那条喷火龙，牙齿还健在吗？
喷火的龙是个经典形象，很多作家都可能思考过，龙到底是怎么喷火的。从现实的生物学角度讲，这种神奇的动物真的能够存在吗?
Most theorists dismiss the notion that dragons actually exhale fire through their throat and lungs. We can't place a furnace inside a living creature without breaking all the rules of biology. Instead, they tend to explain the monsters as a sort of flame thrower. The creature merely spews forth a flammable liquid or gas, which it ignites with a toothsome spark.
很多理论家都没有考虑到，龙确实是从喉咙和肺里呼出火来的。但是，我们真的可以违背生物学规则，往某种生物的喉咙里放上一个火炉吗?按照他们的说法，龙就像是一种喷火器。它们只是喷出一些可燃液体和气体，这些可燃物可以被火花点燃。
"They don't exactly breathe fire. That would kill them. They hold their breaths while flaming. It's swamp gas — methane — from the digestive tract. It's a controlled belch, with a hypergolic effect from an enzyme secreted between the first and second rows of teeth. The gas bursts into flame on the way out."
“龙并不是直接把火喷出来，那会要了它们的命的。龙会在“喷火”的时候屏住呼吸。它们喷出来的是来自消化道的沼气，也就是甲烷。它们控制喷出沼气的过程，沼气会与第一排与第二排牙齿之间的酶发生自燃反应。所以这些气体是在喷出过程中变成火焰的。”
Heinlein imagined a chemical reaction to ignite the dragon's fire, while biologist Frank van Breukelenproposed the physical spark of flintlike scales. But in either case, how would fire breathing affect a dragon's dental health?
海因莱因提出龙喷火是利用了化学反应，而生物学家Frank van Breukelen则提出，它们之所以会喷火，是因为燧石样鳞片产生了火花。但不管怎么说，喷火对龙的牙齿有何影响呢?
While we'd assume dragon teeth to be somewhat fire-resistant, there are limits to how much heat everything can withstand. Hyperthermophiles, such as archaea microorganisms, can take hydrothermal vent temperatures of up to 230 degrees Fahrenheit (110 degrees Celsius). But dragon fire is a whole different type of heat. Wood burns at about around 500 degrees Fahrenheit (260 degrees Celsius) and methane has an adiabatic flame temperature of 3,540 degrees Fahrenheit (1,949 degrees Celsius).
我们可以假设，龙的牙齿能够防火，但任何东西都有一个耐热极限。像古生菌等极端嗜热菌可以忍受深海热泉高达110℃的温度。但龙喷出来的火焰跟深海热泉性质不一样。木头在260℃环境下会燃烧，而甲烷的绝热火焰温度是1949℃。
That's some serious heat. So how would the teeth hold up?
在这样高的温度下，牙齿怎么能坚持得住呢?
Forensic dentists sometimes consider the effects of fire on human teeth. How else might investigators make sense of dental remains after a fire? As pointed out in a 2002 paper in The Journal of Forensic Odonto-Stomatology, human teeth exposed to fire will crack and fragment at between 392 and 752 degrees Fahrenheit (200 and 400 degrees Celsius). By 2,012 degrees Fahrenheit (1,100 degrees Celsius), the roots shatter and the crown is "reduced almost to dust." So even assuming dragon teeth are made of hardier stuff, it's difficult to imagine teeth that could withstand streams of blazing methane.
有牙医研究过人类牙齿的耐火程度。2002年的《法医牙齿口腔学杂志》上有论文提出，如果用火烧，人类牙齿会在200-400℃之间破碎。到了1100℃，牙根就会粉碎，牙冠也“几乎化为灰烬”。即便龙的牙齿是由更坚固的材料组成的，也很难抵抗甲烷燃烧时的温度。
And yet, here's the catch: For most vertebrates, one set of adult teeth doesn't have to last a lifetime. We're diphyodonts, meaning we only have two successive sets of teeth. Other creatures, like sharks, are polyphyodonts. They continually shed and regrow teeth. While elephants, manatees and kangaroos are the only mammalian polyphyodonts, the approach is widespread among fish, reptiles and amphibians.
不过，对大多数脊椎动物来说，并非一辈子只能用一副牙齿。人类是一换性齿动物(diphyodonts)，一生中会用到两副牙齿。而鲨鱼等动物是多换性齿动物(polyphyodonts)，它们的牙齿会不断脱落和再生长。多换性齿哺乳动物只有大象、海牛和袋鼠，而鱼类、爬行类动物和两栖动物中就有很多是多换性齿的。
So perhaps we're better off considering dragons as polyphyodonts, whose teeth continually fracture and flake under the power of their flames. Then, as the old teeth fall away, new ones pop into place to rip apart the still-sizzling meat of their latest kill.Plus, isn't a blast from Smaug all the more terrifying if it's full of fragmented dragon teeth?
不过，对大多数脊椎动物来说，并非一辈子只能用一副牙齿。人类是一换性齿动物(diphyodonts)，一生中会用到两副牙齿。而鲨鱼等动物是多换性齿动物(polyphyodonts)，它们的牙齿会不断脱落和再生长。多换性齿哺乳动物只有大象、海牛和袋鼠，而鱼类、爬行类动物和两栖动物中就有很多是多换性齿的。
