What did the black dogs do when the author and his friends came closer to them?

Read the following passage carefully and answer the questions given below it We passed nomads’ dark tents pitched in splendid isolation, usually with a huge black dog, a Tibetan mastiff, standing guard. These beasts would cock their great big heads when they became aware of our approach and fix us in their sights. As we continued to draw closer, they would explode into action, speeding directly towards us, like a bullet from a gun and nearly as fast.These shaggy monsters, blacker than the darkest night, usually wore bright red collars and barked furiously with massive jaws. They were completely fearless of our vehicle, shooting straight into our path, causing our driver to brake and swerve.