“An Immense World” 這本書，專章講到「Heat」，生物對熱的知覺。
先說說熱死人這件事情。
觀念上，温度太高當然危險，但在相對溼度比較高的狀況，將使得人更難排汗，體感溫度會比較高，那麼絕對是雪上加霜。
譬如，相對溼度60%的時候，氣溫31°C的體感溫度就已經到達要注意的35°C了；如果相對溼度75%，則氣溫30°C的體感溫度會到36°C；而如果相對溼度90%，則氣溫29°C的體感溫度就已經高達37°C了。(資料參考careonline網站)
Rule of thumb：溼度從60%增加到90%，體感溫度會比氣溫高上4°C～8°C！
在相對溼度一般比較高的台灣，一旦氣溫昇高，因此而「熱死」的可能性會更高，大家不得不小心謹慎預防了。
每一種生物，都有一個適合生存的溫度範圍，人類當然也不例外。
人類及其他哺乳動物、鳥類等，都是衡溫動物，必須隨時保持體溫，氣溫太高或太低，都會增加保持體溫𧗾定的難度。
爬蟲類及魚類等，是變溫動物，體溫可以隨著環境的變化而改變，對環境溫度的高低變化的忍受程度，就比較大，但也有一定的耐受範圍。
衡溫動物，活動力較強，但是缺點就是它們的體溫，會讓它們容易成為掠食者或寄生蟲的目標。
某些動物是透過偵測獵物體溫來尋找目標的，諸如吸血蝙蝠和某些蛇類。
而像蟯蟲、鈎蟲這種寄生蟲，也是透過偵測體溫來找到宿主的。
偵測溫度，就是偵測紅外線，是很耗費能量的。俱備這種能力，可是大投資大收穫喔！
摘錄一些, “An Immense World“ 這本書中關於”Heat”的部分，跟大家分享：
“All living things are deeply affected by temperature. If conditions are too cold, chemical reactions slow to a useless crawl. If they are too hot, proteins and other molecules of life lose their shape and fall apart.
These effects constrain most of life to a Goldilocks zone where the temperature is just right. The limits of that zone vary, but they always exist, which is why every animal with a nervous system has a way of sensing and responding to temperature.”
生物深受溫度的影響。溫度太低，生化反應太慢，生物將難以行動；而如果溫度太高，蛋白質及其他分子將變型解離。所以，每一種生物都有一個最適合生存的溫度範圍，它們都有一個敏銳的系統，用來偵測溫度並及時作出回應。
“Animals use a variety of temperature sensors, and the most thoroughly studied of these are a group of proteins called TRP channels. They are found throughout the body on the surface of sensory neurons, where they act as tiny gates that open when they reach the right temperature. When this happens, ions enter the neurons, electrical signals travel to the brain, and we feel the sensations of hot or cold. Some TRP channels are tuned to hot temperatures, and others to cold ones. (Cold isn’t just the absence of hot; it’s a different sense in its own right.)”
生物偵測溫度，有各種方式，其中一種叫做TRP蛋白質的，科學家研究得最為透徹。
TRP蛋白質，常見於生物體表感覺神經末端，就像一個個開關，當達到特定溫度時，離子進入神經元，訊號傳至大腦，就可以感受到冷熱的程度。
有些TRP蛋白質是負責偵測熱，有些則是對冷會產生反應。
“…Warm-blood animals produce their own heat, and their versions of cold sensor TRPM8 alert them if their body temperature start dipping below a narrow comfortable range. In a rat, …at around 24°C(75°F). In a chicken…29°C(84°F)…Cold-blooded animals, by contract, rely on their environment for warm, and their body temperatures fluctuate over a wide range. Consequently, their versions of TRPM8 are typically set much lower——at 14°C(57°F) in frogs…”
衡溫動物在身體的溫度降至某特定溫度的時候，偵測低溫的TRPM8蛋白質就會發出警告的訊號。對老鼠而言，這個溫度是24°C(75°F)；對鷄而言，則是29°C(84°F)。
冷血動物的體溫，會隨著環境的溫度而波動，它們可以忍受的低溫就比較低，譬如青蛙就可以低到14°C(57°F)。
“The TRPV1 sensor, which detects painful heat,…It activates at 45°C(113°F) in chickens, 42°C(108°F) for mice and humans, 38°C(100°F) in frogs, and 33°C(91°F) in zebrafish…the squirrels…55°C(131°F)…The Saharan silver ant…53°C(127°F)…Snow flies…-6°C(21°F)…”
TRPV1蛋白則是偵測高溫用的。鷄在高於45°C(113°F) 時，它的TRPV1蛋白就會發出警告的訊號，老鼠和人都是在42°C(108°F)；青蛙是在38°C(100°F)；斑馬魚是在33°C(91°F)；松鼠可以忍受到55°C(131°F)；撒哈拉銀蟻是53°C(127°F)；雪蠅則是很不耐高溫的-6°C(21°F)。
“…flies could easily stay within air spaces that are kept to 25°C(77°F), which they love, while avoiding neighboring zones of 30°C(86°F), which they dislike, or 40°C(104°F), which kills them…The fly’s ability is called thermotaxi, and it’s common in the animal kingdom…”
蒼蠅最喜歡的溫度是25°C(77°F)，它們會避開超過30°C(86°F)的地方，而一旦溫度高達40°C(104°F)，那會要了它們的命。這種溫度變化的調適能力，在動物界是很普遍的。
“Moths are famously drawn to flames, but it’s the light that attracts them…Below their wings and just behind their middle legs, these insects have a pair of pits..each is filled with fluid and encloses the tip of a pressure-sensitive neuron. When infrared radiation hit the spheres, the fluid inside them heats up and expands. It can’t bulge outward because the spheres have hard exteriors, so instead it squeezes the nerves, causing them to fire…”
