the skin, a process that occurs down a gradient of osmotic concentration. When
the solutions on either side of the skin are osmotically equal, little water is transferred. If the external solution is that of an impermeant solute like sucrose, virtually
no water transfer occurs. If a permeant solute, such as sodium, is present small
movements of water, consistent with any solute transfer, may occur in order to
maintain osmotic equality. Such transfer of water is normally minor. The movement of water across the skin of frogs and toads is directly proportional to the osmotic gradient between the two sides of the skin (SAWYER, 1951). The movement
of water is thus considered to be a passive process, not requiring the direct intervention of any metabolic energy. Normally it is from the outside solution into the
animal, but if amphibians inadvertently enter hyperosmotic solutions they lose water; in Rana pipiens placed in sea-water, water passes out across their skin at the
rate of 55 tlllcm2h (P.]. BENTLEY unpublished observations).
Water transfer across the skin is also influenced by the temperature. The Malayan toad , Bufo melanostictus, when kept in water at 12
0
, gains water through its
skin at the rate of 6.4 flll cm
2
h; at 29
0 this is more than doubled and is 17flllcm
2h,
while at 37
0 it is 34.5 , lIli cm
2
h (DICKER and ELLIOTT, 1967). The latter temperature
cannot be considered a ph ysiological one, but some toads regularly experience temperatures approaching 30
0 and the speed of their hydration, or rehydration, would
be increased under these conditions.
The rate of water transfer across the skin differs by species . SCHMID (1965) has
measured the rates of osmotic water transfer acro ss the skins (in vitro) of six different species of North American frogs and toads. He found that the skins of
aquatic species like the mink frog, Rana septentrionalis , had a lower permeability
to water (6 flllcm
2h)
than those from more terrestrial species like the Dakota toad,
Bufo hemiophrys (20 fll/cm 2h) . Frogs from habitats judged to be intermediate in
their 'dryness' showed correspondingly different permeabilities. The skin from
anurans with low permeability to water was found to have a higher lipid content
than those with a higher permeability (SCHMID and BARDEN, 1965). Other values,
collected from the literature are given in Table 6.5 and generally support the conclusions of SCHMID. Such a relationship is also seen among the terrestrial urodeles.
Such species as the firesalamander and the alpine newt have a greater osmotic permeability than aquatic species, such as the mudpuppy and the congo eel. It is also
interesting that the skin of the aquatic tadpoles of bullfrogs is less permeable than
that of the adults.
When deh ydrated amphibians return to water they regain water by absorbing
it across their skin. They can also take up water in this way from damp surfaces
such as soil. The ability of the Amphibia to gain water from damp earth presumably
depends on the moisture content of the soil and the various forces (osmotic, hygroscopic and capillary) that tend to hold water there.The water content of the soil
may be quite variable, depending on such things as when rain last fell, the presence
of surface vegetation and the proximity of ground water. SPIGHT (1967 a) found
that the six species of salamanders studied by him could absorb water from a soil
sample that contained about 10% water. When the moisture content was lower,
3 to 4%, the salamanders slowly lost water, showing that the forces controlling
water retention by the soil may also have a dehydrating action on the animals.
However, it was apparent that these salamanders, which were mostly native to
169
the solutions on either side of the skin are osmotically equal, little water is transferred. If the external solution is that of an impermeant solute like sucrose, virtually
no water transfer occurs. If a permeant solute, such as sodium, is present small
movements of water, consistent with any solute transfer, may occur in order to
maintain osmotic equality. Such transfer of water is normally minor. The movement of water across the skin of frogs and toads is directly proportional to the osmotic gradient between the two sides of the skin (SAWYER, 1951). The movement
of water is thus considered to be a passive process, not requiring the direct intervention of any metabolic energy. Normally it is from the outside solution into the
animal, but if amphibians inadvertently enter hyperosmotic solutions they lose water; in Rana pipiens placed in sea-water, water passes out across their skin at the
rate of 55 tlllcm2h (P.]. BENTLEY unpublished observations).
Water transfer across the skin is also influenced by the temperature. The Malayan toad , Bufo melanostictus, when kept in water at 12
0
, gains water through its
skin at the rate of 6.4 flll cm
2
h; at 29
0 this is more than doubled and is 17flllcm
2h,
while at 37
0 it is 34.5 , lIli cm
2
h (DICKER and ELLIOTT, 1967). The latter temperature
cannot be considered a ph ysiological one, but some toads regularly experience temperatures approaching 30
0 and the speed of their hydration, or rehydration, would
be increased under these conditions.
The rate of water transfer across the skin differs by species . SCHMID (1965) has
measured the rates of osmotic water transfer acro ss the skins (in vitro) of six different species of North American frogs and toads. He found that the skins of
aquatic species like the mink frog, Rana septentrionalis , had a lower permeability
to water (6 flllcm
2h)
than those from more terrestrial species like the Dakota toad,
Bufo hemiophrys (20 fll/cm 2h) . Frogs from habitats judged to be intermediate in
their 'dryness' showed correspondingly different permeabilities. The skin from
anurans with low permeability to water was found to have a higher lipid content
than those with a higher permeability (SCHMID and BARDEN, 1965). Other values,
collected from the literature are given in Table 6.5 and generally support the conclusions of SCHMID. Such a relationship is also seen among the terrestrial urodeles.
Such species as the firesalamander and the alpine newt have a greater osmotic permeability than aquatic species, such as the mudpuppy and the congo eel. It is also
interesting that the skin of the aquatic tadpoles of bullfrogs is less permeable than
that of the adults.
When deh ydrated amphibians return to water they regain water by absorbing
it across their skin. They can also take up water in this way from damp surfaces
such as soil. The ability of the Amphibia to gain water from damp earth presumably
depends on the moisture content of the soil and the various forces (osmotic, hygroscopic and capillary) that tend to hold water there.The water content of the soil
may be quite variable, depending on such things as when rain last fell, the presence
of surface vegetation and the proximity of ground water. SPIGHT (1967 a) found
that the six species of salamanders studied by him could absorb water from a soil
sample that contained about 10% water. When the moisture content was lower,
3 to 4%, the salamanders slowly lost water, showing that the forces controlling
water retention by the soil may also have a dehydrating action on the animals.
However, it was apparent that these salamanders, which were mostly native to
169