them have been found to react in this way to such pep tides (Table 6.5). These observations are consistent with those showing that Xenopus, along with the urodeles,
rehydrate more slowly than most anurans.
The entire body surface of amphibians may not be uniformly permeable to water or respond equally to neurohypophysial peptides. MCCLANAHAN and BALDWIN
(1969) have recently made the very exciting ob servation that dehydrated toads,
Bufo punctatus, can absorb water across an area of ventral pelvic skin at the rate
of 420 fJ- / crrr' h. In contrast water absorption across skin in the ventral pectoral
area was too small to measure. These observations pose a number of interesting
questions. Can the differences in permeability of the skin be related to its structure?
To what extent does the variability in the speed of rehydration in different amphibians reflect the total area of such specialized regions in each species? The skin in
the ventral pelvic area of many amphibians is an area that is readily apposed to
damp surfaces, so that this could allow the animals to hydrate adequately without
the necessity of it becoming more completely submerged or buried.
P) In Air. Amphibians exposed to the air can lose considerable water by evaporation through the skin. The rate of loss may be 50 to 100 times as rapid as in lizards
of a comparable size (CLAUSSEN, 1969). Evaporation from amphibians occurs at
a rate similar to that from a free water surface (REY, 1937) and, as shown by
ADOLPH (1933), is not altered by the presence of the skin. Small differences in the
rates of evaporative water loss from different amphibians have been described, but
as these often do not include an allowance for differences in the surface area, or
rate of respiration, it is difficult to decide whether they really reflect variations in
cutaneous loss (see BENTLEY, 1966b; WARBURG, 1967; CLAUSSEN, 1969). Water
loss from amphibian skin, like that from a free water surface, is inversely related
to the content of water vapour in the air and increases considerably if air flows
over the animal more rapidly. Amphibians have little physiological ability to limit
evaporation from their integument but they can diminish these losses by appropriate changes in their behaviour. When they are exposed to a dehydrating atmosphere, amphibians often reduce their exposed surface by hunching themselves up
or, as seen in urodeles, by coiling. I have observed that Australian leptodactylid
frogs regularly make vigorous burrowing gestures under such conditions, even
though they were confined to a glass beaker. Amphibians seek refuge from conditions that facilitate evaporation; they may hide away under the bark of trees or
in cracks among rocks. A number of species, including the Australian leptodactylids and many of the Pelobatidae are morphologically well equipped for burrowing
into the soil. We (BENTLEY et al., 1958) have observed Australian frogs, Heleioporus, in burrows 80 cm below the surface and American spadefoot toads probably
go to similar depths (MAYHEW, 1965). During the heat of the day, the temperature
in such burrows is considerably less than that at the surface, while the relative humidity of the air may be greater. Anurans can burrow down into damp layers of
soil as spadefoot toads have been observed to do in the Colorado desert (MAYHEW,
1965).
Amphibians may aestivate for many months in underground burrows. MAYHEW (1965) observed that spadefoot toads, on emerging from such refuges after
rain, were covered with a cocoon-like layer of dry skin. He suggested that this may
173
rehydrate more slowly than most anurans.
The entire body surface of amphibians may not be uniformly permeable to water or respond equally to neurohypophysial peptides. MCCLANAHAN and BALDWIN
(1969) have recently made the very exciting ob servation that dehydrated toads,
Bufo punctatus, can absorb water across an area of ventral pelvic skin at the rate
of 420 fJ- / crrr' h. In contrast water absorption across skin in the ventral pectoral
area was too small to measure. These observations pose a number of interesting
questions. Can the differences in permeability of the skin be related to its structure?
To what extent does the variability in the speed of rehydration in different amphibians reflect the total area of such specialized regions in each species? The skin in
the ventral pelvic area of many amphibians is an area that is readily apposed to
damp surfaces, so that this could allow the animals to hydrate adequately without
the necessity of it becoming more completely submerged or buried.
P) In Air. Amphibians exposed to the air can lose considerable water by evaporation through the skin. The rate of loss may be 50 to 100 times as rapid as in lizards
of a comparable size (CLAUSSEN, 1969). Evaporation from amphibians occurs at
a rate similar to that from a free water surface (REY, 1937) and, as shown by
ADOLPH (1933), is not altered by the presence of the skin. Small differences in the
rates of evaporative water loss from different amphibians have been described, but
as these often do not include an allowance for differences in the surface area, or
rate of respiration, it is difficult to decide whether they really reflect variations in
cutaneous loss (see BENTLEY, 1966b; WARBURG, 1967; CLAUSSEN, 1969). Water
loss from amphibian skin, like that from a free water surface, is inversely related
to the content of water vapour in the air and increases considerably if air flows
over the animal more rapidly. Amphibians have little physiological ability to limit
evaporation from their integument but they can diminish these losses by appropriate changes in their behaviour. When they are exposed to a dehydrating atmosphere, amphibians often reduce their exposed surface by hunching themselves up
or, as seen in urodeles, by coiling. I have observed that Australian leptodactylid
frogs regularly make vigorous burrowing gestures under such conditions, even
though they were confined to a glass beaker. Amphibians seek refuge from conditions that facilitate evaporation; they may hide away under the bark of trees or
in cracks among rocks. A number of species, including the Australian leptodactylids and many of the Pelobatidae are morphologically well equipped for burrowing
into the soil. We (BENTLEY et al., 1958) have observed Australian frogs, Heleioporus, in burrows 80 cm below the surface and American spadefoot toads probably
go to similar depths (MAYHEW, 1965). During the heat of the day, the temperature
in such burrows is considerably less than that at the surface, while the relative humidity of the air may be greater. Anurans can burrow down into damp layers of
soil as spadefoot toads have been observed to do in the Colorado desert (MAYHEW,
1965).
Amphibians may aestivate for many months in underground burrows. MAYHEW (1965) observed that spadefoot toads, on emerging from such refuges after
rain, were covered with a cocoon-like layer of dry skin. He suggested that this may
173