Movement tests

All tests were performed in June and July 2015, after the breeding season when all individuals were in the terrestrial phase. In order to test the crossing capacities of both species, we made them move along two tracks (200 cm long × 10 cm wide × 20 cm high), each filled with a different substrate: smooth concrete slab (human-made) or soil (natural). Tracks were not moistened so that the substrate was unfavorable. During the experiments, all individuals were chased down the tracks and forced to move by gently poking their back with a finger after each stop (3 s between pokes if needed). Only one individual was tested at a time and we recorded the number of stops (stops) and the crossing speed (in cm/s; including stops) to the nearest 0.1 s to travel 200 cm from departure to arrival line. In order to provide reliable estimates of crossing capacity using a repeated-measure design while minimizing stress, every individual was tested three times on each substrate. In toads, tests were randomly spread over 2 days (three trials on day 1 and three trials on day 2; from March 18, 2015 to March 31, 2015) while newts, for which we kept animals during a longer period for another experiment not detailed here, tests were randomly performed over 27.9 ± 8.5 days (mean ± SD; from April 28, 2015 to June 2, 2015). Outside of test periods, animals were returned to the semi-aquatic aquaria. Because locomotion in amphibians is influenced by temperature (Herrel & Bonneaud, 2012; James et al., 2012; Šamajová & Gvoždík, 2010), all tests were performed in a greenhouse under controlled-temperature conditions (mean ± SD: 25 ± 1 °C) with light conditions similar to nature.

Some individuals (n = 32) did not complete the 200 cm run (stopping completely—three successive pokes without moving—or turning back); these individuals and their replicates were removed from the analyses for statistical reasons. We therefore included 77 toads (77 toads × 3 replicates × 2 substrates = 462 tests) and 20 marbled newts (20 newts × 3 replicates × 2 substrates = 120 tests) in the analyses.