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Frog Spit Behaves Like Bug¹-Catching Ketchup²

青蛙唾液竟然类似番茄酱？

You might think frogs catch insects ‘cause their tongues are sticky. "But why is the tongue sticky, and how does it actually adhere to these insects at these very high accelerations³?"

你可能会认为青蛙能够捉到昆虫是因为它们的舌头有粘性。“但为什么青蛙的舌头会很粘，而且它到底是如何做到在非常高的加速度基础上粘住这些昆虫的？”

Those are the questions Alexis Noel, a PhD candidate in mechanical engineering at Georgia Tech, wanted to answer. So she and her team got frog tongues from a dissection⁴ lab, and tested their consistency⁵. Ten times softer than human tongues. A texture⁶ more like brain tissue. "Their tongue is very much like a sponge. It's infused with this thick, viscous⁷ saliva⁸."

这正是乔治亚理工大学机械工程博士生亚历克西斯·诺埃尔想要找出答案的问题。她和她的团队从解剖实验室找来一些青蛙舌头，并对它们的稠度进行了测试。结果发现，青蛙舌头比人类的舌头柔软十倍。其纹理更像脑组织。“它们的舌头很像海绵。它充满了浓浓的、粘性唾液。”

\That saliva was their next study subject. "And in order to test the saliva we had to get about a fifth of a teaspoon⁹ of fluid. Which is a lot of saliva, in a frog's case." They put the saliva in a rheometer, a tool that can measure viscosity¹⁰. And they found that frog saliva is what's called a 'shear¹¹-thinning fluid'—its viscosity changes, depending on conditions.

而这种唾液正是她们的下一个研究对象。“为了测试这种唾液，我们不得不搜集约五分之一茶匙的液体。而这对青蛙来说，是非常多的唾液。”她们把唾液放进一个流变仪内——一种可以测量粘度的工具。然后她们发现，青蛙唾液就是所谓的“剪切稀释液”——它的粘度会随着条件变化而变化。

You might be more familiar with a different shear-thinning fluid. "Ketchup. When you smack¹² the bottom of the ketchup bottle you're actually invoking¹³ shear forces within the ketchup itself. And ketchup, because it's shear thinning, its viscosity actually drops and allows it to slide out of the bottle easily."

你可能对另一种剪切稀释液更熟悉。“番茄酱。当你掌击番茄酱瓶的底部时，实际上就是在激起番茄酱里面的剪切力。由于番茄酱会剪切稀释，因而它的粘度实际上会下降，使它能够轻易地从瓶中滑出。”

So back to our frogs: the tongue shoots out, hits the bug and deforms¹⁴ around it. That impact is like a smack on a ketchup bottle—it changes the saliva from thick and sticky to more watery¹⁵, free to flow all over the bug. Then the tongue bounces back, like a bungee cord, and the saliva thickens up again. What's next is beyond weird¹⁶.

再回到青蛙的研究上来：青蛙吐出舌头，击中昆虫并导致其周围变形。这种效果就像掌击番茄酱瓶底部——它将浓稠粘性的唾液变得湿软，能够自由地将昆虫淹没。然后舌头像一个弹力绳一样反弹回来，唾液又重新变得浓稠。接下来的事情更是不可思议。

"Frogs actually take their bulbous eyeballs and bring them down into their mouth cavity and use their eyeballs to shove the insect down the throat." That force turns the saliva watery again, "and the insect slides down the gullet." The study is in the Journal of the Royal Society Interface¹⁷.

“实际上青蛙会将它们的球状眼球带下来，放到它们的口腔中，然后利用眼球把昆虫推到喉咙里。”那股力量又会把唾液变得湿软。“然后昆虫就会向下滑向食道”。这项研究发表在《英国皇家学会界面杂志》上。

With the case closed on this one, Noel's next inquiries¹⁸ are on grippy fingertips. The dust-collecting properties of earwax. And —wait for it— cat tongues. "Yeah, I get paid to watch cat videos all day."

这项研究完成以后，诺埃尔接下来想要研究的是令人痛苦的指尖、耳垢的收尘特性、以及猫的舌头。“是的，我整天都在看猫的视频。”