Empirical estimation of skin resistance to water loss in amphibians: agar evaluation as a non-resistance model to evaporation.

Total resistance (rt) to evaporative water loss (EWL) in amphibians is given by the sum of the boundary layer (rb) and the skin resistance (rs). Thus, rs can be determined if the rb component is defined (rs=rt-rb). The use of agar models has become the standard technique to estimate rb under the assumption that the agar surface imposes no barrier to evaporation (rs=0). We evaluated this assumption by determining EWL rates and rb values from exposed surfaces of free water, a physiological solution mimicking the osmotic properties of a generalized amphibian, and agar gels prepared at various concentrations using either water or physiological solution as diluent. Water evaporation was affected by both the presence of solutes and agar concentration. Models prepared with agar at 5% concentration in water provided the most practical and appropriate proxy for the estimation of rb.