ary volume and concentration of dogs on a standard diet, in contrast to what they
drink, does not vary over a wide range of environmental circumstances. This suggests that it is drinking, rather than kidney function, that is responsible for regulating the positive water balance. The continual and persistent action of ADH on the
kidney is, however, necessary in order to maintain renal water conservation. Fresh
water, containing at the most only a few m-equiv of salts, is usually the beverage
of choice. However, as already described, brackish water or even sea-water may
be imbibed and tolerated. Prolonged drinking of such solutions can affect the endocrine system. Rabbits living in areas which are deficient in sodium chloride exhibit
an hypertrophy of the adrenal cortex (BLAIR WEST et al., 1968). Rats given sodium
chloride solutions to drink show a decreased storage of neurohypophysial hormones (JONES and PICKERING, 1969) and a decreased rate of secretion from the
thyroid gland (AXELRAD, LEBLOND, and ISLER, 1955).
3. The Monotremes (Egg-Laying Mammals)
The monotremes are confined to Australia and New Guinea where they occupy
a variety of contrasting habitats. The platypus is aquatic, living in lakes and
rivers, while echidnas live in areas ranging from deserts to tropical rain forests.
Most of our knowledge about the physiology of monotremes is based on observation of echidnas, especially Tachyglossus aculeatus . This mammal weighs
about 3 kg and in its native habitat is insectivorous, eating ants and termites. The
echidna, in common with the platypus, has a body temperature of only 30°C (MARTIN, 1902) and a metabolic rate about half that expected in a placental mammal of
similar size (SCHMIDT-NIELSEN, DAWSON, and CRAWFORD, 1966). The extraction
of oxygen from the inspired air is similar to that in other mammals (BENTLEY, HERREID, and SCHMIDT-NIELSEN, 1967 b) so that its respiratory losses of water are expected to be less. Although the echidna can maintain a constant body temperature
at low environmental temperatures, its thermal evaporative cooling at high temperatures is poor, since it neither sweats nor pants under such conditions
(SCHMIDT-NIELSEN, DAWSON, and CRAWFORD, 1966). The overall water balance
of echidnas with a natural diet of termites has been calculated (BENTLEY and
SCHMIDT-NELSEN, 1967); it appears that, in their natural habitat, they probably
require little or no water to drink. The ability of the kidney to form a hypertonic
urine is critical to the maintenance of a positive water balance under such conditions. Dehydration results in a decreased volume of urine, that may have a concentration of 2300 m-osmole/l. This is achieved by tubular reabsorption of water
and a decreased rate of glomerular filtration. The manner of controlling changes
in urine volume and concentration is unknown but the pituitary contains the placental antidiuretic hormone, arginine-vasopressin, as well as oxytocin (SAWYER,
MUNSICK, and VAN DYKE, 1960). The estimated (in a 3 kg echidna) concentration
of vasopressin in the pituitary is about 10.
9
Mzkg body weight, which is less than
that usually found in mammals (see FOLLETT, 1963; Table 3.1). The molecular ratio
of vasopressin/oxytocin is 3, a value, as we shall see, that is similar to that of marsupials, but greater than in most placcntals. The injection of oxytocin into a lactating echidna was shown by GRIFFITHS (1965) to produce a striking increase in
97
drink, does not vary over a wide range of environmental circumstances. This suggests that it is drinking, rather than kidney function, that is responsible for regulating the positive water balance. The continual and persistent action of ADH on the
kidney is, however, necessary in order to maintain renal water conservation. Fresh
water, containing at the most only a few m-equiv of salts, is usually the beverage
of choice. However, as already described, brackish water or even sea-water may
be imbibed and tolerated. Prolonged drinking of such solutions can affect the endocrine system. Rabbits living in areas which are deficient in sodium chloride exhibit
an hypertrophy of the adrenal cortex (BLAIR WEST et al., 1968). Rats given sodium
chloride solutions to drink show a decreased storage of neurohypophysial hormones (JONES and PICKERING, 1969) and a decreased rate of secretion from the
thyroid gland (AXELRAD, LEBLOND, and ISLER, 1955).
3. The Monotremes (Egg-Laying Mammals)
The monotremes are confined to Australia and New Guinea where they occupy
a variety of contrasting habitats. The platypus is aquatic, living in lakes and
rivers, while echidnas live in areas ranging from deserts to tropical rain forests.
Most of our knowledge about the physiology of monotremes is based on observation of echidnas, especially Tachyglossus aculeatus . This mammal weighs
about 3 kg and in its native habitat is insectivorous, eating ants and termites. The
echidna, in common with the platypus, has a body temperature of only 30°C (MARTIN, 1902) and a metabolic rate about half that expected in a placental mammal of
similar size (SCHMIDT-NIELSEN, DAWSON, and CRAWFORD, 1966). The extraction
of oxygen from the inspired air is similar to that in other mammals (BENTLEY, HERREID, and SCHMIDT-NIELSEN, 1967 b) so that its respiratory losses of water are expected to be less. Although the echidna can maintain a constant body temperature
at low environmental temperatures, its thermal evaporative cooling at high temperatures is poor, since it neither sweats nor pants under such conditions
(SCHMIDT-NIELSEN, DAWSON, and CRAWFORD, 1966). The overall water balance
of echidnas with a natural diet of termites has been calculated (BENTLEY and
SCHMIDT-NELSEN, 1967); it appears that, in their natural habitat, they probably
require little or no water to drink. The ability of the kidney to form a hypertonic
urine is critical to the maintenance of a positive water balance under such conditions. Dehydration results in a decreased volume of urine, that may have a concentration of 2300 m-osmole/l. This is achieved by tubular reabsorption of water
and a decreased rate of glomerular filtration. The manner of controlling changes
in urine volume and concentration is unknown but the pituitary contains the placental antidiuretic hormone, arginine-vasopressin, as well as oxytocin (SAWYER,
MUNSICK, and VAN DYKE, 1960). The estimated (in a 3 kg echidna) concentration
of vasopressin in the pituitary is about 10.
9
Mzkg body weight, which is less than
that usually found in mammals (see FOLLETT, 1963; Table 3.1). The molecular ratio
of vasopressin/oxytocin is 3, a value, as we shall see, that is similar to that of marsupials, but greater than in most placcntals. The injection of oxytocin into a lactating echidna was shown by GRIFFITHS (1965) to produce a striking increase in
97