Behavioural evidence of a humidistat: a temperature-compensating mechanism of hydroregulation

The ability to control hydration state essential for terrestrial species, especially amphibians, which are highly susceptible to dehydration. Here, we examined how temperature (17°C vs. 22°C) influenced behavioral hydroregulation in spotted salamanders ( Ambystoma maculatum ) using a laboratory humidity gradient. Salamanders defended a constant vapour pressure deficit (VPD) between temperatures by targeting higher RH 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 the ability to detect rates of EWL. Local cooling of the skin arising from evaporation is a plausible mechanism: if moist-skinned ectotherms allow for a specific, fixed decline in skin relative to core temperature, they can effectively detect the drive for evaporation. Ultimately, our study underscores the complexity of amphibian hydroregulation and emphasises the role of behaviour in maintaining hydration state.