為什麼霸王龍跑不快？又為什麼最大的動物不一定是最迅速的？

原文網址：https://www.uni-jena.de/en/Research+News/FM170717_Speed_en.html

為什麼霸王龍跑不快？又為什麼最大的動物不一定是最迅速的？

生物多樣性研究人員陳述了體型和速度之間的關係

甲蟲走得比老鼠慢，老鼠跑得比兔子慢，兔子又比獵豹慢，獵豹又比……大象慢？錯！沒有一種陸上動物跑得比獵豹還快。大象是比較大沒有錯，但牠的速度比獵豹慢多了。對於小型至中型的動物來說，體型較大代表跑得更快；但對於相當大型的動物來說，談到速度牠們又變得越來越慢。對於這種體型和速度之間的拋物線性關係從何而來，首度有科學家描述了可能原因。德國綜合生物多樣性研究中心(iDIV)和德國耶拿市弗里德里希˙席勒大學主持的研究團隊，利用新的數學模型得出此成果，並將他們的發現刊登於頂尖期刊《自然――生態學與演化》(Nature Ecology and Evolution)。

此模型的單純程度令人驚訝：需要「餵」給它的資訊只有某種動物的體重以及牠在什麼媒介中移動，也就是陸地、空中或水裡。單靠這些基本資料，就可以計算出某種動物所能到達的最高速度，精準度將近有90%。研究主要作者，iDiv研究中心和耶拿大學的Myriam Hirt表示：「我們的模型最優秀的特點便是它可以適用於各種情況。它可以用於從塵蟎至藍鯨各種體型的動物，以及跑步、游泳到飛行的任何一種移動方式，還能用在各種棲地。」此外，這個模型不限於只能用在現存的動物種類，就連已經滅絕的動物也同樣適用。

霸王龍的最高速度只能到達每小時27公里

Hirt解釋：「為了驗證我們的模型能否計算已經滅絕的動物所能到達的最快速度，我們把某些種類的恐龍代入到我們的模型之中，牠們的速度目前已經可以用相當複雜的生物力學方法模擬出來。」結果顯示這項簡單的模型對三角龍、霸王龍、腕龍和其他恐龍的速度傳遞出來的結果跟相當複雜的模擬一致――霸王龍的速度完全沒有快到破表，大約只達到每小時27公里。科學家表示：「這代表我們的模型在未來可以讓我們用非常簡單的方式，來預估其他已經滅亡的動物跑起來的速度可以到多快。」

質量必須克服慣性

這項模型基於兩種基本理論。跟第一個理論相關的是，動物是在一段相對短時間的衝刺中達到最高速度，而非在長距離的跑動期間。動物在長距離跑動時身體會持續供應給肌肉能量(有氧代謝)；與此不同，動物在衝刺時利用的是肌肉本身儲存的能量，但卻會在相對較短的時間內就消耗殆盡(無氧代謝)。因此這看起來合乎邏輯：動物體型越大，擁有的肌肉就越多，因此衝刺的速度也越快。然而，牛頓運動定律也適用於動物王國。我們知道物體移動時必須要克服自身慣性，因此一頭5噸重的大象運動初始速度不可能跟一隻2.5克重的小臭鼩一樣快。當大象之類的大型動物在衝刺中到達牠們的最高速度時，牠們可以快速使用的儲備能量也幾乎用光了。結合這兩項理論便可以得到之前所提的拋物線關係：甲蟲走得比老鼠慢，老鼠跑得比兔子慢，兔子又比獵豹慢――而獵豹又比大象跑得快。

Why Tyrannosaurus was a slow runner and why the largest are not always the fastest

Biodiversity Researcher described the relationship between body size and speed

A beetle is slower than a mouse, which is slower than a rabbit, which is slower than a cheetah...  which is slower than an elephant? No! No other animal on land is faster than a cheetah - the elephant is indeed larger, but slower. For small to medium-sized animals, larger also means faster, but for really large animals, when it comes to speed, everything goes downhill again. For the first time, it is now possible to describe how this parabola-like relationship between body size and speed comes about. A research team under the direction of the German Centre for Integrative Biodiversity Research (iDiv) and the Friedrich Schiller University Jena (Germany) have managed to do so thanks to a new mathematical model, and also published their findings in the prestigious journal "Nature Ecology and Evolution".

The model is amazingly simple: The only information that it must be 'fed' with is the weight of a particular animal as well as the medium it moves in, so either land, air or water. On this basis alone, it calculates the maximum speed that an animal can reach with almost 90% accuracy. "The best feature of our model is that it is universally applicable," says the lead author of the study, Myriam Hirt of the iDiv research centre and the University of Jena. "It can be performed for all body sizes of animals, from mites to blue whales, with all means of locomotion, from running and swimming to flying, and can be applied in all habitats."  Moreover, the model is by no means limited to animal species that currently exist, but can be applied equally well to extinct species.

Tyrannosaurus reached a speed of only 17 miles/hour

"To test whether we can use our model to calculate the maximum speed of animals that are already extinct, we have applied it to dinosaur species, whose speed has up to now been simulated using highly complex biomechanical processes," explains Hirt. The result is that the simple model delivered results for Triceratops, Tyrannosaurus, Brachiosaurus and others that matched those from complex simulations - and were not exactly record-breaking for Tyrannosaurus, who reached a speed of only 27 km/h (17 mi/h). "This means that in future, our model will enable us to estimate, in a very simple way, how fast other extinct animals were able to run," says the scientist.

Mass has to overcome inertia

Two assumptions are the basis of the model. The first assumption is related on the fact that animals reach their maximum speeds during comparatively short sprints, and not while running over long distances. Unlike running over longer distances, where the body constantly resupplies the muscles with energy (aerobic metabolism), sprinting uses energy that is stored in the muscles themselves but which is exhausted relatively quickly (anaerobic metabolism). It seems logical enough: the larger the animal, the more muscle it has - and thus the faster it can sprint. However, Newton's laws of motion also apply in the animal kingdom, we know mass has to overcome inertia, and so a five-tonne African elephant simply cannot start moving as quickly as a 2.5-gramme Etruscan shrew. By the time large animals such as the elephant get up to full speed while sprinting, their rapidly available energy reserves also soon run out. Taken together, these two assumptions result in the previously mentioned curve: A beetle is slower than a mouse, which is slower than a rabbit, which is slower than a cheetah - which is faster than an elephant.

原始論文：Myriam R. Hirt, Walter Jetz, Björn C. Rall, Ulrich Brose. A general scaling law reveals why the largest animals are not the fastest. Nature Ecology & Evolution, 2017; DOI: 10.1038/s41559-017-0241-4

引用自：Friedrich Schiller University Jena. "Why Tyrannosaurus was a slow runner and why the largest are not always the fastest."