By Michael Dhar
Dinosaur bones aren't lying: animals really did use to be bigger.
恐龍的骨頭沒有說謊：動物的確曾經更大。
圖1. 2019年3月5日，在紐約市美國自然歷史博物館舉行的“霸王龍：終極捕食者”媒體預覽期間，一個男孩在試用互動霸王龍。 (圖援用自原文)
Prehistoric giants used to populate the Earth. These behemoths included mighty dinosaurs, airplane-size pterosaurs, massive crocodiles and snakes, and even armadillos the size of cars. But today, there are just a few big animals on our planet.
史前巨大的動物曾適應生存於地球上。此些龐然大物包括，巨大的恐龍、飛機大小的翼龍、巨大的鱷魚及蛇，甚至汽車大小的犰狳。不過今天，咱們星球上僅有少數大型動物。
What happened? Why aren't there many giants left anymore?
發生了什麼？為何不再有很多巨大動物存活下來？
First of all, there's plenty of fossil evidence that the ancient past really did have larger animals — beasts that were humongous but also larger, on average, than today's creatures, Greg Erickson, a vertebrate paleobiologist at Florida State University in Tallahassee who specializes in ancient reptiles, told Live Science.
美國佛羅里達州立大學(位塔拉哈西市)專攻古代爬行類動物的脊椎動物古生物學家，Greg Erickson告訴Live Science科學新聞網站，首先，有大量年代久遠之過去，確實有更大動物(平均比當今生物更大的巨大野獸)的化石證據。
Ever since scientists unearthed the first known stash of dinosaur bones, in the 19th century, researchers have put forth ideas to explain why giants were common millions of years ago but less so today. But no one can point to one definitive answer, Erickson said. "It's so multifactorial."
Erickson表示，打從19世紀科學家們發現，第一個已知的恐龍骨骼藏匿處以來，研究人員們已經提出一些，解釋為何巨大的動物，在數百萬年前很常見，不過在當今卻不那麼常見的見解。不過，無人能指出一個明確的答案。“這是很多因素造成的”。
Several major differences between dinosaurs and today's largest animals, the mammals, may help explain the loss of behemoths, however. Along with other giant reptiles, dinosaurs could adapt to different niches as they grew bigger over life, hunting smaller prey as juveniles and larger victims as adults.
不過，恐龍與當今最大動物(哺乳動物)之間的若干主要差異，可能有助於解釋巨獸的消失。連同其他巨大爬行類動物，恐龍可能隨著其生存期間變得更大時，適應了不同的生態位。在幼年時捕獵較小的獵物，在成年時捕獵較大的獵物。
In part, they could do this because they swapped out sets of teeth over a lifetime. "They replace their teeth constantly, just like sharks do. But along the way they could change the type of teeth," Erickson said. Crocodiles, for instance, go from "needle-like teeth to more robust teeth. Mammals don't have that luxury."
在某種程度上，牠們可能做到這一點。因為，牠們在一生中，更換了數組牙齒。Erickson宣稱：「牠們不斷更換牙齒，就像鯊魚般。」不過，在此過程中，牠們能改變牙齒類型。譬如，鱷魚從"針狀的更換到更強壯的牙齒。哺乳動物沒有那種難得的過程"。
Put another way, as some reptilian youngsters ballooned into hulking adults, they traded their relatively puny juvenile teeth for bigger weapons, allowing them, in turn, to hunt bigger meals to fuel their larger bodies.
換句話說，隨著一些爬行類動物的幼獸，迅速成長成笨重的成獸時，牠們用相對較弱的幼齒換取更大的牙齒，依序使牠們得以獵取更大的食物，來為其更大的身驅提供能量。
In dinosaurs, too, air sacs likely extended from their lungs to their bones, creating sturdy but light scaffolding, Edinburgh University paleontologist Steve Brusatte told Scientific American. That gave dinosaurs skeletons that were "still strong and still flexible, but lightweight.
英國蘇格蘭愛丁堡大學古生物學家，Steve Brusatte告訴《科學美國人》雜誌，在恐龍氣囊中，也可能從肺部延伸到骨骼，形成堅固卻輕的骨架。
That gave dinosaurs skeletons that were "still strong and still flexible, but lightweight. That helped them get bigger and bigger and bigger," Brusatte said. "The same way that skyscrapers are getting bigger and bigger and bigger because of the internal support structures."
Brusatte宣稱：「那賦予了恐龍依然堅固、靈活卻重量輕的骨骼。這有助於牠們變得越來越大。這類似，由於內部支撐結構，摩天大樓正變得越來越大的方式。」
Of course, though air sacs helped make for strong, lightweight bones, no animal could actually get as big as a skyscraper. That's because body weight grows much faster than bone strength as animals increase in size, as physicist Neil deGrasse Tyson has explained.
當然，雖然氣囊有助於形成堅固、重量輕的骨骼。不過，實際上沒有動物能長得像摩天大樓那麼大。那是因為，當動物在身驅變大時，體重成長的速度比骨骼強度更快得多。正如美國天體物理學家，Neil deGrasse Tyson已經解釋那樣。
Mammals lack such air sacs, though, "that can invade the bone and lighten up the bone," Brusatte said, " So elephant size or a little bit bigger, that might be the limit as to where mammals, at least on land" can get.… You can't really get mammals, it doesn't seem, to be the size of dinosaurs."
Brusatte宣稱：「不過，哺乳動物缺乏，能充溢骨骼而減輕骨骼重量的氣囊。因此，至於在哺乳動物方面，大象或稍大一點的身驅，至少在陸地上，那可能是極限點。無人能真正捕獲，看來不像且是恐龍大小的哺乳動物。」
圖2. 人們認為，大象的身驅可能是陸地哺乳動物能長到的最大極限。 (圖援用自原文)
As warm-blooded, or endothermic creatures, mammals also need a lot of fuel. "Elephants are full endotherms, and the dinosaurs, at least the herbivorous dinosaurs, probably mostly were not," Geerat Vermeij, a professor of geobiology and paleobiology at the University of California, Davis, told Live Science. "So the food requirement for, say, a gigantic elephant would be … perhaps 5 times greater than that of even the very largest dinosaurs."
身為溫血動物，也就是吸熱性動物，哺乳動物也需要大量食物。美國加州大學戴維斯分校的地球生物學及古生物學教授，Geerat Vermeij告訴科學新聞網站Live Science：「象是完全溫血的動物，而恐龍，至少草食性恐龍，可能幾乎都不是。因此，譬如說，一頭巨大的象，食物需求量會是…或許比甚至最大的恐龍大5倍。」
Paleontologists have debated whether dinosaurs were cold- or warm-blooded. But current science places many animal species on a gradient between cold- and warm-bloodedness, and dinosaurs were probably "on the low end of the warm-blooded range," Erickson said. That made a large body less energetically expensive for dinos.
古生物學家一直在爭論，恐龍是冷血動物還是溫血動物。不過，目前的科學將諸多動物物種，置於冷血與溫血之間的生理梯度上。因此，Erickson表示，恐龍可能"處於溫血動物範疇的低端"。對恐龍而言，那使得大身驅能量上能較少負擔。
Huge size also requires the right environment. In a 2016 study published in the journal PLOS One, Vermeij concluded that giantism depends mostly on sufficient resources produced and recycled by "highly developed ecological infrastructure."
巨大的身驅需要適宜的環境。在2016年，發表於《PLOS One》雜誌的一項研究中，Vermeij斷言，巨型化趨向主要依賴，由"高度發達之生態基礎結構"生產及循環的充足資源。
In other words, the ecology needs to produce sufficient oxygen, food and habitat to grow a truly giant creature. Such ecologies had seen great development by the middle Triassic period, near the beginning of the age of dinosaurs, Vermeij wrote. In one potentially important environmental change, ancient atmospheres had higher concentrations of oxygen.
