關於動物保護的程度，於道德層面的考量，我們往往是基於，到底該動物能不能感受到痛苦，做為主要參考的標準。
能夠感受到痛苦的動物，我們認為是和人類一樣，是「高一個等級」的生物，值得我們更善心良意地對待。
因此，以往會認為，像老鼠、𤠣子等哺乳動物，對痛苦有感，亦是動物試驗的大宗，我們因此立法，要儘量避免對它們增加不必要的痛苦。
對於家禽家畜，牛羊豬鷄鴨鵝等等的飼養和屠殺的過程，也是希望減少它們的痛苦。
至於貓、狗等等寵物，那就更不用說了。
而像魚類、無脊椎的頭足類等等的「冷血動物」，以前我們認為它們應該不知道疼痛為何物，所以不用太過於在意。
或許，就是基於這個迷思，有一種素食法叫做「魚素」，吃魚和植物，把魚視為和植物一樣，是沒有疼痛感覺的生物，放心吃它們也沒有什麼關係，認為也是一種「素食」。
這些「冷血」生物，難道真的沒有疼痛的感覺嗎？
根據科學家的研究，魚、寄居蟹、頭足類中的章魚和魷魚等等，在面對刺激的時候，都會有各種有不同程度的疼痛反應。
對於疼痛的反應，不能針對大類，譬如魚類、甲殼類、頭足類等等，整個一大類就遽然下簡單的結論。其實，在同一類中，不同物種之間的差異也是蠻大的。
《An Immense World》這本書，關於疼痛(Pains)有專章在討論，很有意思。
還是要回到《An Immense World》這本書的思路主軸，我們固然可以將生物歸類，但要避免從「擬人論」(Anthropomorphism) 的角度去想生物，而是要從「Umwelt 」的角度，也就是回歸到生物本身怎麼看世界、怎麼和世界互動的角度，來研究生物。
莊周夢蝶，醒來弄不清楚自己，是變成莊周的蝴蝶，還是夢中變成蝴蝶的莊周。《An Immense World》這本書告訴我們的是，要真正了解蝴蝶，非變成蝴蝶的莊周不可了。
節錄一些書中的段落，與大家分享：
Our experience of pain depends on a class of neurons called nociceptor.(The word is pronounced with a soft c, and comes from the Latin word nocere, meaning “to harm.”) The naked tips of these neurons pervade our skin and other organs. They are loaded with sensors that detect harmful stimuli-intense heat or cold, crushing pressures, acids, toxins, and chemicals released by injuries and inflammation. Nociceptors vary in their size, how excitable they are, and how quickly they transmit information–qualities that collectively sculpt a landscape of pricks, stabs, burns, throbs, cramps, and aches that we are unfortunate enough to experience.”
神經的末端的傷害感受器，分佈在皮膚表面及身體的其他器官裏。依照傷害所造成刺激的大小，而會感受到大小不同程度的疼痛感。
刺激的種類有：燙、凍、重壓、酸、毒素、受傷和發炎所引發的化學物質等等。
“…Nociception is the sensory process by which we detect damage. Pain is the suffering that ensues. Last week, when I accidentally touched a hot pan, the nociceptors in my skin sensed the scalding temperatures. That’s nociception, which triggered a reflex that forced my arm to withdraw before I realized what was happening. Shortly after, signals from those nociceptors reached my brain, which produced feelings of discomfort and distress. That’s pain.
The two are intimately linked but also distinct. Nociception occurred in my hand (and spinal cord); the pain was produced by my brain. They are the sensory and emotional halves of a process that, to most of us, feel inseparable.”
傷害感受是我們偵測到傷害的過程，而隨著察覺到傷害，腦袋接著就產生疼痛的感覺。
以被茶壺燙到為例，我們皮膚上的傷害感受器感受到高溫的燙，訊息傳到脊椎隨即產生反射動作，手馬上彈離熱源，接下來訊息傳到腦部，然後感到不舒服和壓力，這就是疼痛。
感覺受到傷害的是皮膚上傷害感受器的作業，而疼痛的感覺則是腦袋產生的。
“…Some painkillers duplicate this effect by acting on the central nervous system to dull pain without affecting nociception…
”I would still be fully aware that the sensation was there, but I felt very serene about it….”
