This pronghorn buck tried to run away from me by crossing my field of vision at a more-or-less constant distance of 25 yards -- so I was lucky to get a couple of shots of the world's second-fastest land mammal in action.

He clearly had at least another gear that he wasn't using, by the way.

Picture was taken on northeast side of Mount Everts, just off the Rescue Creek Trail, Yellowstone NP, late September 2007.