Animals are usually the "guinea pigs" for human medicine, but a recent case at Smithsonian's National Zoo worked the other way around. After inventing a procedure that reverses vasectomies and performing it on about 13,000 patients, St. Louis-based urologist Sherman Silber successfully operated on a critically endangered species at the zoo. "Minnesota," a Przewalski's horse kept there since 2006, had been vasectomized to keep him from horsing around in the co-ed enclosure at his previous home. The species became extinct in their native Central Asian habitats about 40 years ago, and there are only 1,500 are alive today. When the zoo's Budhan Pukazhenthi, a reproductive specialist, realized that Minnesota's genes made him an ideal sire, he asked Silber to team up with the zoo's veterinary staff. "To maybe bring an extinct animal back from extinction was pretty thrilling," Silber says. Minnesota will be introduced to a mate this summer.