Effects of predators and conspecific chemical cues on the swimming activity of Rana temporaria and Bufo bufo tadpoles

Behavioural observations were performed in an olfactometer to examine the effects of chemical stimuli from predators and conspecifics on the swimming activity of two larval anurans, the common frog, Rana temporaria, and the common toad, Bufo bufo. Both species of tadpoles showed a slight but non significant reduction in swimming behaviour when confronted to chemical cues from starved sympatric predators (larval spotted salamander, Salamandra salamandra; larval dragonfly, Anax imperator). In contrast, test solutions from starved Astacus leptodactylus, a recently introduced predator, produced no change at all in behaviour. As a whole, significant results indicate that indirect chemical signals resulting from the preying activity of the predator are mainly used for the chemical assessment of predation risk by tadpoles. Swimming activity of R. temporaria and B. bufo tadpoles was also found to be significantly decreased in response to chemical cues released by crushed conspecifics, or by A. leptodactylus within the day following ingestion of conspecific tadpoles. Our results suggest that the presence of conspecific alarm substances in water and predators' waste products play a pre-eminent role in the chemical detection of predators by these larval anurans.