By Martin McGuigan
Smaller animals have larger brains relative to their bodies.
相對於身軀，較小動物有較大的大腦。
圖1. 於一所博物館展示的樹懶大腦橫截面。 (圖援用自原文)
If terms like "bird brain" and "harebrained" are any indication, animal brains present fascinating and diverse examples of one of the most complex organs known to science. Animal brains differ not only in overall size but also in their size relative to the animal's body mass.
倘若像"鳥腦"及"欠思考的"這樣用語是任何暗示，那麼動物大腦就呈現了，科學已知最複雜器官之一，吸引人且多樣化的例子。動物的大腦，不僅在整體大小上不同，而且在相對於動物身軀質量大小上也不同。
At 18 pounds (8 kilograms), on average, the sperm whale (Physeter macrocephalus) has the largest brain, but has an overall body mass of 45 tons (40 metric tons), giving it a brain-to-body-mass ratio of 1:5,100. But which animal has the biggest brain relative to its body size?
抹香鯨(Physeter macrocephalus)具有，平均重18磅(8公斤)的最大大腦。不過，具有整體身軀質量45噸(40公噸)。因此，使其具有大腦對身軀質量1:5100的比。不過，相對於身軀大小，哪種動物具有最大的大腦？
A 2009 study in the journal Brain, Behavior and Evolution found that an especially tiny genus of ant has the largest brain for its body size.
在2009年，發表於《大腦，行為與演化》期刊的一項研究發現，一種特別細小的螞蟻屬具有，就其身軀大小而言，最大的大腦。
Brachymyrmex has an average body mass of up to 0.049 milligrams and an average brain mass of 0.006 milligram. That means its brain is roughly 12% of its body mass, giving it a brain-to-body-mass ratio of about 1:8.
Brachymyrmex具有平均身軀質量高達0.049毫克，及平均大腦質量0.006毫克。這意味著，其大腦大約其身軀質量的12%，使其大腦對身軀質量比大約1:8。
In absolute terms, animals' brain size tends to increase with the size of the animal. Bigger brains are typically related to three factors: "maternal investment, complexity of behavior and the sheer size of the body," said Sophie Scott, a professor of cognitive neuroscience at University College London.
從絕對值來看，動物大腦的大小傾向，隨著動物大小的增加。較大的大腦通常與三個因素有關：英國倫敦大學學院認知神經科學教授，Sophie Scott宣稱：「母親的投資(母親單位時間對每一後代成長的投資)、行為的複雜性及身軀的絕對大小。」
"Having a bigger body means needing to control that more," Scott told Live Science. "Apex predators tend to be big. And because of a need for more complex behavior, like being able to outwit your prey, they benefit from having a bigger brain."
Scott告訴Live Science：「具有較大的身軀意味著，更多需要控制。頂級掠食者傾向是大的。也就是，由於需要更複雜的行為，譬如能夠智勝，受益於具有較大大腦的獵物。」
But, brain size isn't a perfect predictor of animal intelligence. The brain of an African elephant (Loxodonta africana) weighs 10 pounds (4.6 kg), on average, according to a 2014 study in the journal Frontiers in Neuroanatomy, three times larger than the human brain.
不過，大腦大小並非動物智力的完美預測指標。根據2014年，於《神經解剖學前沿》期刊的一項研究，非洲象(Loxodonta africana)的大腦，平均重10磅(4.6公斤)，比人腦大三倍。
Their large brain size is due in part to their massive cerebellum, used to coordinate the muscle activity in their trunk and ears, according to Scott.
根據Scott的說法，牠們大的大腦，部分由於其被用來協調，軀幹及耳朵中，肌肉活動的粗大小腦。
圖2. Brachymyrmex屬螞蟻的大腦與身體質量比為1:8。 (圖援用自原文)
Just as absolute brain size isn't a good predictor of animal intelligence, comparing brain-to-body mass ratios can also be a red herring. Humans and rodents have a roughly similar brain-to-body mass ratio (1:40), according to a 2009 study in the journal Frontiers in Human Neuroscience.
正如絕對的大腦大小，並非動物智力的良好預測指標。因此，比較大腦對身軀的質量比，也可能是一種轉移注意力的事。根據2009年，於《人類神經科學前沿》期刊的一項研究，人類及囓齒動物具有，大致上相似之大腦對身軀的質量比(1:40)。
However, the same study argues that if a rat were human-size, it wouldn't be as intelligent because it has a smaller cerebral cortex (the outermost area of the brain, which is associated with the most complex mental functions) and fewer neurons there than humans do.
不過，該相同的研究認為，即使老鼠是人類的大小，它也不會有同樣智力。因為，其具有較小的大腦皮層(大腦與最複雜之心理功能有所關聯的最外層)，及在那裡具有比人類少的神經元。
"If you look at the brain of a rabbit, a cat and a small monkey, they're not that different in size, but their behavior is going to be very different because of the nature of the brain cells," Scott said. "By the time you get to the monkey, you're seeing a primate brain, with proportionally larger frontal lobe areas and more curiosity-driven behavior."
Scott宣稱：「倘若檢視兔子、貓及小猴子的大腦，它們大小並無上述的不同。不過，由於腦細胞的性質，它們的行為會非常不同。當著手處理猴子時，看到的是一種，具有比例上較大額葉區域的靈長類動物大腦。因此，有較多好奇心驅動的行為。」
Scott explained that evolutionary adaptations change the brain's structure to increase the size of certain areas and favor certain neural connections.
Scott解釋說，演化上的適應改變了大腦結構，以增加某些區域的大小，而有利於某些神經連接。
In humans, the size of our cerebral cortex and the density of cortical neurons (the number of neurons present there) explain our intelligence more than the size of our brains relative to our bodies. Compared with other animals "we have quite a small body for the size of our brains," Scott said.
於人類中，咱們大腦皮層的大小及皮層神經元的密度(在那裡存在的神經元數量)解釋了，比咱們大腦相對於咱們身軀大小，更能解釋咱們的智力。與其他動物相較下，Scott宣稱：「就咱們大腦的大小而言，咱們具有相當小的身軀。」
When comparing brains of different species, it's important to consider brain architecture as well as brain size.
當比較不同物種的大腦時，除了大腦大小之外，考慮大腦結構也很重要。
Because brain-to-body-mass ratio does not take into account the evolutionary development of the cerebral cortex and the density of neural connections found there, scientists look to encephalization quotient (EQ) as a more accurate measure of animal intelligence.
因為，大腦對身軀質量比，沒有將大腦皮層的演化發展及在那裡發現之神經連接的密度納入考量。科學家們將腦發展指數(EQ：指動物身軀質量與腦質量比的指數)視為，動物智力的一種更準確測量法。
The encephalization quotient is the relative brain size observed in a particular species, compared with the expected brain size of other species of a similar body size. One key factor in EQ is the relative size of the cortex compared with the rest of the brain.
腦發展指數是在，與身軀相似之其他物種預期大腦大小，相較之特定物種中，觀察到的相對大腦大小。於EQ中的一個關鍵因素，是皮層與大腦其他部分相較的相對大小。
Comparing animals based on their EQ provides a more accurate view of their intelligence than brain-to-body-mass-ratio, according to the Encyclopedia of Behavioral Neuroscience, though not as accurate as measuring the absolute size and interaction of individual brain regions.
根據行為神經科學百科全書，以動物EQ為基礎來比較動物，提供了有關其智力，比大腦對身軀質量更準確的見解。雖然，不如測量個體大腦區域的絕對大小及相互作用那樣準確。
Then, there's a concept known as Haller's rule: The bigger the animal is, the smaller the brain-to-body ratio will be. "Because brain size scales relative to body size, the smallest animals have relatively the largest brains," Wulfila Gronenberg, a professor of neurobiology at the University of Arizona, told Live Science.
之後，有一種被稱為哈勒法則的概念：動物越大，大腦對身軀的比例會是越小。美國亞利桑那大學神經生物學教授，Wulfila Gronenberg告訴Live Science：「因為，大腦大小按比例與身軀大小相應。因此，最小的動物具有相對最大的大腦。」
For example, ants have relatively small brains compared with other hymenoptera, a class that includes bees, wasps, hornets and sawflies. "We think this is because … ant workers don't fly," Gronenberg said.
譬如，與其他膜翅目昆蟲(包括蜜蜂、黃蜂、大黃蜂及鋸蜂)相較下，螞蟻具有相對小的大腦。Gronenberg宣稱：「我們認為，這是因為…工蟻不會飛。」
Flying requires a lot of visual processing, so many flying insects typically have large eyes, thus leading to larger optical lobes. "In some insects, like a dragonfly, visual processing is more than half of their entire brain," Gronenberg said.
飛行需要大量的視覺處理，因此許多飛行的昆蟲通常具有大眼睛，從而導致較大的視葉(負責處理視覺信息，位於節肢動物眼睛後面的原腦結構)。Gronenberg宣稱：「在某些昆蟲中，譬如蜻蜓，視覺處理多於其整個大腦的半數。」
網址：https://www.livescience.com/largest-brain-body-size
翻譯：許東榮
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