Abstract:
Inter-population disparities in a species have been shown to occur as an adaptation to different thermal regimes
in the environment. We investigated the thermal sensitivities of the tropical toad Duttaphrynus melanostictus (Asia
Common Toad) from two populations at different altitudes: Nuwara-Eliya – 1870 m, and Polonnaruwa – 25 m,
above mean sea level. The two locations were separated by what may be considered a short direct distance – 110
km. Thermal sensitivity trials were conducted at six temperatures between 12 and 39 �C. Assessments were made
using the performance indicators jump distance, jump force, contact time on the test plate following stimulus to
jump, and righting time after being overturned. Optimum performance is taken to be the greatest jump distance
and jump force, the least contact time on the test plate, and the least righting time. The populations at the two
altitudes had markedly different thermal sensitivities – toads in the cool area (Nuwara-Eliya) performed at an
optimal level under low temperatures, whereas the toads in the warm area (Polonnaruwa) performed optimally
under high temperatures. The finding that the thermal optima (i.e., the temperatures at which the optimal
performance for the four performance indicators was recorded) of the toads in Polonnaruwa were below the
mean maximum ambient temperature at this location suggests that these toads would be more susceptible to
global warming than those in Nuwara-Eliya whose thermal optima were above the mean maximum ambient
temperature in that location. This was consistent with the narrower thermal safety margin (i.e., difference between the mean optimum temperature and mean ambient temperature) of toads in Polonnaruwa, compared to
those in Nuwara-Eliya. Importantly, these findings demonstrate that, although thermal sensitivity is considered a
conservative trait, differentiation does occur even over a small spatial scale presumably because it offers an
adaptive advantage to the population concerned.
