Data set to accompany manuscript \"Behavioural evidence of a humidistat: a temperature-compensating mechanism of hydroregulation\"

1. Water availability on land fluctuates both spatially and temporally, making the ability to control hydration state essential for terrestrial species. This is especially true in amphibians, which are more susceptible to dehydration than other vertebrates. Here, we used a laboratory humidity gradient to test how temperature (17°C vs. 22°C) affected behavioural hydroregulation in the spotted salamander (Ambystoma maculatum). 2. We found that salamanders defended a constant vapour pressure deficit (VPD), which was achieved by altering relative humidity (RH) selection between temperatures. Targeting a higher RH at 22°C than at 17°C possibly compensates for increased evaporative demand at higher temperatures, indicating a temperature-compensating behavioural mechanism. 3. Salamanders that selected higher VPDs experienced greater rates of evaporative water loss (EWL), with larger individuals showing higher EWL rates than smaller ones, even after accounting for temperature effects. Together, these results highlight a trade-off among body size, humidity preference, and desiccation tolerance. Salamanders also rehydrated faster at 22°C than at 17°C, showcasing temperature-dependent differences in water uptake rates. 4. Our findings demonstrate that field measurements such as RH alone are insufficient to predict behaviours of A. maculatum, since their behavioural preferences are mediated by how they manage water loss biophysically, rather than through simple meteorological measurements. 5. Therefore, ecophysiological measures should be considered in any field studies assessing habitat or microhabitat usage. Ultimately, our study underscores the complexity of amphibian hydroregulation and emphasises the critical role of behavioural strategies in maintaining hydration state.