to conserve this urea in the body fluids is vital for its osmoregulation as it is used
to balance the body fluids osmotically with the external sea-water. The skin is presumably relatively impermeable to this solute, but precise measurements would
be interesting as neurohypophysial peptides can increase the permeability of this
anuran membrane to urea (see USSING, 1960). The kidney is potentially an important avenue for urea loss, but as much as 99% of that filtered at the glomerulus
is' reabsorbed across the renal tubule of the crab-eating frog (SCHMIDT-NIELSEN and
LEE, 1962). It is unknown whether the actions of hormones on the kidney can interfere with this process.
4. Behaviour
The importance of behaviour in limiting the evaporation of water has already been
stressed. Amphibians may hide in dark places under rocks or vegetation (termed
cryptozoic) and so reduce their water losses. Others, provided with stout hind legs
for digging, may burrow into the earth. Frogs like Heleioporus have been found
80 em below the ground's surface while others like Neobatrachus only appear to
dig down to 30 em (MAIN et al., 1959). The depth is probably largely determined
by the friability of the soil in the area in which the animals live, rather than the industry of each group of frogs. Such anurans may burrow down to regions of damp
earth where the humidity is elevated and they may even be able to absorb moisture.
All burrowing frogs do not occupy arid areas, but most of the animals in such
regions are efficient burrowers. There are exceptions like the small Hyla rubella,
which is widespread throughout the dry areas of Australia and owes its survival
largely to its cryptozoic habits (see WARBURG, 1965 b).
During prolonged periods of drought frogs may aestivate in burrows. This process is common among desert species but little is known about its physiological
basis. MCCLANAHAN (1967) has given an account of some of the physiological processes that occur during aestivation in spadefoot toads, Scapbiopus couchi. This
work is, I think, the most complete account of the life of a single anuran living
in a desert environment. I strongly recommend it to those interested in this subject.
During aestivation MCCLANAHAN found that these toads sustain a large increase
in the concentration of their body fluids due largely to the accumulation of urea.
This is excreted when the toads emerge after rain has fallen. Such rain may fall only
rarely and it is thought that these animals may aestivate for periods of longer than
a year. Attempts to measure the oxygen consumption of the aestivating toads were
not successful, as they were aroused by the experiment, but the rates of oxygen
consumption of resting animals were recorded. From these values, and the fat content of the body, it was estimated that the aestivating toads could survive for at
least one year with the available food stores in the body. If the metabolic rate of
aestivating toads were reduced further they could survive even longer, providing
that their ability to withstand the osmotic changes are also equal to the task. The
endocrinology of such aestivating toads, both with respect to their osmoregulation
and their general metabolism should be most interesting.
185
to balance the body fluids osmotically with the external sea-water. The skin is presumably relatively impermeable to this solute, but precise measurements would
be interesting as neurohypophysial peptides can increase the permeability of this
anuran membrane to urea (see USSING, 1960). The kidney is potentially an important avenue for urea loss, but as much as 99% of that filtered at the glomerulus
is' reabsorbed across the renal tubule of the crab-eating frog (SCHMIDT-NIELSEN and
LEE, 1962). It is unknown whether the actions of hormones on the kidney can interfere with this process.
4. Behaviour
The importance of behaviour in limiting the evaporation of water has already been
stressed. Amphibians may hide in dark places under rocks or vegetation (termed
cryptozoic) and so reduce their water losses. Others, provided with stout hind legs
for digging, may burrow into the earth. Frogs like Heleioporus have been found
80 em below the ground's surface while others like Neobatrachus only appear to
dig down to 30 em (MAIN et al., 1959). The depth is probably largely determined
by the friability of the soil in the area in which the animals live, rather than the industry of each group of frogs. Such anurans may burrow down to regions of damp
earth where the humidity is elevated and they may even be able to absorb moisture.
All burrowing frogs do not occupy arid areas, but most of the animals in such
regions are efficient burrowers. There are exceptions like the small Hyla rubella,
which is widespread throughout the dry areas of Australia and owes its survival
largely to its cryptozoic habits (see WARBURG, 1965 b).
During prolonged periods of drought frogs may aestivate in burrows. This process is common among desert species but little is known about its physiological
basis. MCCLANAHAN (1967) has given an account of some of the physiological processes that occur during aestivation in spadefoot toads, Scapbiopus couchi. This
work is, I think, the most complete account of the life of a single anuran living
in a desert environment. I strongly recommend it to those interested in this subject.
During aestivation MCCLANAHAN found that these toads sustain a large increase
in the concentration of their body fluids due largely to the accumulation of urea.
This is excreted when the toads emerge after rain has fallen. Such rain may fall only
rarely and it is thought that these animals may aestivate for periods of longer than
a year. Attempts to measure the oxygen consumption of the aestivating toads were
not successful, as they were aroused by the experiment, but the rates of oxygen
consumption of resting animals were recorded. From these values, and the fat content of the body, it was estimated that the aestivating toads could survive for at
least one year with the available food stores in the body. If the metabolic rate of
aestivating toads were reduced further they could survive even longer, providing
that their ability to withstand the osmotic changes are also equal to the task. The
endocrinology of such aestivating toads, both with respect to their osmoregulation
and their general metabolism should be most interesting.
185