Biology session

In response to a predator attack, many prey species release chemical alarm signals that alert other potential prey to the threat. These chemicals, called Schreckstoff, are present in many aquatic vertebrates. While the chemicals and their release mechanisms have been identified in a few species, these seem to vary among species and we know little about how these alarm substances are released in larvae of anuran amphibians. Also, it has been suggested, that Schreckstoff may not only function to alert other individuals, but may also enhance the releaser’s survival probability by attracting further predators that interfere with the primary attacking predator. Using tadpoles of the agile frog (Rana dalmatina), we experimentally investigated if disturbed individuals release Schreckstoff actively, and if common predators of tadpoles are attracted to chemical alarm signals. As predators, we used larvae of the southern hawker (Aeshna cyanea) and of the great diving beetle (Dytiscus marginalis), young individuals of the western pike (Esox lucius), and adult males of the smooth newt (Lissotriton vulgaris). We exposed predators and groups of tadpoles to (1) homogenized tadpoles (all types of cues released by prey), (2) the smell of tadpoles disturbed with a plastic stirrer (only actively secreted chemical cues), or to (3) the smell of undisturbed tadpoles (no alarm substances) and recorded their behaviour. Our results suggest that R. dalmatina tadpoles can not actively relase alarm pheromones. We contradicted some previous studies, that they found actively secreted chemical alarm cues in a North-American Rana species and other Anuran tadpoles. Pikes, dragonfly larvae and beetle larvae did not seem to be attracted to any type of stimuli originating from tadpoles. However, our results suggest that newts do use chemical cues to locate prey. Newts were attracted to the smell of disturbed tadpoles, and tended to show attraction to the smell of undistrubed tadpoles, but were not attracted to the smell of homogenized tapoles. It is possible that the smell of injured tadpoles does not attract newts, because it may indicate the presence of another predator, potentially also posing a threat to them as well. It will be interesting to see in future studies to what degree the predator attractant property of alarm pheromones enhances the survival probability of attacked individuals under realistic onditions.