Behavioural evidence of a humidistat: a temperature-compensating mechanism of hydroregulation in spotted salamanders.

The ability to control hydration state is essential for terrestrial species, especially amphibians, which are highly susceptible to dehydration. Here, we examined how temperature (17°C versus 22°C) influenced behavioural hydroregulation in spotted salamanders (Ambystoma maculatum) using a laboratory humidity gradient. Salamanders defended a constant vapour pressure deficit (VPD) between temperatures by targeting higher relative humidity at 22°C than at 17°C, possibly to compensate for increased evaporative demand at warmer temperatures. Individuals selecting higher VPDs experienced greater evaporative water loss (EWL), with larger salamanders losing more water than smaller ones after accounting for temperature. Together, these results highlight a trade-off among body size, humidity preference and desiccation tolerance. Salamanders also rehydrated faster at 22°C than 17°C, highlighting temperature-dependent water uptake rates. Our finding that salamanders regulated a constant driving force of evaporation between temperatures suggests they are able to detect rates of EWL. Local evaporative cooling of the skin is a plausible mechanism: if moist-skinned ectotherms show local evaporative cooling on exposed surfaces but not on those in contact with the substrate, then the temperature gradient between dorsal and ventral skin could act as an effective cue to detect the drive for EWL. Ultimately, our study underscores the complexity of amphibian hydroregulation and emphasises the role of behaviour in maintaining hydration state.