Acoustic enrichment in wildlife passages under railways improves their use by amphibians

Linear transport infrastructure can impact wildlife directly or indirectly, and amphibians are particularly affected. In some cases, mitigation measures – such as tunnels below the linear transport infrastructure – are implemented, but an evaluation of their effectiveness is often lacking, especially for small animals such as amphibians. Few studies have focused on crossing behaviour in underpasses, yet this information may be crucial to improve mitigation measures. In this study, we conducted experiments in three tunnels below a high-speed railway with individually marked amphibians (great crested newt Triturus cristatus, marbled newt Triturus marmoratus, alpine newt Ichthyosaura alpestris, fire salamander Salamandra salamandra, spined toad Bufo spinosus, agile frog Rana dalmatina and edible/pool frog Pelophylax kl. esculentus/lessonae) which were released at the entrance of tunnels varying in length. Behaviours (e.g. completed crossing, U-turn, not moving, and stay) and crossing speed were recorded. R. dalmatina, S. salamandra and T. cristatus (but not B. spinosus) exhibited crossing rate differences between tunnels 21 m and 40 m in length. More individuals performed “complete crossing” through the shorter tunnel. The crossing speed of the S. salamandra and T. cristatus did not differ between the tunnels of different lengths. In a second experiment, we evaluated the effect of acoustic enrichment on the crossing behaviour of one frog and three newt species by broadcasting a soundtrack of mating calls of syntopic anuran species in the tunnels. Pelophylax kl exhibited a large increase in complete crossings and in speed. T. cristatus showed an increase in tunnel complete crossings (but not speed) in one of the acoustically enhanced tunnels. These results indicate that acoustic enrichment can improve the crossing of underpasses. This low cost and easily implementable method could help to inform the design of wildlife passages as mitigation measures for impacted amphibian populations, though more experiments on the effect of tunnel characteristics on amphibian behaviour would be valuable.