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.