Abstract:
The thermal performance of an animal is directly linked to its thermal sensitivity. Information on
thermal sensitivity would assume special significance in interpreting the vulnerability of
amphibian species to climate change. Ectotherm species with narrow temperature tolerance
ranges, would be particularly susceptible to predicted changes in ambient temperature. The
present study therefore attempts to assess the degree of thermal sensitivity of two anuaran
species. It is expected that the study would provide an insight into the ability of the species to
adapt to climate change. Two native anurans, Duttaphrynus melanostictus (common Asian toad)
and Euphlyctis cyanophlyctis (Indian skipper frog) were selected as they are widely distributed
across the country. Toads and frogs were collected from four locations: Matara-(mean ambient
temperature (MAT=30.2° C), Nuwara Eliya (MAT=17.6° C), Colombo (MAT=28.4° C),
Polonnaruwa (MAT=34.2° C). The animals were empirically exposed to six test temperatures
(between 12° C and 38° C) at field stations in the locations of capture to assess four performance
parameters (Jump distance, Righting time, Contact time and Jump force). Thermal Performance
Curves (TPCs) were constructed to obtain values for the endpoints-thermal niche breadth (NB),
thermal optima (TO) and optimal performance (OP).The TPCs for each species differed
considerably across the different populations. Both D. melanostictus and E. cyanophlyctis in
cooler areas performed better at lower temperatures, while those from hotter areas performed
better at warmer temperatures. Negative skewedness of TPCs show drastic drops in performance
once optima is reached. There was good congruence between thermal optima and the MATs of
the capture regions (F3, 20=26.87 p<0.05). Niche breadth was narrowest in toads and frogs from
the coldest region (Nuwara Eliya). The results were consistent across both species. The findings
are in agreement with previous observation for disparities in thermal sensitivity across latitudinal
and altitudinal gradients in other anuran species. These trends suggest that amphibian
populations show a degree of adaptability to specific ambient temperatures to which they are
exposed to in their natural environment, enabling them to perform at an optimal level. It was also
noted that in the hottest region (Matara) the thermal optima of E. cyanophlyctis was slightly
below the MAT. This and its narrower thermal niche breadth would make it more susceptible to
increases in ambient temperature than D. melanostictus.
