WEBVTT

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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.