Look carefully at this mule deer's back leg. How is its knee different from ours?
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There are several differences. First, the joint that seems to be its knee is really its ankle. The knee is much higher on the leg. It's knee is always bent. In fact, the deer cannot extend its leg far enough for the knee to be straight. That may seem strange at first, but think about what you have to do to prepare to jump. The first thing you have to do is bend your knees, so that you can straighten them to jump. That takes time, which can mean the difference between life and death if there is a predator trying to catch you. Since the deer's knee is always at least partially bent, it is always ready to leap or run. The same is true for horses and many other animals that need to run quickly.