那條噴火龍，牙齒還健在嗎？

Fire-breathing dragons really capture the imagination — leading many writers to ponder just how such a creature might spew forth a torrent of flame. How do we square such a fantastic adaptation with real-life biology?

噴火的龍是個經(jīng)典形象，很多作家都可能思考過，龍到底是怎么噴火的。從現(xiàn)實的生物學(xué)角度講，這種神奇的動物真的能夠存在嗎?

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.

很多理論家都沒有考慮到，龍確實是從喉嚨和肺里呼出火來的。但是，我們真的可以違背生物學(xué)規(guī)則，往某種生物的喉嚨里放上一個火爐嗎?按照他們的說法，龍就像是一種噴火器。它們只是噴出一些可燃液體和氣體，這些可燃物可以被火花點燃。

"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."

“龍并不是直接把火噴出來，那會要了它們的命的。龍會在“噴火”的時候屏住呼吸。它們噴出來的是來自消化道的沼氣，也就是甲烷。它們控制噴出沼氣的過程，沼氣會與第一排與第二排牙齒之間的酶發(fā)生自燃反應(yīng)。所以這些氣體是在噴出過程中變成火焰的。”

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?

海因萊因提出龍噴火是利用了化學(xué)反應(yīng)，而生物學(xué)家Frank van Breukelen則提出，它們之所以會噴火，是因為燧石樣鱗片產(chǎn)生了火花。但不管怎么說，噴火對龍的牙齒有何影響呢?

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).

我們可以假設(shè)，龍的牙齒能夠防火，但任何東西都有一個耐熱極限。像古生菌等極端嗜熱菌可以忍受深海熱泉高達110℃的溫度。但龍噴出來的火焰跟深海熱泉性質(zhì)不一樣。木頭在260℃環(huán)境下會燃燒，而甲烷的絕熱火焰溫度是1949℃。

That's some serious heat. So how would the teeth hold up?

在這樣高的溫度下，牙齒怎么能堅持得住呢?

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.

有牙醫(yī)研究過人類牙齒的耐火程度。2002年的《法醫(yī)牙齒口腔學(xué)雜志》上有論文提出，如果用火燒，人類牙齒會在200-400℃之間破碎。到了1100℃，牙根就會粉碎，牙冠也“幾乎化為灰燼”。即便龍的牙齒是由更堅固的材料組成的，也很難抵抗甲烷燃燒時的溫度。

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.

不過，對大多數(shù)脊椎動物來說，并非一輩子只能用一副牙齒。人類是一換性齒動物(diphyodonts)，一生中會用到兩副牙齒。而鯊魚等動物是多換性齒動物(polyphyodonts)，它們的牙齒會不斷脫落和再生長。多換性齒哺乳動物只有大象、海牛和袋鼠，而魚類、爬行類動物和兩棲動物中就有很多是多換性齒的。

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?

不過，對大多數(shù)脊椎動物來說，并非一輩子只能用一副牙齒。人類是一換性齒動物(diphyodonts)，一生中會用到兩副牙齒。而鯊魚等動物是多換性齒動物(polyphyodonts)，它們的牙齒會不斷脫落和再生長。多換性齒哺乳動物只有大象、海牛和袋鼠，而魚類、爬行類動物和兩棲動物中就有很多是多換性齒的。