換句話說，生態需要產生充足的氧氣、食物及棲息地，才能蘊育出真正的巨大生物。Vermeij記述，到了接近恐龍時代開始的三疊紀中期，此類生態已經經歷顯著的發展。在一個潛在上重要的環境變化中，遠古時期之大氣裡，具有較高濃度的氧氣。
This may have played a role in gigantism, particularly among insects. Wingspans among prehistory's biggest bugs tracked ancient increases in oxygen concentration, a 2012 study in the journal Proceedings of the National Academy of Sciences reported.
在巨型化趨向中，這可能曾經扮演一種角色，特別是在昆蟲中。2012年，發表於《美國國家科學院院刊》雜誌的一項研究報告，記述了在史前最大的昆蟲中，翼幅留下了遠古時期，氧氣濃度增加的痕跡。
Brewers of gigantism shouldn't forget the crucial ingredient of time, either. Though animal lineages tend to get larger over the generations, it takes a vast amount of evolutionary time to reach giant sizes, Erickson said. And mass extinction events tend to wipe out larger creatures, Vermeij said, so these events can leave giant-animal slots unfilled for tens or hundreds of millions of years.
巨型化趨向的倡議者們，也不應該疏忽時間的關鍵性要素。Erickson表示，雖然動物譜系傾向於，在若干世代間變得較大。不過，要達到巨大的身驅，需要大量的演化時間。Vermeij 表示，大規模滅絕事件傾向於，消滅更大的生物。因此，這些事件會遺留下，無法填補之巨型動物的空缺，達數千萬或數億年。
In the case of woolly mammoths, decimated by climate change and human hunters just 10,000 years ago, it may not be a coincidence that we modern humans don't see such huge creatures: Our own ancestors helped kill them off not so long ago.
他宣稱：「首批哺乳動物達到一噸重，大約花了2千5百萬年。」就僅一萬年前，因氣候變遷及人類狩獵而滅絕的毛茸茸猛獁象而言，我們現代人類沒有看到如此巨大的生物，可能並非巧合。因為，咱們自己的祖先，不久前助長消滅了牠們。
For Vermeij, the most comprehensive explanation for decreasing size comes not from physiology or environment, but from social structure. " The evolution of … organized social behavior, not just herds but really organized hunting" in mammals introduced a new form of dominance, he said.
對Vermeij而言，有關縮小身驅，最全面的解釋不是來自生理或環境，而是來自社會結構。他宣稱：「…有組織之社會行為的演變，不僅僅有牧人，而且在哺乳動物方面，引進了一種真正有組織之狩獵的新支配形式。」
"Group hunting by relatively small predators makes even very large prey vulnerable. Individual gigantism has in effect been replaced on land by gigantism at the group level," he wrote in the 2016 study.
他在2016年的研究中記述：「靠著諸多相對上少之捕食者們的群體狩獵，使得即使非常大的獵物也變脆弱。在陸地上，個體巨型化趨向，實際上已經被群體層面的巨型化趨向所取代。」
That is, smaller individuals working together, as happens with wolves and hyenas for example, may constitute a more effective way of getting big than building a huge body. As a result, "gigantism lost its luster on land," Vermeij wrote.
也就是說，較小的個體一起合作，譬如像包括狼及鬣狗等發生的，可能構成比發展出龐大身驅之變大更有效的方式。結果，Vermeij記述：「在陸地上，巨型化趨向失去了其光彩。」
Social organization may also help explain a rather, ahem, giant exception to the timeline traced here: In the ocean, the biggest animals to ever live still exist today: blue whales.
社會結構也可能有助於解釋，這裡追溯之年表，一個恰恰相反的巨大例外。在海洋中，迄今最大的動物，藍鯨目前仍然存在。
Sea life, Vermeij said, makes long-distance communication more difficult, hindering the development of complex hunting groups. The evolution of such groups "has happened on land much more than, at least until recently, that has happened in the ocean," such as with killer whales, he said.
Vermeij表示，海洋生物使遠距離溝通變得更困難，這阻礙了複雜狩獵群體的發展。他宣稱：「此類群體的演化，至少直到最近，在陸地上比在包括諸如虎鯨等的海洋中，已經發生更多得多。」
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網址：https://www.livescience.com/why-no-more-giant-animals
翻譯：許東榮
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