Physiological responses of ectotherms to daily temperature variation

Pippa Kern, Rebecca L. Cramp, Craig E. Franklin

Daily thermal fluctuations (DTF) impact the capacity of ectotherms to maintain performance and energetic demands due to thermodynamic effects on physiological processes. Mechanisms which reduce the thermal sensitivity of physiological traits may buffer ectotherms from the consequences of DTF. Species which experience varying degrees of DTF in their environments may differ in their responses to thermally variable conditions, if thermal performance curves reflect environmental conditions. We tested the hypothesis that in response to DTF tadpoles from habitats characterised by small DTF would show greater plasticity in the thermal sensitivity physiological processes than tadpoles from environments characterised by large DTF. We tested the thermal sensitivity of physiological traits in tadpoles of three species which differ naturally in their exposure to DTF raised in control (24°C) and DTF treatments (20-30°C and 18-38°C). DTF reduced growth in all species. Development of tadpoles experiencing DTF was increased for tadpoles from highly thermally variable habitats (∼15%), and slower in tadpoles from less thermally variable habitats (∼30%). In general, tadpoles were unable to alter the thermal sensitivity of physiological processes, although DTF induced plasticity in metabolic enzyme activity in all species, although to a greater extent in species from less thermally variable environments. DTF increased upper thermal limits in all species (between 0.89-1.6°C). Our results suggest that the impact of increased thermal variability may favour some species while others are negatively impacted. Species that cannot compensate for increased variability by buffering growth and development will likely be most affected.