Reading plaques at zoo exhibits is so last century. Now you can call up behind-the-scenes videos of zookeepers in action, test your knowledge of animal trivia and navigate the grounds with hand-held multimedia stations. Using global positioning satellite, or GPS, technology to detect your location, the small devices trigger video introductions on 4-inch LCD screens when you approach an exhibit. You can see how grizzly bears get their teeth cleaned and rhinos get pedicures. Designed by Bar Z Adventures in Austin, Texas, the so-called Zoo Rangers rent for $9.95 at the San Francisco and Dallas zoos. In San Francisco, up to 50 visitors a day rent them, spokesman Paul Garcia said.