英语听力：探索发现 2012-11-25 美洲大平原 American Serengeti—13

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First they get the bison on the run, then filter out the weak and vulnerable and select the perfect target. Striking together, wolves can bring down prey³ many times their own size. A million such chases must have taken place across these plains and we can still find echoes of these distant life-or-death encounters.

 

Not all evidence lies locked in bone and rock. These pronghorn antelopes⁴ among the great survivors⁵ of the ice age reveal a lot about the distant past. As well as being tough enough to stand extremes of temperature, they're famous for their speed. 

 

A sprinting⁶ pronghorn can top 60 miles an hour and cruise at 30 for several hours. This kind of speed requires a very finely tuned⁷ physique. Pronghorn have a massive heart and run with their mouths gaping⁸ open, forcing extra air into their huge lungs.

 

But what's the point? No predator⁹ can run this fast. Even the wolves can only manage 40 miles an hour. So why do pronghorn feel this need for speed? This is why—once there was a predator here that could outrun the pronghorn a cheetah¹⁰.

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