Search

This study was funded by the Agence Francaise pour la Biodiversite (research program supervised by J.M.P.) and the Ministere de l'Education Nationale, de l'Enseignement Superieur et de la Recherche (Ph.D. grant to N.B.). This work was carried out under the licenses 08/2015 and 07/2016 delivered by the Prefecture de la Loire-Atlantique. We thank the Parc Natural Regional de Briere, notably J.P. Damien, for supporting our research activities on amphibian ecology. The authors declare that they have no conflict of interest. We are also grateful to two anonymous reviewers for their constructive comments on an earlier version of the manuscript.; International audience; Stable isotope analyses (SIA) are now widely used in ecology research to quantify the trophic implications of a large range of ecological processes. Non-destructive sampling methods have been successfully developed for most vertebrates, but the validation of such methods is missing in amphibians. Filling this methodological gap is critical to reduce the significant lack of knowledge on the trophic ecology of this imperilled group. The relevance of tail fin as a surrogate for muscle, a tissue routinely used for SIA, was assessed in four amphibian species, including tadpoles and adult newts, by (1) testing the link between the isotope ratios of carbon and nitrogen in fin and muscle, and addressing the additional practical issues of (2) the effects of a topical anaesthetic on isotope ratios, and (3) the minimum quantities of fin needed for SIA to limit the severity of sampling. The isotope ratios were highly correlated between the two tissues in all four species. Equations were provided to predict muscle estimates from fin values for each species separately and grouped by their taxonomic order (anurans or urodeles), which represent a valuable solution for any other species. Topical anaesthetics had little influence on isotope ratios, and the surface of tail fin needed for SIA was small. We conclude that the tail fin of amphibians can be used as a non-lethal surrogate for muscle in SIA, and this offers promising prospects for research on the trophic ecology of amphibians, and potentially on their conservation.; Les analyses d'isotopes stables (AIS) sont maintenant largement utilisées en recherche en écologie pour quantifier les implications trophiques d'une large gamme de processus écologiques. Des méthodes d'échantillonnage non-létales ont été développées chez la plupart des vertébrés, mais une telle validation est absente chez les amphibiens. Il est essentiel de combler cette lacune de façon à réduire le manque de connaissances sur l'écologie trophique de ce groupe menacé. La pertinence de la nageoire caudale comme substitut au muscle, un tissu régulièrement utilisé pour les AIS, a été évaluée chez quatre espèces d'amphibiens, incluant des têtards et des tritons, (1) en testant le lien entre les ratios isotopiques du carbone et de l'azote du muscle et de la nageoire, et en répondant à des questions pratiques comme (2) l'effet d'un anesthésique local sur les ratios isotopiques, et (3) les quantités minimales de nageoire nécessaires aux analyses isotopiques, dans le but de limiter la sévérité de l'échantillonnage. Les ratios isotopiques étaient hautement corrélés entre les deux tissus chez les quatre espèces. Des équations ont été fournies pour prédire les valeurs du muscle à partir de celles de la nageoire en considérant chaque espèce séparément et en les regroupant selon leur ordre taxonomique (anoures ou urodèles) ce qui représente une solution intéressante pour d'autres espèces. L'anesthésique topique a eu une faible influence sur les ratios isotopiques, et la surface de nageoire nécessaire pour les AIS était faible. Nous concluons que la nageoire caudale des amphibiens peut être utilisée en tant que substitut au muscle pour conduire des AIS, ce qui offre des perspectives prometteuses pour la recherche en écologie trophique des amphibiens, et potentiellement pour leur conservation.

In a laboratory experiment, we investigated the role played by conspecific chemical cues in the acquisition of chemical recognition of an unfamiliar predator by common toad tadpoles, Bufo bufo. During a ten-day period, we assigned one group of tadpoles to a control (unexposed) and five other groups to different conditioning treatments involving exposure to either an allopatric predatory crayfish (Astacus leptodactylus), or a sympatric harmless gastropod (Lymnaea stagnalis). We kept the caged crayfish under three conditions: i) starved, ii) starved in combination with cues from crushed tadpoles, or iii) fed with toad tadpoles. In the same way, we kept the caged pond snails either alone or paired with cues from crushed tadpoles. After exposure, we carried out behaviour measurements to assess the effects of conditioning treatments on the behavioural responses to chemical cues from starved crayfish, pond snails and crushed conspecifics. Test solutions from starved crayfish and snails produced a significant decrease in the percentage of moving tadpoles, a known antipredator response, but only in the two groups of tadpoles previously exposed to these organisms in combination with cues from crushed conspecifics. These results indicate that Bufo bufo tadpoles can learn to recognize an organism as representing a predation risk based on a prior association between the chemical stimuli from that organism and the chemical cues from damaged conspecifics.

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.