Patrolling the Border on Four Legs

LA GRULLA, Tex. — Manuel Torresmutt, a Border Patrol agent, pulls his white and green Chevy Tahoe to the side of a deserted gravel road, framed on one side by railroad tracks and on the other by thick green brush.

The South Texas sun streams brightly as Mr. Torresmutt, a stocky, 24-year veteran of the United States Border Patrol, steps from his truck to meet with his three-man team. A radio dispatcher