Lv.3 Ability A_Eyesight

The animal kingdom boasts an incredible diversity of eyes. Some rotate independently, while others have squiggly-shaped（くねった形の）pupils（瞳孔）. Some have protective lid（まぶた）, others squirt（流す）blood. But which creature has the best sight? Which sees best in the darkness? Which sees the most detail? Which animal sees the most color? And finally, which detects motion the fastest?

All eyes take in sensory stimuli（感覚刺激）in the form of waves of light. To convert these into everything we see, eyes focus incoming（入ってくる）light onto photoreceptors（感受器官）. These cell（細胞）translate light into neural（神経の）signals and send them to the brain, where they’re finally processed（処理される）. The eyes that see best in darkness are those that capture as much available light as possible.

Colossal squids（ダイオウホウズキイカ）have soccer-ball-sized eyes– the largest known in existence. These may help them spot the faint（かすかな）glow（光）of sperm whales（マッコウクジラ）as they disturb light-producing organisms（生物）. Some fish have eyes that are unique among vertebrates（脊椎動物）because they use mirrors. For the brownsnout spookfish（茶色のゴーストフィッシュ）, each eye has an upward-facing lens and a downward-pointing mirror composed of tiny crystal plates that efficiently gather light. They can see up and down simultaneously（同時に）, and may perceive distinct shapes, even in the ocean’s depth.

Back on solid ground, arctic reindeer（北極のトナカイ）have adaptations（適応能力）to deal with months of darkness. The backs of reindeer eyes（眼底）change color, from gold in summer to blue in winter. Their blue-backed eyes are about 1,000 times more sensitive（敏感な）to light. This may allow reindeer to recognize important things in the snow like urine（尿）and lichen（地衣植物）.

When it comes to the sharpest vision, birds of prey（猛禽類：肉食で荒々しい鳥類）soar（高くそびえる）above the competition. To capture the most detail, an animal must have lots of photoreceptors（光受容体）in its eye, as well as increased visual processing power. Raptors（=birds of prey, 猛禽類）have an especially deep fovea（眼底にあるもっとも視力が良い部分）– a depression（くぼみ）in the back of their eye that fits more photoreceptors. So, Peregrine falcons（ハヤブサ）have vision that’s more than twice as sharp as human’s. They’re able to zero（狙いを定める）in on a rabbit from more than three kilometers away.

When crowning（王位を授ける） the creature with the best color vision, the picture（全体の状況）gets complicated. Different photoreceptors are sensitive to specific waves of light, meaning the colors we see are largely determined by what kinds of photoreceptors we have. Presumably（おそらく）, the more types of color photoreceptors an animal has, the better its color vision. Dogs have just two types. Humans have three. And we are far outdone（上回られている）by some birds, fishes, and insects. Bluebottle butterflies（アオスジアゲハ）have at least 15 types of photoreceptors. Seven of them are attuned（慣れた）to distinct blues and greens, which researchers think might help them track each other during high-speed chases. Mantis shrimp（シャコ）have a whopping（とてつもない）16 kinds of photoreceptors, with five reserved（確保する）just for the ultraviolet（紫外線の）, or UV, spectrum（スペクトル）, which humans can’t see.

But experiments suggest that the mantis shrimp’s ability to discriminate（区別する）between colors is more limited than you might expect. Exactly how they use their complex eyes is a mystery. Meanwhile, with just four kinds of color photoreceptors, goldfish（金魚）actually excel at discerning（見分ける）subtle differences in shades.

Finally, insects have mastered the ability to see the world… on the fly. The fastest motion vision requires photoreceptors that quickly sense changes in light, and a brain that rapidly processes the information. A movie shot at 24 frames（コマ）per second gives us the perception of near seamless（途切れない）motion. But insects would see a slideshow. Fly photoreceptors register（認識する）changes 10 times faster than we do, making them especially hard to catch.

These animals（*ビデオ参照）have some of the best vision we know of, but there’s no winner across the board（全域的の）. Each category has different top contenders（競争者）because vision requires tradeoffs（交換の関係）. So, some eyes are highly specialized（適応する、特化する）, while others, like ours, perform decently（ある程度満足できるほど）in many categories. From eyes the size of soccer balls to those that see in UV– the ways of looking at the world are as varied as the life forms in it.