我們說飛蛾撲火，其實它們是受到光和熱的吸引，而不是火……在這些昆蟲翅膀下，位於中間的那一對腳上，有一對孔洞，裏面敏感的神經上充滿了液體，當紅外線照射到那些孔洞裏的液體加熱之後，對下面的神經產生壓力，就發出了熱源的訊號了。
“Nematodes are one of the most diverse and numerous groups of animals, including tens of thousands of species that are mostly harmless to human…The exceptions includes species…—Strongyloides stercoralis, the threadworm. Its larvae abound in soil and water that are contaminated by feces. If an unlucky person stands or walks through such places, the worms swim toward them and penetrate their skin. Threadworms, along with hookworms and other skin penetrating nematodes, infect around 800 million people around the world, from Vietnam to Alabama. They case gastrointestinal problems, stunted development, and sometimes death…”
線蟲動物門下，有各種不同的生物，包含成千上萬對人無害的種類，但蟯蟲是例外。它的幼蟲會在被糞便污染的水或土壤中，如果有人不幸恰巧赤腳站在那上面或走經過那兒，幼蟲就可能穿過他的皮膚感染。蟯蟲、蚼蟲及其他會穿過皮膚感染的線蟲動物門的生物，全球感染人數超過8億人，它們會引發腸胃道的問題，造成發育不良，甚至促使宿主死亡。
“The ancestors of birds and mammals independently evolved the ability to produce and control their own body heat,…The ability, known technically as endothermy and colloquially as warm-bloodedness,…Their unwavering body heat made them perpetually blaring beacons, which parasites could use to find hosts, and especially blood vessels…Blood…It’s no surprise that at least 14,000 animal species have evolved to feed on it…”
鳥類和哺乳類各自演化出保持身體衡溫的能力。它們的體溫，就是寄生蟲用來尋找宿主的線索。溫熱血管中營養的血液，對它們最有吸引力，至少有14,000種的動物是靠吸食它過活的。
“Among mammals, only three species of vampire bats feed exclusively on blood…The vampire’s heat sensors lie in its nose…Among infrared-sensing animals, vampire bats have a unique problem because they are themselves warm-blooded. The neurons in their pits ought to be bamboozled by their own body heat, but a dense network of tissue insulates them and keeps them 9°C(16°F) cooler than the rest of the bat’s face.”
在哺乳動物中，有三種吸血蝙蝠只以鮮血為生。它們偵測溫度的組織在鼻子。在可以感知紅外線的動物中，吸血蝙蝠有一個特別的問題，那就是它們本身就是衡溫動物。鼻子的孔洞中的感熱神經，會被它自身的體溫所遮蔽，它們因此發展出隔熱的組織，使得鼻子的溫度，硬是比它臉部的體溫低上9°C(16°F)。
“Ticks also suck blood, but their heat sensors are found on the tips of their first pair of legs…ticks can detect body heat from up to 13 feet away…common repellent like DEET and citronella don’t disrupt tick’s sense of smell but do stop them from tracking heat…”
蜱蟲也是吸血的。它偵測熱源的感覺器官在第一對腳的末端。蜱蟲可以偵測到13英呎之外的體溫，防不勝防。一般的驅蟲劑和香茅油固然不能阻卻它的嗅覺，但是卻可以阻卻它對熱源的偵測。
“Heat-sensitive pits have evolved independently among three groups of snakes. Two of these, pythons and boas, are non-venomous constrictors that kill with suffocating coils. The third are the highly venomous and aptly named pit vipers ——cottonmouths, copperheads, moccasins, and rattlesnakes…”
蛇身上偵測熱源的孔洞，在不同蛇類間各自演化。其中有兩種，蟒蛇和蚺蛇，是無毒腺的大蛇，以纒綣的方式勒死獵物。第三種是有劇毒的蛇，諸如棉口蝮、銅頭蛇、百步蛇、響尾蛇等等。
“Whatever advantage the pits provide, it must be significant. The nerves in their membranes are loaded with tiny batteries called mitochondria, far more than exist in typical sense organs. This suggests that the infrared sense demands a lot of energy, so it must provide benefits that are worthy of that cost.“
(蛇)感熱孔洞的效益，應該要很大。它包膜中的神經負載著產生能量的粒腺體，比一般的器官中的還要多很多。這表示偵測紅外線需要耗費很多的能量，所以它產生的效益，應該要大於所使用的能量成本才對。
“When thinking about another Umwelt, distance always matters.
Under the right conditions, smell and vision operate over vast scales.
Infrared senses work over shorter distances,…”
我們應該站在生物的角度思考，距離永遠是關鍵。在合適的條件下，嗅覺和視覺可以及擴及很大的範圍。而針對紅外線(熱源)，則只能在短距上才有效用。
*：ED Yong, “An Immense World,” 2022, Random House/New York
2023/9/5 Heat Damakey
