Abstract
1. Research on water exchange in frogs has historically assumed that
blood osmotic potential drives water exchange between a frog and its
environment, but here we show that the “seat patch” (the primary site
of water exchange in many anurans), or other sites of cutaneous water
uptake, act as an anatomic “compartment” with a water potential
controlled separately from water potential of the blood, and the water
potential of that compartment can be the driver of water exchange
between the animal and its environment. 2. We studied six frog species
(Xenopus laevis, Rana pipiens, R. catesbeiana, Bufo boreas, Pseudacris
cadaverina and P. regilla) differing in ecological relationships to
environmental water. We inferred the water potentials of seat patches
from water exchanges by frogs in sucrose solutions ranging in water
potential from 0 to 1000 -kPa. 3. Terrestrial and arboreal species had
seat patch water potentials that were more negative than the water
potentials of more aquatic species, and their seat patch water
potentials were similar to the water potential of their blood, but the
water potentials of venters of the more aquatic species were different
from (and less negative than) the water potentials of their blood. 4.
These findings indicate that there are physiological mechanisms among
frog species that can be used to control water potential at the sites of
cutaneous water uptake, and that some frogs may be able to adjust the
hydric conductance of their skin when they are absorbing water from very
dilute solutions. 5. Largely unexplored mechanisms involving aquaporins
are likely responsible for adjustments in hydric conductance, which in
turn, allow control of water potential at sites of cutaneous water
uptake among species differing in ecological habit and the observed
disequilibrium between sites of cutaneous water uptake and blood water
potential in more aquatic species.