about 30% of its body weight, while the spadefoot toad, Scaphiopus hammondi,
can surv ive a loss of 50% in its body weight. A similar relationship between the
habitat and survival of dessication has been shown by SCHMID (1965) in North
American anurans and we have also observed this to be so in some Australian tree
frogs (MAIN and BENTLEY, 1964). WARBURG (1967) found that some frogs from
dry regions in central Australia withstand dehydration better than those from wet -
ter habitats but he has emphasized the import ance of the speed of water loss in
determining the animals' survival. Frogs seem to be able to live longer when the
rate of dehydration is slow, than when it is fast, and although I have made no precise
measurements of this , I would agree that this is probably an important factor. Differences in ability to withstand dehydration are not invariably seen among different
groups of anurans. The Australian genera Neobatrachus and Heleioporus each have
4 or 5 species that have adopted habitats ranging from wet forests to dr y deserts
and they all survive a water loss equivalent to 40 to 45% of the ir body weight
(BENTLEY, LEE, and MAIN, 1958). These Australian frogs (family Leptodacrylidae)
thus all tolerate water losses as great as those of the best adapted North American
species or of Australian Hylidae. The Australian deserts are far older than those
in North America and the leptodactylids are an ancient anuran family that probably
gave rise to the now more widely distributed Bufonidae. The Australian leptodactylids may thus have evolved into a basically xerophilous group which in the
course of long periods of time have adapted more uniformly to dry conditions.
As we shall see, these leptodactylids also have a remarkable ability to rehydrate
rapidly and can store large volumes of water in their urinary bladders.
U rodeles do not live in conditions as arid as those experienced by many anurans.
It is interesting that 5 species of salamander from temperate areas in North Carolina
died after losing water equivalent to only 18 to 30% of their body weight (SPIGHT,
1968). It would be interesting to know if any urodeles can withstand the extremes
of dehydration tolerated by some anurans.
Amphibians may live in three contrasting types of osmotic environment; fresh
water, dry land or, very occasionally, in salt water. Larval amphibians, certain urodeles, such as the mudpuppy and th e congo eel, and anurans like the South African
clawed toad are almost entirely aquatic. Many species habitually live in ponds and
streams from which they make periodic excursions and are thus truly amphibious.
The fresh water where such species live, or visit, is hypoosmotic to their body fluids
so that there is a continual accumulation of water occurring across their skin. This
may be equivalent to 30 to 40% of their body weight in a day and is excreted by
the kidneys as a dilute (hypoosmotic) urine. This urine contains only small , but
nevertheless, significant amounts of solutes and the loss of some of these, especially
sodium, may be physiologically significant. In addition, as shown by the classical
studies of KROGH (see KROGH, 1939), amphibian skin is permeable to sodium and
chloride, so that on simple physico-chemical grounds one would expect further
losses to occur across the integument. While in certain unphysiological circumstances, like bathing in distilled water, such losses can be demonstrated, they are
usually prevented by an active transport, by the skin, of sodium (and sometimes
chloride) from the bathing medium.
DARWIN (1859) noted that frogs have a poor tolerance to salt solutions and only
two or three species have been discovered for which this is not true. When leopard
164
can surv ive a loss of 50% in its body weight. A similar relationship between the
habitat and survival of dessication has been shown by SCHMID (1965) in North
American anurans and we have also observed this to be so in some Australian tree
frogs (MAIN and BENTLEY, 1964). WARBURG (1967) found that some frogs from
dry regions in central Australia withstand dehydration better than those from wet -
ter habitats but he has emphasized the import ance of the speed of water loss in
determining the animals' survival. Frogs seem to be able to live longer when the
rate of dehydration is slow, than when it is fast, and although I have made no precise
measurements of this , I would agree that this is probably an important factor. Differences in ability to withstand dehydration are not invariably seen among different
groups of anurans. The Australian genera Neobatrachus and Heleioporus each have
4 or 5 species that have adopted habitats ranging from wet forests to dr y deserts
and they all survive a water loss equivalent to 40 to 45% of the ir body weight
(BENTLEY, LEE, and MAIN, 1958). These Australian frogs (family Leptodacrylidae)
thus all tolerate water losses as great as those of the best adapted North American
species or of Australian Hylidae. The Australian deserts are far older than those
in North America and the leptodactylids are an ancient anuran family that probably
gave rise to the now more widely distributed Bufonidae. The Australian leptodactylids may thus have evolved into a basically xerophilous group which in the
course of long periods of time have adapted more uniformly to dry conditions.
As we shall see, these leptodactylids also have a remarkable ability to rehydrate
rapidly and can store large volumes of water in their urinary bladders.
U rodeles do not live in conditions as arid as those experienced by many anurans.
It is interesting that 5 species of salamander from temperate areas in North Carolina
died after losing water equivalent to only 18 to 30% of their body weight (SPIGHT,
1968). It would be interesting to know if any urodeles can withstand the extremes
of dehydration tolerated by some anurans.
Amphibians may live in three contrasting types of osmotic environment; fresh
water, dry land or, very occasionally, in salt water. Larval amphibians, certain urodeles, such as the mudpuppy and th e congo eel, and anurans like the South African
clawed toad are almost entirely aquatic. Many species habitually live in ponds and
streams from which they make periodic excursions and are thus truly amphibious.
The fresh water where such species live, or visit, is hypoosmotic to their body fluids
so that there is a continual accumulation of water occurring across their skin. This
may be equivalent to 30 to 40% of their body weight in a day and is excreted by
the kidneys as a dilute (hypoosmotic) urine. This urine contains only small , but
nevertheless, significant amounts of solutes and the loss of some of these, especially
sodium, may be physiologically significant. In addition, as shown by the classical
studies of KROGH (see KROGH, 1939), amphibian skin is permeable to sodium and
chloride, so that on simple physico-chemical grounds one would expect further
losses to occur across the integument. While in certain unphysiological circumstances, like bathing in distilled water, such losses can be demonstrated, they are
usually prevented by an active transport, by the skin, of sodium (and sometimes
chloride) from the bathing medium.
DARWIN (1859) noted that frogs have a poor tolerance to salt solutions and only
two or three species have been discovered for which this is not true. When leopard
164