本篇文章共分 6 段，將蛇類獨特的分叉舌尖的可能功能自古到今幾項重要的科學假說一一陳述，並且仔細說明現今主要理論的支持證據。

本篇考題英文原文與對應之中文翻譯整理如下。練習作答解題時若有對語意不清楚之處，請仔細查閱對照，以提升閱讀理解能力。

Why Snakes Have Forked Tongues 蛇為什麼有分叉的舌頭

1. 蛇的分叉舌尖引人好奇

   The forked tongue of snakes has intrigued people for millennia, inspiring many hypotheses. In many cultures and religions, the forked tongue symbolizes malevolence and deceit. The first person known to inquire about the functional significance of the forked tongue was Aristotle; he suggested that it would double the pleasure of sensations of taste. By the beginning of the twentieth century, the consensus was that the snake’s tongue is a tactile organ, that is, the snake uses it to tap the ground much as a blind person uses a cane.

   幾千年來，蛇的分叉舌頭一直引發人們的好奇，激發了許多假說。在許多文化和宗教中，分叉的舌頭象徵著惡毒和欺騙。現今所知，第一個詢問分叉舌頭的功能意義的人是亞里斯多德；他認為分叉的舌頭會使味覺的快樂加倍。到二十世紀初，人們一致認為蛇的舌頭是一種觸覺器官，也就是說，蛇用它來輕觸地面，就像盲人用枴杖一樣。

2. 探測費洛蒙說

   In 1920, Broman suggested what seemed to be a winning hypothesis: When the snake retracts its tongue, the tips (or tines) of the forked tongue are inserted into openings on both sides of the roof of the mouth, through these openings chemical stimuli reach special organs that help snakes detect smells – the vomeronasal organs (VNO). These organs are highly developed in snakes, lizards, and many mammals. They are a second system for detecting smells that appear to have evolved specifically to detect pheromones, the chemical signals that animals secrete as messages to other animals of their species. Broman suggested that forked tongue flicks out, picking up chemical signals, and then delivers these to the VNO. This hypothesis was widely accepted into the 1980s. Then X-ray movie studies of tongue flicks in snakes and lizards with forked tongue disproved the hypothesis; they showed that when the tongue is withdrawn into the mouth, it enters a sheath and the tips do not go into the openings to the VNO. Instead, the chemical molecules are deposited on pads at the bottom of the mouth, and closing the mouth presses the pads and molecules against the opening.

   1920 年，布羅曼提出了一個似乎是正確的假說：當蛇縮回舌頭時，分叉的舌尖（或稱分叉）插入嘴頂端兩側的開口，通過這些開口，化學刺激到達幫助蛇探測氣味的特殊器官—鋤鼻器官 (VNO)。這些器官在蛇、蜥蜴和許多哺乳動物中高度發達。它們是探測氣味的第二個系統，似乎是專門為探測費洛蒙（外激素）而進化的，費洛蒙是動物分泌的化學信號，作為向其物種的其他動物傳遞的訊息。布羅曼提出，分叉的舌頭彈出，拾取化學信號，然後將這些信號傳遞給 VNO。這一假說在 20 世紀 80 年代被廣泛接受。後來對蛇和蜥蜴分叉舌頭的 X 射線影片研究推翻了此一假說；影片顯示，當舌頭被收回口腔時，它進入了一個鞘，舌尖並沒有進入通向 VNO 的開口。相反，化學分子沉積在口腔底部的墊子上，封閉口腔會將墊子和分子壓在開口處。

3. 探測氣味差異說

   If the tongue is not forked to fit into the VNO, then what function could the forked shape serve? Schwenk proposes a solution that encompasses observations from several fields – animal behavior, ecology, sensory physiology, and neuroanatomy. He hypothesizes that the forked tongue allows the snake to sense chemical stimuli at two points simultaneously, thereby giving it the ability to detect differences in an odor trail. Obtaining two simultaneous readings enhances the ability of the snake to detect the edges of odor trails, and thus to follow pheromone trails accurately. This ability is important in seeking both prey and mates.

   如果舌頭分叉不是為了伸入 VNO，那麼分叉的形狀能有什麼功能呢？施文克提出了一個解答，其中包含了幾個領域的觀察—動物行為學、生態學、感覺生理學和神經解剖學。他主張，分叉的舌頭使蛇能夠同時感知兩點的化學刺激，從而使它有能力檢測到氣味線索的差異。獲得兩個同時的讀數增強了蛇探測氣味蹤跡邊界的能力，從而準確地跟蹤費洛蒙蹤跡。這種能力對於尋找獵物和配偶都很重要。

4. 立體知覺

   This spatial chemical perception is like other systems for spatial perception that are based on simultaneous stimulation of two separated sense organs – for example, auditory localization, which depends on differential stimulation at the two ears. Similarly, the use of two eyes permits stereovision.

   這種空間化學知覺與其他基於同時刺激兩個分離的感覺器官的空間知覺系統一樣—例如，聽覺定位，它取決於兩個耳朵的不同刺激。同樣地，使用兩隻眼睛可以實現立體視覺。

5. 氣味差異說之支持

   Several kinds of evidence support the hypothesis that forked tongues evolved as chemosensory edge detectors to enhance the ability to follow odor trails: Snakes and lizards spread the tines of their tongue apart when they retrieve odor molecules, then draw the tines together when retracting the tongue. The greater the distance between sampling points, the better the animals sample differences within an odor trail. Lizards that forage widely have forked tongues, whereas lizard species without forked tongues tend not to forage widely. Forked tongues have evolved independently at least twice in different families of reptiles, indicating their value as an adaption. In the snake nervous system, each tine of the tongue is linked to a nucleus in the other side of the brain, and the two nuclei are linked across the two hemispheres. This arrangement is similar to the anatomy of auditory centers in mammals and birds that permits the computation of differences between what one ear hears and what the other ear hears and thus mediates auditory localization.

   有好幾種證據支持這樣的假設：分叉的舌頭進化成了化學感覺的邊緣探測器，以提高跟蹤氣味的能力。蛇和蜥蜴在檢索氣味分子時，會將舌頭的舌尖張開，然後在縮回舌頭時將舌尖併攏。採樣點之間的距離越大，動物對氣味線索中的差異採樣就越好。廣泛覓食的蜥蜴有分叉的舌頭，而沒有分叉舌頭的蜥蜴物種往往不會廣泛覓食。分叉的舌頭在不同的爬行動物家族中至少獨立進化了兩次，顯示它們在適應上的價值。在蛇的神經系統中，舌頭的每根齒與另一側大腦中的一個核相連，兩個核在兩個半球中相連。這種安排類似於哺乳動物和鳥類的聽覺中心的解剖結構，允許計算一隻耳朵聽到的東西和另一隻耳朵聽到的東西之間的差異，從而調整聽覺定位。

6. 其他類似物種

   Species in other orders have also evolved paired chemical receptors to guide individuals to mates or prey. For example, male gypsy moths have large, elaborate, odor-detecting antennae with which they track potential mates over large distances, and the ant nest beetle has spoon-shaped antennae extending from each side of the head with which it detects and follows the pheromones of the ants that are its food.

   其他目中的物種也進化出成對的化學受體，以引導個體找到配偶或獵物。例如，雄性吉普賽蛾有大而精緻的氣味探測觸角，它們用這些觸角在長距離的範圍中追蹤潛在的配偶，而蟻巢甲蟲有勺狀的觸角從頭部兩側延伸出來，它用這些觸角探測並追蹤作為其食物的螞蟻的費洛蒙。