Short-term changes in air humidity and water availability weakly constrain thermoregulation in a dry-skinned ectotherm

Abstract

Thermoregulation is critical for ectotherms as it allows them to maintain their body temperature close to an optimum for ecological performance. Thermoregulation includes a range of behaviors that aim at regulating body temperature within a range centered around the thermal preference. Thermal preference is typically measured in a thermal gradient in fully-hydrated and post-absorptive animals. Short-term effects of the hydric environment on thermal preferences in such set-ups have been rarely quantified in dry-skinned ectotherms, despite accumulating evidence that dehydration might trade-off with behavioral thermoregulation. Using experiments performed under controlled conditions in climatic chambers, we demonstrate that thermal preferences of a ground-dwelling, actively foraging lizard (Zootoca vivipara) are weakly decreased by a daily restriction in free-standing water availability (less than 0.5°C contrast). The influence of air humidity during the day on thermal preferences depends on time of the day and sex of the lizard, and is generally weaker than those of of free-standing water (less than 1°C contrast). This shows that short-term dehydration can influence, albeit weakly, thermal preferences under some circumstances in this species. Environmental humidity conditions are important methodological factors to consider in the analysis of thermal preferences.

Fig 1. Preferred body temperatures (mean daily body temperatures, °C) of yearling common lizards from thermal gradients are slightly higher with than without access to drinking water.

Mean, standard errors and dotplots are obtained from combined raw data of males and females.

Fig 2. Individual records of body temperatures (T_b, °C) of yearling common lizards during daytime.

A. Mean, standard errors and dotplots for female and male lizards in neutral arenas from Experiment 1 as a function of availability of free-standing water and air humidity (dry: VPD = 10.66 kPa, wet: VPD = 0.46 kPa). B. Mean, standard errors and dotplots for female and male lizards in neutral arenas from Experiment 1 per daytime period. C. Mean, standard errors and dotplots for lizards of both sexes in neutral arenas from Experiment 2 per daytime period in each air humidity treatment (wet: VPD = 0.70 kPa, super wet: VPD = 0.12 kPa). D. Mean, standard errors and dotplots for female and male lizards in neutral arenas from Experiment 2 per daytime period.

Fig 3. Boxplots of daily body mass change of yearling common lizards in three independent experiments.

A. Effects of water availability (thermal gradient with or without water) and air humidity (dry: VPD = 1.87 kPa, wet: VPD = 0.46 kPa) in neutral arenas from Experiment 1. B. Effects of air humidity (wet: VPD = 0.70 kPa, super wet: VPD = 0.12 kPa) in neutral arenas from Experiment 2. Data are residual body mass change after controlling for sex differences and initial body mass. C. Effects of air humidity (same as B) in neutral arenas provided with shelters from Experiment 3. Letters indicate statistically different groups according to contrasts from the statistical models explained in the main text. Note that differences in mean mass loss between experiments are difficult to interpret because those were performed sequentially and lizards might acclimate to laboratory conditions.