3 RESULTS
3.1 Water exchange with the environment
Evaporative water losses from agar model frogs in the experimental apparatus (Table 1) ranged from 0.42 mg min-1 inX. laevis to 4.7 mg min-1 in R. catesbeiana , representing, respectively, 0.88% to 0.17% of body mass lost per hour. These rates of evaporative water loss were added to species water uptake rates to account for water lost through cutaneous evaporation while in the experimental setup (Fig. 1). Overall, the water uptake rates ranged from 43.01 ± 10.44 mg min-1 at 0 kPa for the terrestrial R. pipiens to -14.94 ± 13.85 mg min-1 at -400 kPa in R. catesbeiana (Fig. 1).
3.2 Water potential of seat patches and blood
Seat patch water potentials were generally related to the ecological habit of the species (Fig. 2). The seat patch water potential of the fully aquatic X. laevis was -279 ± 21 kPa and the semi-aquaticR. catesbeiana was -322 ± 76 kPa. The terrestrial R. pipiens had a seat patch water potential of -494 ± 67 kPa, and the terrestrial B. boreas was -634 ± 144 kPa. The seat patch water potentials of the arboreal species were -685 ± 70 kPa for P. cadaverina and -911 ± 114 kPa for P. regilla .
The osmotic potentials of blood (Table 2) were lower (more negative) at 90% hydration than at 100% hydration, indicating that they had a greater concentration of osmolytes. An ANOVA evaluating differences in water potentials of the seat patches and blood of all six species was highly significant (F11, 103 = 53.46, P< 0.0001) with a significant difference between water potential of seat patch and blood (F5, 109 = 95.36,P < 0.0001), and a significant interaction between species and source of water potential (viz., seat patch or blood); (F5, 109 = 17.89, P < 0.0001). A posthoc analysis (Tukey HSD) showed that the more aquatic speciesX. laevis, R. catesbieana, and R. pipiens had significant differences (Q = 3.34, P < 0.05) between their seat patch water potential, and their blood water potential (Fig. 3), with seat patch water potentials being higher (less concentrated) than blood water potentials. However, in the remaining terrestrial and arboreal species, the seat patch water potentials were not different from that of their blood.