help retard evaporation during aestivation. A number of Australian leptodactylids,
which also aestivate during periods of drought, similarly have been shown to lorm
such cocoons (LEE and MERCER, 1967). A careful examination showed that these
sacs completely cover the animals, with the exception of small channels leading
to the nares. These cocoons are composed of compacted layers of the shed epidermis. Evaporation across these membranes was found to be considerably less
than across the integument of the living frogs . These epidermal structures may thus
limit evaporation of water from the skin of aestivating frogs and are reminiscent
of the cocoons that surround the African lungfishes, Protopterus, during similar
periods of retirement.
b) The Kidney
Amphibians in fresh water excrete the water absorbed through their skin as a dilute
urine. An anuran like the European frog, Rana esculenta, which weighs 100 g, forms
about 60 ml of urine during a single day in fresh water (lARD and MOREL; 1963).
Urodeles, in the same situation, secrete similar amounts of urine each day; the mudpuppy (200 g) forms about 50 ml while the alpine newt (10 g) produces 16 ml, 160%
of the body weight (BENTLEY and HELLER, 1964). It can be seen that the daily volume of urine that-anamphibian in fresh water may secrete is considerable, especially
in small animals that have a relatively large surface area in relation to their body
weight. When living in sea-water, Rana cancrivora forms far less urine ; this frog,
weighing 50 g, forms only about 2 ml of urine each day (SCHMIDT-NIELSEN and
LEE, 1962). When amphibians are exposed to a drying atmosphere, urine formation
is too small to measure and in leopard frogs amounts to an anuria (ADOLPH, 1933).
The relationship of the processes of glomerular filtration and tubular water
reabsorption to the eventual urine volume has been examined in a number of anurans but in no urodeles. Both of these mechanisms playa prominent role in the
regulation of the urine volume. The Rana esculenta, referred to above, have a GFR
of 125 ml per day, corresponding to the 60 ml of urine per day, so that about 50%
of the filtered water is reabsorbed under these conditions. In other circumstances
tubular water reabsorption may be greater. The urine volume in Rana esculenta
is almost directly related to the GFR, and in some individual frogs that form urine
at twice the average rate, the GFR is also doubled (lARD, 1966). Crab-eating frogs
(50 g) in sea-water filter 30 ml of water per day across their glomeruli but produce
only 2 ml of urine, so that more than 90% of the filtrate is reabsorbed. The cessation
of urine secretion, that is observed in dehydrated frogs probably results from the
almost complete failure to form a glomerular filtrate.
Amphibians, in common with reptiles and fishes , cannot secrete urine that is
osmotically more concentrated than their body fluids, so that the ability to prevent
water loss in this way is limited. The urine can attain isotonicity with the plasma,
as seen, for instance, in aestivating spadefoot toads (MCCLANAHAN, 1967). In fresh
water the urine is markedly hypoosmotic, less than 30 m-osmole/I (see for instance
GORDON, 1962; JARD and MOREL, 1963). In sea-water the urine of crab-eating frogs
is intermediate in concentration, being slightly hypoosmotic to the body fluids,
600 m-osrnole/l compared to 830 m-osmole/l in the plasma (GORDON et al., 1961).
174
which also aestivate during periods of drought, similarly have been shown to lorm
such cocoons (LEE and MERCER, 1967). A careful examination showed that these
sacs completely cover the animals, with the exception of small channels leading
to the nares. These cocoons are composed of compacted layers of the shed epidermis. Evaporation across these membranes was found to be considerably less
than across the integument of the living frogs . These epidermal structures may thus
limit evaporation of water from the skin of aestivating frogs and are reminiscent
of the cocoons that surround the African lungfishes, Protopterus, during similar
periods of retirement.
b) The Kidney
Amphibians in fresh water excrete the water absorbed through their skin as a dilute
urine. An anuran like the European frog, Rana esculenta, which weighs 100 g, forms
about 60 ml of urine during a single day in fresh water (lARD and MOREL; 1963).
Urodeles, in the same situation, secrete similar amounts of urine each day; the mudpuppy (200 g) forms about 50 ml while the alpine newt (10 g) produces 16 ml, 160%
of the body weight (BENTLEY and HELLER, 1964). It can be seen that the daily volume of urine that-anamphibian in fresh water may secrete is considerable, especially
in small animals that have a relatively large surface area in relation to their body
weight. When living in sea-water, Rana cancrivora forms far less urine ; this frog,
weighing 50 g, forms only about 2 ml of urine each day (SCHMIDT-NIELSEN and
LEE, 1962). When amphibians are exposed to a drying atmosphere, urine formation
is too small to measure and in leopard frogs amounts to an anuria (ADOLPH, 1933).
The relationship of the processes of glomerular filtration and tubular water
reabsorption to the eventual urine volume has been examined in a number of anurans but in no urodeles. Both of these mechanisms playa prominent role in the
regulation of the urine volume. The Rana esculenta, referred to above, have a GFR
of 125 ml per day, corresponding to the 60 ml of urine per day, so that about 50%
of the filtered water is reabsorbed under these conditions. In other circumstances
tubular water reabsorption may be greater. The urine volume in Rana esculenta
is almost directly related to the GFR, and in some individual frogs that form urine
at twice the average rate, the GFR is also doubled (lARD, 1966). Crab-eating frogs
(50 g) in sea-water filter 30 ml of water per day across their glomeruli but produce
only 2 ml of urine, so that more than 90% of the filtrate is reabsorbed. The cessation
of urine secretion, that is observed in dehydrated frogs probably results from the
almost complete failure to form a glomerular filtrate.
Amphibians, in common with reptiles and fishes , cannot secrete urine that is
osmotically more concentrated than their body fluids, so that the ability to prevent
water loss in this way is limited. The urine can attain isotonicity with the plasma,
as seen, for instance, in aestivating spadefoot toads (MCCLANAHAN, 1967). In fresh
water the urine is markedly hypoosmotic, less than 30 m-osmole/I (see for instance
GORDON, 1962; JARD and MOREL, 1963). In sea-water the urine of crab-eating frogs
is intermediate in concentration, being slightly hypoosmotic to the body fluids,
600 m-osrnole/l compared to 830 m-osmole/l in the plasma (GORDON et al., 1961).
174