Correlated trait responses to multiple selection pressures in larval amphibians reveal conflict avoidance strategies

1. Larval amphibians frequently experience simultaneous, conflicting selection pressures from ultraviolet-B (UV-B) radiation and predation risk, two stressors that can select for opposing defence strategies. When both UV-B and predators are present, individuals can reconcile potential conflicts by correlating particular trait responses (e.g. combining dark or light body colouration with increased or decreased activity rates to create an appropriate multiple stressor strategy). Optimal combinations of body colour and activity rate are predicted to change across an elevation gradient with increasing UV-B exposure. 2. In this study, we tested how larval amphibians combine changes in body colouration and activity rates to create a correlated response to potentially conflicting selection pressures. We quantified activity and colour response in two amphibian species, the pacific treefrog (Hyla regilla) and the long-toed salamander (Ambystoma macrodactylum), from both high and low elevation populations, and exposed individuals to both a common predator and naturally relevant levels of UV-B. 3. Hyla regilla and A. macrodactylum individuals from low elevation populations responded with correlated response strategies while high elevation populations did not. Low elevation H. regilla coupled decreased activity rates to reduce predator detection with dark body colouration to screen out UV-B. Low elevation A. macrodactylum adopted cryptic colouration when predators were present and decreased activity in response to UV-B. Individuals from high elevation H. regilla and A. macrodactylum populations responded only with changes in activity and not colour change. 4. The observed population differences may reflect variation in selection strengths across an elevation gradient. High elevation habitats may require individuals to focus defence efforts on the greatest potential risk. Additionally, pigmentation changes may not be an adequate defence in these UV-B intense habitats.