How do animals see in the dark? - Anna Stöckl

譯者: Helen Chang 審譯者: S Sung

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

人眼看見的夜間世界 是個隱約的灰色帆布。

Many nocturnal animals, on the other hand,

另一方面，許多夜行動物體驗

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

有著豐富細節、形狀、顏色， 多姿多彩的世界。

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

人與飛蛾的差異是什麼呢？

Moths and many other nocturnal animals see at night

飛蛾和其他許多夜行動物 夜裡能看得見，

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

是因為牠們的眼睛已經適應， 彌補了光線的不足。

All eyes, whether nocturnal or not,

無論是否是夜行的動物，所有的眼睛

depend on photoreceptors in the retina to detect light particles,

都靠「視網膜」的「受光體」 來偵測光的粒子，

known as photons.

稱為「光子」。

Photoreceptors then report information about these photons to other cells

受光體把光子的資訊

傳給視網膜的其他細胞和大腦。

in the retina and brain.

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

大腦篩選資訊，

用來建立眼睛感知的環境形象。

of the environment the eye perceives.

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

光越亮，就有越多的光子進入眼睛。

On a sunny day,

在陽光明媚的日子，

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

眼睛接收到的光子 是無月陰夜的數億倍。

than on a cloudy, moonless night.

Photons aren't just less numerous in darkness,

在黑暗中，不只光子的數量少很多，

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

光子進入眼睛的方式也不可靠。

This means the information that photoreceptors collect

這意味著受光體接收的資訊

will vary over time,

會隨時間而變化，

as will the quality of the image.

改變圖像的品質。

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

在黑暗中，試圖偵測稀疏、 隨機抵達的散射光子

is too difficult for the eyes of most daytime animals.

對於大部分的白天動物而言， 太困難了。

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

但對於夜間生物而言， 只是個適應的問題。

One of these adaptations is size.

其中之一的適應機制是眼睛的大小。

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

以眼鏡猴為例，眼球和腦一般大，

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

牠的眼頭比例， 是所有哺乳動物中最大的。

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

如果人類有同樣的腦眼比例， 我們的眼睛將會大如葡萄柚。

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

眼鏡猴演化的大眼球， 不是為使它變得可愛，

but to gather as much light as possible.

而是要盡可能地多收集光子。

Bigger eyes can have larger openings, called pupils,

大的眼睛有更大的開口， 也就是「瞳孔」，

and larger lenses,

和更大的鏡頭，

allowing for more light to be focused on the receptors.

允許更多的光聚焦在受光體上。

While tarsiers scan the nocturnal scene with their enormous peepers,

眼鏡猴以巨大的眼睛掃描夜景，

cats use gleaming eyes to do the same.

而貓用的是閃閃發亮的眼睛。

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

貓眼的亮光來自 「脈絡膜毯」這個結構，

that sits behind the photoreceptors.

位於受光體的後方。

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

這結構由層層含有像鏡子模樣、 晶體結構的細胞所組成，

that send incoming light bouncing back towards the photoreceptors

把射入光反彈給受光體，

and out of the eye.

然後從眼睛出去。

This results in an eerie glow,

如此產生了詭異的亮光，

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

並給受光體二次偵測光子的機會。

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

事實上，貓眼啟發我們設計出 嵌在馬路上的反光人工貓眼。

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

另一方面，蟾蜍適應了慢活。

They can form an image

牠們能看見成形的圖像，

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

即使其受光體每秒只收到一粒光子。

They accomplish this with photoreceptors

比人慢25倍的受光體

that are more than 25 times slower than human ones.

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

意味著它收集光子的間隔長達4秒，

allowing them to gather many more than our eyes do

使得眼睛能在 每個間隔4秒的視覺時間裡

at each visual time interval.

比人收集更多的光子。

The downside is that this causes toads to react very slowly

缺點是使得蟾蜍反應遲緩，

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

因為每四秒才更新一次圖像。

Fortunately, they're accustomed to targeting sluggish prey.

還好牠們習慣獵食動作緩慢的獵物。

Meanwhile, the night is also buzzing with insects,

同時，夜間嗡嗡飛著的昆蟲，

such as hawk moths,

如鷹蛾，

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

能看到喜愛的花朵色彩， 即使在只有星光照明的夜空。

They achieve this by a surprising move -

牠們通過異乎尋常的演化：

getting rid of details in their visual perception.

省略細節的受光體。

Information from neighboring photoreceptors is grouped in their brains,

鄰近的受光體所接收的信息 在腦裡組合成群，

so the photon catch of each group is higher

因此每個群組捕獲的光子 較個別受光體的更多。

compared to individual receptors.

However, grouping photoreceptors loses details in the image,

然而，群組的受光體 損失了圖像的細節，

as fine details require a fine grid of photoreceptors,

因為更多的細節 需要更細密的受光體網絡，

each detecting photons from one small point in space.

網絡的每一點都要有受光體。

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

關鍵是要平衡細節與光子數目

to still find their flowers.

讓牠仍能找到花朵。

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

無論眼睛是緩慢的、巨大的、 有光的，或粗略看的，

it's the combination of these biological adaptations

這些生物適應的組合

that gives nocturnal animals their unique visual powers.

給予夜行動物獨特的視覺能力。

Imagine what it might be like to witness through their eyes

想像透過牠們的眼睛看到

the world that wakes up when the Sun goes down.

太陽下山後的甦醒世界，是何模樣。