They could know it in three ways: they could smell the danger, or hear it, or see it. I shall tell you how they do all that.
First, if the danger came from the direction in which the wind was blowing, they would sniff the air, and so smell the danger. If the danger were a tiger, the buffaloes could smell him half a mile off; that is about as far as ten blocks in a city. And if the wind were not blowing that way, the buffaloes could still smell the tiger five blocks away. They could smell the tiger, or any other danger, even if it came from behind.
The second way of finding out the danger is to hear it. As I said a little while ago, if you should put your foot on a rotten twig, the buffaloes could hear the sound of it as far off as five blocks. And even if the danger came from behind, or from the side, or from anywhere, they could still hear it coming, if it made the least bit of sound that you and I could not hear.
The third way of finding out the danger is to see it. The buffaloes do that by keeping a lookout nearly all the time. I shall show you how.
Just watch for a minute the buffalo in the middle of the crescent; he is the leader of the herd. We can see him only from the back; but as he is the biggest and tallest buffalo there, we can make him out quite easily. He is grazing quietly, and then moving along.
But see, what is he doing now? Why, he is looking up, straight ahead of him! No, he sees no danger there. So he gives a glance to his right, and then to his left. No, there is no danger there either. So he puts down his head, and starts feeding again.
Thus, you see, every now and again he looks to see that no danger is coming from anywhere in front of the herd.
But what if any danger came from the side of the herd,—right near the end of the crescent,—or even from the back of the herd?
Buffalo Sentinels
Let us see what the two buffaloes at the two ends of the crescent are doing. They are the watchers, or sentinels, as they are sometimes called. They keep a lookout nearly all the time.