the animal, rather than the trainer; therefore the discriminative stimulus is generally gestural (a hand sign) and sparse in nature. Unobtrusive dog whistles are used as bridges, and positive reinforcers are either primary (food) or tactile (rub downs), and not vocal. However, pinnipeds and mustelids (sea lions, seals, walruses, and otters) can hear in our frequency, so most of the time they will receive vocal reinforcers during shows and performances. The shows are turned into more of a play production because of this, instead of just a run through of behaviors like cetaceans generally do in their shows. Guests can often hear these vocal reinforcers when attending a SeaWorld show. During the Clyde and Seamore show, the trainers may say something like: "Good grief, Clyde!" or "Good job, Seamore". The trainers substitute the word "good" in the place of food or rubdowns when teaching a specific behavior to the animals so that the animals no longer need constant feeding as praise for achieving the appropriate behavior. Field research On an experimental basis, wildlife researchers have employed animal trainers in their interactions with animals in the field. List of notable animal trainers Known for their influence on the circus: Dolores Vallecita (1877–1925), American vaudeville entertainer, and circus animal trainer. Hanno Coldam (1932–1992) Chief Animal Trainer with the 1960–1990. Brothers Vladimir Durov (1863–1916) and Anatoly Durov (1887–1928) Russian circus animal trainers and founders of the Durov Animal Theater in Moscow. Carl Hagenbeck (1844–1913) a merchant of wild animals who introduced "natural" animal enclosures. Gunther Gebel-Williams (1934–2001) trained animals for the Ringling Bros. and Barnum & Bailey Circus. Martin Lacey, (born 1947), animal trainer, owner of the Great British Circus, trained most of the tigers used in the ESSO TV advertisements in the 1970s. Martin Lacey Jr., (born 1977), son of Martin, an animal trainer and performer with Circus Krone in Munich. Known for scientific research: Ivan Pavlov (1849–1946) studied the psychology of animal training and described the phenomenon of classical conditioning. Known for earliest commercial application of Skinner's operant conditioning: Keller and Marian Breland, Animal Behavior Enterprises Known for work in television and film: Nell Shipman (1892–1970) a Canadian film maker in early Hollywood. Frank Inn (1916–2002) trained dogs used in the Benji series. Brothers Frank Weatherwax and Rudd Weatherwax trained the collie Pal, which portrayed the first Lassie. Ralph Helfer Richard (Ric) O'Barry trained dolphins for the original 1960s Flipper television