look right through me

I hiked up a mesa following pronghorn and got about 25 yards away from their herd. They did not notice me for about five minutes as I watched them graze, but when they did notice me they formed a defensive line and began snorting aggressively. I held still and continued to take pictures for about three minutes before they took off in a mock charge towards me. They broke around 10 yards away from me before scattering.
The defensive line can be viewed here: