Thermal Acclimation and Tolerance in The Túngara Frog (Engystomops pustulosus)

Understanding species thermal sensitivity is fundamental to predict species distribution patterns and the potential effects of climate change. A challenge in assessing thermal sensitivity is to make the measurements in the laboratory relevant to field conditions. Short-term exposures to extreme temperatures are commonly used to assess the upper (CTmax) and the lower (CTmin) thermal tolerances. Usually, these extreme temperatures can be tolerated for an acute period of time, and these events are uncommon in field conditions. Environmental conditions that are required for continuous growth and development, such as optimal performance temperature (To) and a performance breadth, are also important ecological parameters to assess thermal sensitivity. Furthermore, experimental studies have rarely considered the effects of temperature acclimation in CTmax and CTmin. In this study, our goals were to determine 1) the thermal performance breadth, 2) thermal tolerance, and 3) acclimation capacity in the túngara frog tadpoles (Engstomopus pustulosus), a tropical species distributed in Central America. We exposed the tadpoles to four acclimation temperatures (21ºC, 28ºC, 31ºC, and 33ºC) and estimated the CTmax and CTmin under for each acclimation temperature. We found that CTmax (42ºC) and CTmin (13ºC) were extremely above and below respectively the temperature they experience in the field. The optimum temperature (28ºC) is close to the average environmental temperature (27.4ºC), and the thermal performance breadth is in between 21ºC and 31ºC. Furthermore, they are able to acclimate better to colder than warmer temperatures. Our results indicate that in addition to the thermal tolerances, optimum temperature, thermal performance breadth and acclimation capacity give a more realistic measure of thermal sensitivity and climate change vulnerability.