The energetic cost of exposure to UV radiation for tadpoles is greater when they live with predators

1. Global increases in ultraviolet-B radiation (UVBR) associated with stratospheric ozone depletion are thought to be contributing to the rapid disappearance of amphibian populations from pristine habitats around the world. Much research has been dedicated to understanding the effects of ultraviolet radiation (UVR) alone and in combination with other environmental stressors on the early life stages of amphibians. Little is known, however, of how UVR affects amphibian metabolism, or how amphibian metabolism may change in response to exposure to other stressors in addition to UVR. 2. Using a controlled laboratory experiment, we examined the independent and interactive effects of UVR and non-lethal predatory chemical cues (PCC) on the tissue and whole-animal metabolic rate (MR) and activity of striped marsh frog Limnodynastes peronii tadpoles. PCC signal risk of predation, which is a natural stressor that can cause tadpoles to alter their behaviour, morphology, and life history, and for which the metabolic cost remains little explored. 3. We found that exposure to UVR caused tissue MR to increase by 36%, but whole-animal MR to decrease by 14%, which is most likely due to tadpoles reducing their activity levels by 56%. Exposure to PCC had no significant effect on tissue or whole-animal MR, but caused tadpoles to reduce their activity levels by 36%, indicating that the whole-animal MR of tadpoles exposed to PCC is elevated relative to their activity levels. Compared with tadpoles exposed to neither stressor, tadpoles exposed simultaneously to UVR and PCC showed no change in whole-animal MR despite reducing their activity levels by 62%. 4. This research shows that, for tadpoles, there is an energetic cost associated with being exposed to UVR and PCC independently, and that this cost is greater when they are exposed to both stressors simultaneously. Our previous research has shown that exposure of tadpoles to PCC enhances the lethal effects of UVR, and the present study suggests that this synergistic interaction may arise as a consequence of the effect of these combined stressors on MR. Global increases in UVBR may therefore be contributing to amphibian population declines by compromising energy allocation towards growth and development as well as energy allocation towards coping with additional environmental stressors.