Interactive effects of experimental heating rates, ontogeny and body mass on the upper thermal limits of anuran...

Biological and methodological factors influence the upper thermal limits (UTL) of ectothermic animals, but most factors have been studied independently. Few studies have integrated variables, so our understanding about sources of UTL variation remains fragmentary. Thereby, we investigated synergic effects of experimental protocols (heating rates, ΔTs) and biological factors (ontogeny and body mass) on the UTL on the larvae of two anuran species (Physalaemus nattereri and Boana pardalis), specifically their Critical Thermal Maximum (CT_max). The species displayed slightly different responses to ΔTs: In B. pardalis tadpoles both average and variance of CT_max increased at a fastest ΔT, the same response happened in P. nattereri tadpoles at slow and moderate ΔTs. Also, the CT_max of P. nattereri declined at the end of metamorphosis independently of ΔT, but tadpoles at all developmental stages still displayed higher heat tolerance at the slow ΔT. Finally, we detected small, synergic effects of body mass and ΔTs on the CT_max of both species. In small B. pardalis tadpoles and premetamorphic P. nattereri tadpoles, body mass had a positive effect on CT_max, but only at slow and moderate ΔTs, probably indicating physiological responses. A similar trend was observed in large B. pardalis tadpoles at the fast ΔT, but this result is likely to be influenced by thermal inertia. Our findings contribute to integrate the understanding of factors influencing UTL in small ectothermic animals. This understanding is critical to discuss the physiological component of vulnerability to climate change that is related to acute temperatures.