托福听力练习：青蛙

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　　托福听力练习：青蛙

　　If you liked the colorful animals we just saw, you're going to love these next animals.Frogs.

　　You might not normally think of frogs as being colorful, but these frogs definitely are.

　　They are the dart-poison frogs of Central and South America.

　　Look at their striking colors often yellow with black stripes or deep blue with black spots.

　　Beyond being nice to look at, these markings have a purpose.

　　They warn predators that these frogs are poisonous.

　　When threatened, these frogs secrete a substance through their skin that would easily kill whatever animal might try to eat them.

　　Their bright colors communicate this, and so most animals tend not to hunt them.Now, speaking of hunting, for centuries these frogs were sought after by hunters.

　　As you might think, the hunters didn't want to eat the frogs, but rather, they captured them for their poison.

　　They would add the poison to the tips of their hunting arrows of course, nowadays most hunters use guns.These days dart-poison frogs are of less interest to hunters than to medical researchers.

　　Researchers believe that they can make new heart medicine from the poison, because it acts as a stimulant on the body's nervous system.

　　Researchers think they could use it to stimulate a weak heart.There is, however, a problem with doing research on these frogs.

　　Those that are caught in the wind will produce their poison until they die.However, those that are born in captivity, like the ones you see here, will not produce any poison at all.

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