Short-term responses of Rana arvalis tadpoles to pH and predator stress: adaptive divergence in behavioural and physiological plasticity?

Environmental stress is a major driver of ecological and evolutionary processes in nature. To cope with stress, organisms can adjust through phenotypic plasticity and/or adapt through genetic change. Here, we compared short-term behavioural (activity) and physiological (corticosterone levels, CORT) responses of Rana arvalis tadpoles from two divergent populations (acid origin, AOP, versus neutral origin, NOP) to acid and predator stress. Tadpoles were initially reared in benign conditions at pH 7 and then exposed to a combination of two pH (acid versus neutral) and two predator cue (predator cue versus no predator cue) treatments. We assessed behavioural activity within the first 15 min, and tissue CORT within 8 and 24 h of stress exposure. Both AOP and NOP tadpoles reduced their activity in acidic pH, but the response to the predator cue differed between the populations: AOP tadpoles increased whereas NOP tadpoles decreased their activity. The AOP and NOP tadpoles differed also in their CORT responses, with AOP being more responsive (CORT levels of NOP tadpoles did not differ statistically across treatments). After 8 h exposure, AOP tadpoles had elevated CORT levels in the acid-predator cue treatment and after 24 h exposure they had elevated CORT levels in all three stress treatments (relative to the benign neutral-no-cue treatment). These results suggest that adaptation to environmental acidification in R. arvalis is mediated, in part, via behavioural and hormonal plasticity.