyeballs
are each as big as its brain,
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giving it the biggest eyes compared
to head size of all mammals.
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If humans had the same brain to eye ratio,
our eyes would be the size of grapefruits.
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The tarsier's enlarged orbs haven't
evolved to make it cuter, however,
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but to gather as much light as possible.
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Bigger eyes can have larger openings,
called pupils,
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and larger lenses,
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allowing for more light to be focused
on the receptors.
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While tarsiers scan the nocturnal scene
with their enormous peepers,
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cats use gleaming eyes to do the same.
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Cats' eyes get their shine from
a structure called the tapetum lucidum
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that sits behind the photoreceptors.
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This structure is made from layers
of mirror-like cells containing crystals
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that send incoming light
bouncing back towards the photoreceptors
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and out of the eye.
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This results in an eerie glow,
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and it also gives the photoreceptors
a second chance to detect photons.
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In fact, this system has inspired the
artificial cats eyes we use on our roads.
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Toads, on the other hand, have adapted
to take it slow.
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They can form an image
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even when just a single photon
hits each photoreceptor per second.
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They accomplish this with photoreceptors
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that are more than 25 times slower
than human ones.
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This means toads can collect photons
for up to four seconds,
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allowing them to gather many more
than our eyes do
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at each visual time interval.
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The downside is that this causes toads
to react very slowly
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because they're only receiving
an updated image every four seconds.
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Fortunately, they're accustomed
to targeting sluggish prey.
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Meanwhile, the night is also buzzing
with insects,
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such as hawk moths,
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which can see their favorite flowers
in color, even on a starlit night.
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They achieve this by a surprising move -
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getting rid of details
in their visual perception.
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Information from neighboring
photoreceptors is grouped in their brains,
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so the photon catch of each group
is higher
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compared to individual receptors.
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However, grouping photoreceptors
loses details in the image,
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as fine details require a fine grid
of photoreceptors,
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each detecting photons from one
small point in space.
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The trick is to balance the need
for photons with the loss of detail
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to still find their flowers.
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Whether eyes are slow, enormous,
shiny, or coarse,
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it's the combination
of these biological adaptations
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that gives nocturnal animals their unique
visual powers.
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Imagine what it might be like to witness
through their eyes
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the world that wakes up
when the sun goes down.