The Buenos Aires National Wildlife Reserve in southern Arizona is a treacherous, wild terrain filled with patches of tall whip-like Ocotillo cactus and bristling bunches of cotton-spired Jumping Chollas. The reserve’s southern edge flattens against the Mexican border and makes it a favored entry point for migrants and smugglers.

The horses are already slick with sweat and we’re only halfway through the patrol. We are looking for "bodies" -- agent slang for blips picked up from the multitude of electronic motion and heat sensors that dot the area. The technology can spot people crossing the border, but retrieving them is where the horse patrol comes in.

The U.S. Customs and Border Protection has been buying horses from the inmate program since its inception. The animals are well-suited for the slow tracking of smugglers; the “chase” creeps along in the rugged terrain. There are no high-speed chases out here on the rocks.

Horses can also move further distances much faster and more quietly than other forms of vehicular movement or humans on foot can. Plus, horses are naturally intelligent. While an agent may think he can tear through a wash on an ATV and then end up misjudging the depth and getting stuck, a horse knows its limitations and can gauge which path is best.