One year ago, a very special prairie dog appeared at the Saint Louis Zoo. When the pup surfaced, zookeepers saw that it had only one arm, evidently the result of a birth defect. Because prairie dogs' survival is largely dependent on their ability to dig, things didn't look good for the youngster. Yet here we are one year later, and she has overcome the Darwinian odds. The zoo doesn't typically name its prairie dogs, but this li'l fighter burrowed her way into their hearts, and so she was christened Henrietta. She has learned to eat with an arm and a stump; her only evident handicap is a slight limp. In fact, Henrietta is so healthy, she became a mom this year.