Meeting Abstract
The measurement of heat and water flux in amphibians has been problematic due to significant rates of cutaneous water loss. Physical surrogates, such as agar replicas of amphibians, have been used as a replacement for complex calculations to understand water loss physiology. Unfortunately, these physical surrogates likely do not measure the processes for which they were meant. Here, we present theoretical and empirical methods that improve estimating water loss traits in amphibians. Skin resistance (Rs) is the gold standard for water loss studies because it reflects the physiological mechanisms underlying water loss rates. To evaluate the efficacy of physical surrogates, we compared theoretical calculations to empirical methods for measuring Rs. Empirically, we measured water loss of agar models in five body sizes, two temperatures, and three vapor pressure deficits using a flow system capable of precisely controlling temperature and vapor pressure. We then calculated the same biophysical parameters under free and forced convection using dimensionless analysis. Upon applying these parameters to data collected from salamanders, empirical estimates underestimated Rs and even resulted in negative values of Rs. Contrary to previous research, our experiments revealed that agar replicas exhibit a temperature-dependent skin resistance. Underestimates of Rs are likely due to evaporative cooling which reduces of the water vapor density gradient. Based on our analyses, we recommend that agar replicas are an inadequate medium to characterize skin resistance to water loss. Instead, first principles provide better estimates of the biophysical parameters necessary to characterize water loss physiology.
