Early in the day we saw a bright, limber young bullsnake, the diameter of a thick pencil, its body many tiny wiggles instead of the sober curves of an adult. And late in the day I almost stepped on a two-foot Western diamondback.

Its coon-tail and coon-mask were a dustier color than the bright scales in the middle of its back. It didn’t buzz, and seemed quite unbothered by our admiration. No wonder—halfway down its fat, spread-flat body was a mouse-sized bulge.

Though it didn’t react much, it knew we were there: its tongue was busy, tasting our airborne molecules. In spite of this “tongue smelling,” a rattler hunts largely by heat detection. On a summer evening, how does it tell a warm rock from a warm mouse?