The mystery of why glass frogs have see-through skin has been solved, scientists say: the unusual feature is a type of camouflage.

    为什么玻璃蛙的皮肤是透明的？科学家们说：这个不寻常的特征是一种伪装。

    Glass frogs are found in tropical Central and South America, and get their name from their skin.

    玻璃蛙生活在热带的中美洲和南美洲，它们的名字来源于它们的皮肤。

    However, the frogs are not truly transparent but translucent, with the skin on their backs typically a vivid green and their intestines and heart visible through their underbelly. This has led to a question that has kept scientists on the hop.

    然而，这种蛙并不是完全透明的，而是半透明的，它们背上的皮肤通常是鲜绿色的，却可以从腹部看到它们的内脏。这就引出了一个让科学家们措手不及的问题。

    “If predators cannot see straight though the frogs, why do glass frogs have transparent skin at all, and not the opaque camouflaged patterns of other tree frog species?” said Dr James Barnett, a postdoctoral researcher at McMaster University, Canada, who co-authored the study.

    “如果捕食者的目光不能直接穿过青蛙，那么为什么玻璃蛙的皮肤是透明的，而不是像其他树蛙一样是不透明的伪装图案？” 加拿大麦克马斯特大学博士后詹姆斯·巴尼特博士说，他是这项研究的合著者。

    Barnett and colleagues say they have cracked the conundrum. “The frog is always green to generally match leaves, but leaves will differ in their brightness,” said Barnett. The team say that while the colour of the frog’s body changes little against dark or light foliage, the legs are more translucent and hence shift in brightness, helping the amphibians to blend in.

    不过，巴尼特和他的同事称他们已经解决了这个难题。“青蛙通常是绿色的，与树叶颜色类似，但叶子的亮度会有所不同。”巴尼特说。研究小组称，尽管青蛙身体的颜色在树叶变暗或变亮的情况下变化不大，但它的腿却更透明，因此亮度也会发生变化，这有助于两栖动物融入环境。

    “By having translucent legs and resting with the legs surrounding the body, the frog’s edge is transformed into a softer, less contrasting gradient from the leaf to the legs, and again from the legs to the body,” said Barnett, noting that this makes the frog’s outline less recognisable to predators.

    “通过拥有半透明的腿，用腿环绕着身体休息时，从树叶到腿部，再从腿部到身体，青蛙的边缘被转换成更柔和、对比度更小的梯度。” 巴尼特说，值得注意的是，这使得青蛙的轮廓更不容易被捕食者识别。

    Writing in the US journal Proceedings of the National Academy of Sciences, Barnett and colleagues report how they carried out three experiments.

    巴尼特和他的同事在《美国国家科学院院刊》上报告了他们是如何进行三个实验的。

    In the first, they photographed 55 glass frogs both on leaves and on a white background and then used computer models to compare the colour of the frog in each scenario. “We found that the colour of the frogs’ bodies did not change much between backgrounds, but the legs did change significantly,” said Barnett, adding that the change was down to a shift in brightness, not hue.

    在第一个实验中，他们在树叶和白色背景上拍摄了55只玻璃青蛙，然后用电脑模型来比较每种场景中青蛙的颜色。巴尼特说：“我们发现青蛙的身体颜色在不同背景下变化不大，但它们的腿确实有很大变化。”他补充说，这种变化是亮度的变化，而不是色调的变化。

    The results were the same when they modelled how different species may see these frogs, including a snake, a bird and a human. “The camouflaging effect is interpreted in a similar manner between humans and the frogs’ natural predators,” said Barnett.

    当他们模拟不同物种如何看待这些青蛙（包括蛇、鸟和人）时，结果是相同的。巴尼特说：“在人类和青蛙的自然捕食者之间，伪装效应的解释是相似的。”

    The team then produced computer-generated images of glass frogs with different patterns of translucency against leafy backgrounds. Twenty people were each presented with 125 such images and asked to point out the frog as quickly as possible. The team found participants were quicker to spot the frog when it was fully opaque compared with frogs with a natural pattern of translucency.

    然后，研究小组用电脑生成了玻璃蛙的图像，在树叶背景下呈现出不同的半透明模式。给参与研究的20个人每人125张这样的图片，并要求他们尽快找出这只青蛙。研究小组发现，与天然半透明的青蛙相比，完全不透明的青蛙更容易被参与者发现。

    Finally, the team made 180 translucent and 180 opaque frogs out of gelatine and put them in vegetation in Ecuador, monitoring over the course of 72 hours whether the frogs were attacked by birds. Overall, 53 opaque and 24 translucent frogs were eaten during the experiment.

    最后，研究小组用明胶制作了180只半透明和180只不透明的青蛙，并把它们放在厄瓜多尔的植被中，在72小时的时间里监测青蛙是否受到了鸟类的攻击。在实验过程中，总共吃掉了53只不透明和24只半透明的青蛙。

    “Our study shows that being translucent does help glass frogs camouflage themselves from predators, but not necessarily in the way expected by comparison to fully transparent species,” said Barnett.

    “我们的研究表明，半透明确实有助于玻璃蛙伪装自己免受捕食者的攻击，但不一定是完全透明的物种所期望的那样。”巴尼特说。

    Prof Devi Stuart-Fox, an expert on animal colour and behaviour at the University of Melbourne, who was not involved in the research, said: “This is a fascinating study because it shows yet another form of camouflage in animal – the sheer diversity of camouflage strategies in nature is truly remarkable.”

    墨尔本大学动物色彩和行为研究专家德维·斯图尔特-福克斯教授没有参与这项研究，她说：“这是一项令人着迷的研究，因为它展示了动物身上的另一种伪装形式，自然界中伪装策略的多样性真的很了不起。”

    Stuart-Fox said all three of the experiments had limitations, but taken together the evidence was compelling that the frogs’ translucency is a form of camouflage.

    斯图尔特-福克斯说，这三个实验都有局限性，但综合起来的证据是令人信服的，青蛙的半透明是一种形式的伪装。

    “Interestingly, the legs are more translucent than the body, making the edges of the body harder to distinguish,” she said. “Predators form a search image for the shape of their prey, so masking the body’s outline is a very effective strategy to enhance camouflage.”

    “有趣的是，腿比身体更透明，使得身体的边缘更难分辨。”她说，“捕食者会根据猎物的形状形成搜索图像，所以掩盖身体的轮廓是增强伪装的一种非常有效的策略。”

原文链接：https://www.theguardian.com/science/2020/may/25/why-glass-frogs-have-see-through-skin-becomes-clear-study-legs-translucent

（责任编辑：陈潭）

（版权说明，转载自：科学猫科普公众号）