Limited physiological capacity for response to warming in lowland tropical frogs

Background/Question/Methods

Lowland tropical habitats are among the warmest environments on a warming planet.  Many ectotherms found in such climates are living close to the upper limits of their thermal tolerances.  As climate warms further, such organisms must respond physiologically or behaviorally to increasing temperatures or suffer local extirpations.  Lowland tropical ectotherms would be particularly susceptible to increasing temperatures if they show limited or no capacity for physiological acclimation to warming.  We estimated the field substrate temperatures and critical thermal maxima (“CTmax”) for an assemblage of lowland tropical frogs.  We estimated critical thermal maxima by placing frogs in water baths and slowly increased water temperatures until individual frogs lost their righting reflex. Further, we tested three common species of frogs for acclimation capacity by holding frogs in stable, temperature-controlled environments at 20°C, 26°C, 28°C, 30°C, and estimated the CTmax every 5 days for 20 days.

Results/Conclusions

We estimated substrate temperatures and CTmax for 16 species of frogs from 8 families.  Substrate temperatures varied widely among species, between 23-28°C.  Critical thermal maxima ranged from 28°C to 38°C.  We found strong phylogenetic signature to CTmax, with direct developing strabomantid and craugstorid frogs characterized by the lowest CTmax values.  None of the three species we tested for acclimation response showed any increase in CTmax after 20 days.  Our study indicates that many species of lowland tropical frogs currently exist very close to their thermal upper limits and that these species show no capacity for physiological acclimation to higher temperatures.  We discuss the results of this study in relation to amphibian population declines, emerging infectious diseases, and lowland biotic attrition.