TAILED FROG TADPOLES DIFFERENTIALLY ALTER THEIR FEEDING-BEHAVIOR IN RESPONSE TO NONVISUAL CUES FROM 4 PREDATORS

Tadpoles of the tailed frog (Ascaphus truei) are common in riffles wit hin many small, high-gradient streams of the Pacific Northwest (United States and southern Canada), where they typically graze periphyton fr om exposed cobbles. We conducted field observations and experiments in Clearwater Creek, southwestern Washington, to determine if tadpoles w ould reduce their feeding activity (i.e., emergence from crevices to g raze periphyton) in the presence of non-visual cues released from each of four aquatic predators: giant salamanders (Dicamptodon spp.), cutt hroat trout (Salmo clarki), brook trout (Salvelinus fontinalis), and s horthead sculpin (Cottus confusus). In absence of predators, tadpoles usually emerged from under cobbles to feed at night (2000-0100 h), and spent the remainder of the 24-h interval hidden in crevices. In the p resence of giant salamanders, cutthroat trout, and brook trout that we re all confined within separate, in situ enclosures immediately upstre am of tadpoles, tadpole activity was reduced two- , three- , and six-f old, respectively, compared with predator-free controls. In contrast, tadpoles appeared unable to detect upstream sculpins. Subsequent consu mption experiments in the laboratory showed that salamanders, sculpins , and cutthroat trout all were capable of consuming tadpoles in both s tructurally simple and complex habitats. We hypothesize that the inabi lity of tadpoles to detect predaceous sculpins may explain why tailed frog tadpoles are largely absent from lower-gradient streams where scu lpins are often abundant.