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  • To human eyes, the world at night is a formless canvas of grey.

  • Many nocturnal animals, on the other hand,

  • experience a rich and varied world bursting with details, shapes, and colors.

  • What is it, then, that separates moths from men?

  • Moths and many other nocturnal animals see at night

  • because their eyes are adapted to compensate for the lack of light.

  • All eyes, whether nocturnal or not,

  • depend on photoreceptors in the retina to detect light particles,

  • known as photons.

  • Photoreceptors then report information about these photons to other cells

  • in the retina and brain.

  • The brain sifts through that information and uses it to build up an image

  • of the environment the eye perceives.

  • The brighter the light is, the more photons hit the eye.

  • On a sunny day,

  • upwards of 100 million times more photons are available to the eye

  • than on a cloudy, moonless night.

  • Photons aren't just less numerous in darkness,

  • but they also hit the eye in a less reliable way.

  • This means the information that photoreceptors collect

  • will vary over time,

  • as will the quality of the image.

  • In darkness, trying to detect the sparse scattering of randomly arriving photons

  • is too difficult for the eyes of most daytime animals.

  • But for night creatures, it's just a matter of adaptation.

  • One of these adaptations is size.

  • Take the tarsier, whose eyeballs are each as big as its brain,

  • giving it the biggest eyes compared to head size of all mammals.

  • If humans had the same brain to eye ratio, our eyes would be the size of grapefruits.

  • The tarsier's enlarged orbs haven't evolved to make it cuter, however,

  • but to gather as much light as possible.

  • Bigger eyes can have larger openings, called pupils,

  • and larger lenses,

  • allowing for more light to be focused on the receptors.

  • While tarsiers scan the nocturnal scene with their enormous peepers,

  • cats use gleaming eyes to do the same.

  • Cats' eyes get their shine from a structure called the tapetum lucidum

  • that sits behind the photoreceptors.

  • This structure is made from layers of mirror-like cells containing crystals

  • that send incoming light bouncing back towards the photoreceptors

  • and out of the eye.

  • This results in an eerie glow,

  • and it also gives the photoreceptors a second chance to detect photons.

  • In fact, this system has inspired the artificial cats' eyes we use on our roads.

  • Toads, on the other hand, have adapted to take it slow.

  • They can form an image

  • even when just a single photon hits each photoreceptor per second.

  • They accomplish this with photoreceptors

  • that are more than 25 times slower than human ones.

  • This means toads can collect photons for up to four seconds,

  • allowing them to gather many more than our eyes do

  • at each visual time interval.

  • The downside is that this causes toads to react very slowly

  • because they're only receiving an updated image every four seconds.

  • Fortunately, they're accustomed to targeting sluggish prey.

  • Meanwhile, the night is also buzzing with insects,

  • such as hawk moths,

  • which can see their favorite flowers in color, even on a starlit night.

  • They achieve this by a surprising move -

  • getting rid of details in their visual perception.

  • Information from neighboring photoreceptors is grouped in their brains,

  • so the photon catch of each group is higher

  • compared to individual receptors.

  • However, grouping photoreceptors loses details in the image,

  • as fine details require a fine grid of photoreceptors,

  • each detecting photons from one small point in space.

  • The trick is to balance the need for photons with the loss of detail

  • to still find their flowers.

  • Whether eyes are slow, enormous, shiny, or coarse,

  • it's the combination of these biological adaptations

  • that gives nocturnal animals their unique visual powers.

  • Imagine what it might be like to witness through their eyes

  • the world that wakes up when the sun goes down.

To human eyes, the world at night is a formless canvas of grey.


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B2 中上級

TED-ED】動物はどうやって暗闇を見るのか?- アンナ・シュテックル (【TED-Ed】How do animals see in the dark? - Anna Stöckl)

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    吳D に公開 2021 年 01 月 14 日