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第1段
1 .<-NARRATOR:-> Now listen to part of a lecture on this topic in a zoology class.
现在请听动物学课上关于这个话题的部分内容。
第2段
1 .<-MALE PROFESSOR:-> Let's start with an example you may be familiar with.
在这堂课开头，我先讲一个你们可能都觉得熟悉的例子吧。
2 .What's one thing dogs typically do when they when they feel threatened...
当狗感觉自己受到了威胁，
3 .when they want to protect themselves?...
当它们想要保护自己的时候，它们通常会做的一件事是什么呢？
4 .that's right, they show their teeth.
没错，它们会露出自己的牙齿。
5 .And when we see this, we know this behavior means, basically, "stay away!"
而当我们看见这一现象时就知道，这种行为基本上是在说 “走开！”
第3段
1 .Now, here's how this behavior became a recognizable warning.
下面我要讲的就是这种行为是如何成为一种极具辨识度的警告的。
2 .A long, long time ago,
很久很久以前，
3 .a dog sensing danger would get ready to bite whatever animal was threatening it.
有这么一只狗，无论面前威胁着它的是什么动物，一旦察觉到危险，它都会准备好上前撕咬。
4 .It started by baring its teeth,
它首先会露出牙齿，
5 .but the purpose of doing this was something very simple...
不过狗这么做的原因很简单：
6 .the dog instinctively was making sure it wouldn't bite its own lips when it bit the other animal. OK?
它只是下意识地确保自己在咬其他动物的时候不会咬到自己的嘴唇，懂了吗？
7 .But what happened ...
但是接下来发生的事情就是：
8 .other dogs, other animals ...over time,
其他的狗，其他的动物，
9 .they learned that this teeth baring always came before a dog was going to bite ...
慢慢发现在这只狗咬它们之前总是会先把牙齿露出来，
10 .and so they started to read it as a signal to be careful...
于是它们就开始把这种行为理解成要小心的信号，
11 .to keep away or risk getting bitten.
这种行为一出现就说明它们该走开了，不然就要承担被咬的风险。
12 .This scenario, repeated time after time,
这种情节一次次重复上演之后，
13 .also led to the dog's awareness that by simply showing its teeth,
这只狗也意识到了，很多时候单单把牙齿露出来
14 .a lot of the time, that was protection enough.
就足够保护它自己了。
15 .What had been a simple physical preparation to bite had developed into a kind of defense mechanism in and of itself.
曾经只是一种撕咬之前所做的简单身体准备，后来便发展成了狗的一种防御机制。
16 .The dog learned it did not need to actually attack,
这只狗发现它并不需要真正进行攻击，
17 .but could get the right result from just communicating the possibility of an attack through showing its teeth.
反而仅仅通过露出牙齿向其他生物展示攻击的可能性，就能获得满意的结果。
18 .It started to use this behavior as a warning signal.
接着它便开始把这种行为用作一种警告信号。