Integrative thermal and hydric responses to dynamic heat-desiccation challenge in an endangered tepui summit...

Small amphibians active on dry, sunlit substrates face a physiological conflict: exposure to solar radiation and heat transfer from sun-warmed substrates can increase body temperature and activity, but permeable skin makes such exposure costly through rapid evaporative water loss. Most amphibians reduce this cost by restricting water loss or by remaining in moist microhabitats. The endangered tepui summit toad Oreophrynella quelchii, however, is frequently observed basking for extended periods on dry, exposed sandstone, raising the question of whether this species copes with coupled heat-desiccation stress by suppressing evaporative water loss or by sustaining high evaporative flux and recovering from acute body mass loss. We investigated upper thermal tolerance, behavioural thermal avoidance, and water-loss dynamics in O. quelchii during a standardised dynamic heat-desiccation challenge. To provide ecological context, we also report limited observations from the sympatric frog Pristimantis aureoventris, which is associated with more buffered vegetation microhabitats. Oreophrynella quelchii exhibited CT_max values averaging 32.6 °C. Although this value is not exceptional among amphibians as a whole, it combines with previously reported extremely low CT_min values to produce a broad thermal tolerance breadth for a tropical amphibian. Behavioural avoidance occurred below CT_max, indicating a margin between voluntary retreat and critical thermal failure. During dynamic heat-desiccation exposure, evaporative flux increased with body temperature and vapour pressure deficit and approached passive evaporation estimates from a plaster-model reference, suggesting limited restriction of water loss under fixed-posture exposure. Despite this high evaporative flux, individuals reached the predefined acute body-mass-loss endpoint of approximately 10% initial mass and recovered after rehydration. Limited observations from P. aureoventris indicated lower thermal limits and lower evaporative flux under the same experimental framework, but trials were stopped at a lower safety endpoint and these data are interpreted as contextual rather than comparative because of the small sample size. Overall, our results suggest that O. quelchii does not cope with exposed tepui summit conditions primarily by strongly suppressing evaporative water loss. Instead, its physiological profile appears to combine broad thermal tolerance, behavioural avoidance of critical overheating, high evaporative flux, and the capacity to sustain and recover from acute body mass loss. This tolerance-based strategy may help explain how a small amphibian remains active on exposed tepui sandstone, while highlighting the need to integrate thermal and hydric traits when interpreting amphibian activity in extreme microhabitats.