People can also learn to ignore or even enjoy things that trigger nociceptors, like mustard, chilies, or intense heat.”
有些止痛劑的原理，就是作用於鈍化中樞神經。我們(的末梢神經)還是會感到受傷害，但是(腦袋中的)疼痛感卻會舒緩很多。
我們也可以透過學習，來忽視甚至享受傷害感受器所承受的刺激，諸如芥末、辣椒、高溫等等。
“Nociception is an ancient sense. It is so widespread and consistent across the animal kingdom that the same chemicals, opioids, can quell the nociceptors of humans, chickens, trout, sea slugs, and fruit flies creatures separated by around 800 million years of evolution.
But since pain is subjective, it is difficult to tell which creatures have it. Humans can barely do that with each other….”
感覺傷害是一種很古老的感知能力。在動物界，縱然經歷了8億年分開的演化，但是人類、鷄、鱒魚、海蛞蝓、果蠅等等，某些化學物質，譬如嗎啡，對他們的傷害感受器都有鎮定的效果。
但是疼痛是主觀的，很難了解哪些生物會感到疼痛。甚至在人跟人之間，也很難理解對方疼痛的程度。
“…in recent decades, and most scientists would now agree that mammals can feel pain. But fierce debates are still raging around other animal groups, including fish, insects, and crustaceans. At the core of these lingering controversies is the distinction between nociception and pain….“
在近幾十年，大部分的科學家都同意哺乳動物是感覺得到疼痛的，至於其他的動物，譬如魚類、昆蟲、甲殼類，則存在很大的爭議。主要的爭議點是在於傷害感受和疼痛的差別。
“Elwood studied the common hermit crab, which frequents European beaches and tucks its soft abdomen into empty seashells. These shells are valuable property, and the crabs are vulnerable without them. But Elwood and his colleague Mirjam Appel found that they will nonetheless evacuate if given a small electric shock. These flights looked reflexive, but the crabs didn’t always flee. It took a stronger shock to force them out of their favored periwinkle shells than it did to evict them from the less desirable flat-top shells. And they were half as likely to abandon their shells if they could smell the scent of predators in the water. “That told me that this isn’t a reflex,” Elwood says. In-stead, evacuation is a decision the crabs make after weighing up several sources of information.”
“The crabs also behaved differently long after the shocks. After fleeing, they wouldn’t return to their shells, despite being dangerously ex-posed. They groomed the part of their abdomens that got shocked.
And even when they didn’t relinquish their shells, they were quicker to accept a new one without the usual careful investigations. These data, Elwood says, are consistent with the idea of pain, but it’s impossible to know what crustaceans are really feeling. “I’m often asked if crabs and lobsters feel pain,” he tells me,
“and after 15 years of research, the answer is maybe.”
寄居蟹被電擊之後，雖然最終會離開它的寄居的殼，但是要它離開比較喜愛的殼，需要更大的電擊。而當它偵測到有掠食者出現的時候，被電擊後會離開殼的機率會減半，這些情況顯示，寄居蟹對電擊的反應並不完全是反射動作。
在接受電擊之後又過了一段時間，寄居蟹的行為依舊不同，它們不再回到蟹殼內，除非要躲避危險。它們對受電擊的肚子，會悉心照料。縱使它們沒有放棄本來的殼，但是一旦有新的殼，它們則會不經仔細檢查就搬到新殼內。這些和疼痛所產生的反應是一致的。
寄居蟹可以感到疼痛嗎？經過15年的研究，答案是模稜兩可的「或許」。
In September 2010, the European Union extended its regulations on animal research to cephalopods the group that includes octopuses, squid, and cuttlefish. Being invertebrates, cephalopods aren’t usually covered by laws that protect the welfare of backboned lab animals like mice or monkeys. But they also have much larger nervous systems than most invertebrates- 500 million neurons in an octopus, compared to 100,000 in a fruit fly. They show intelligent and flexible behaviors that surpass those of some vertebrates like reptiles and amphibians… “
歐盟在2010年，把動物實驗中對動物福利的保障範圍，從老鼠和𤠣子等這些主要的實驗脊椎動物，進一步把頭足類包含了進來，包括章魚、魷魚、烏賊等等。
“(Longfin squids) …,they never touched, groomed, or cradled their wounds, the way humans, rats, and even hermit crabs do. They could easily reach their stump with any of their other seven arms, but they didn’t try.
Even more surprisingly, …that injured squid behave as if their entire bodies were sore. When humans and other mammals get cut or bruised, the damaged area is painful but the rest of the body isn’t. If I singe my hand, it hurts when I prod the burn but not when I poke my foot. But when Crook damaged one of the squid’s fins, the nociceptors on the opposite fin were just as excitable as those on the wounded side…”
長鰭魷魚的實驗發現，它對受傷的部位，不像鼠類、人類或寄居蟹一樣，它不會刻意去照拂。而更令人驚訝的是，長鰭魷魚一旦受傷了，不只是受傷的地方受到影響，而是看起來好像全身都受傷了一樣。
“Octopuses will sometimes break off an arm if its tip is injured. When that happens, the stump will be more sensitive than the arms around it, and octopuses will cradle that stump in their beaks….that octopuses will avoid places where they’ve been injected with acetic acid, but gravitate to places where they receive painkillers. And once they’re injected with local anesthetic, they stop grooming their injured arms….Octopuses are capable of experiencing pain…”
章魚有時候會自斷臂膀以放棄受傷的觸手，斷臂處會變得特別敏感，章魚會把它放到嘴喙裏照拂。章魚會避開曾經注射它乙酸的地方，但會去接近得到止痛劑的地方。受傷的觸手一旦注射了麻藥，它就𣎴再刻意去照拂它了。從各種跡象顯示，章魚是會感受到到疼痛的。
Consider also the squid and octopuses. Both are cephalopods, but they’ve been evolving separately for more than 300 million years, roughly the same amount of time that separates mammals and birds. Their bodies and lifestyles are utterly different, so it’s no surprise that their nervous systems function very differently after injury.
魷魚和章魚雖然都屬於頭足類動物，但是從3億年前就分開各自演化，大概和哺動物與鳥類分開的時間差不多久。它們身體結構和棲息型態差異很大，神經系統在面對傷害的反應會很不一樣，就𣎴足為奇了。
Rather than asking if cephalopods experience pain, we might ask which ones experience it, and how. The same goes for the 34,000 known species of fish, the 67,000 known species of crustaceans, and the who-knows-how-many-million species of insects. It’s ridiculous to treat these groups as monolithic when we know, from other senses like vision and smell, that even closely related animals differ in how they perceive the world.
與其問頭足類能不能感到疼痛，我們不如問其中哪些能感到疼痛，然後是怎樣感到疼痛的。對於已知的34,000種魚類和67,000種甲殼類動物，以及種類數不知凡幾百萬的昆蟲，我們都要用同一種態度來研究。把那麽多物種視為相同，是很荒謬的。一如我們已經知道的，在視覺和嗅覺上面，就算很近親的物種，它們的表現都可能很不一樣呢！
“Instead of focusing on whether pain even exists, we might ask, as physiologist Catherine Williams told me, “In which conditions and from which stimuli is it an advantage to have it, experience it, and display it?”
結論就是，不會再研究到底疼痛存不存在，而是要研究，到底在什麼樣的情形下的怎麼樣的刺激，在生物擁有、經歷、並展現之下，(在演化上)是最有益處的。
*：ED Yong, “An Immense World,” 2022, Random House/New York
2023/9/4 Pains Damakey
