is regulated, as in placentals, by the release of antidiuretic hormone that acts on
the kidney. The injection of vasopressin also increases the total sodium and chloride
excretion in the urine of Setonix (P.]. BENTLEY; P. WOOLLEY, quoted by WARING
et al., 1966), but the physiological significance of these observations is not clear .
These actions of neurohypophysial peptides in Setonix are similar to those observed in the American marsupial, Didelphis virginiana. The posterior pituitary
gland of this opossum contains arginine-vasopressin and oxytocin (SAWYER et al.,
1960). Posterior pituitary extracts have an antidiuretic action and increase excretion
of urinary chloride in this species (SILVETTE and BRITTON, 1938).
The young Setonix at birth weighs about 0.25 g, about one-fifteen thousandth
of the maternal weight, and is deficient in many morphological and physiological
characters when compared with the parent, or with new-born placental mammals
(see SHIELD, 1961). Such deficiencies include an inability to form a hyperosmotic
urine or respond to the injection of vasopressin (BENTLEY and SHIELD, 1962). It
is not until an age of more than 120 days that they respond to dehydration, or injected vasopressin, and form a hypertonic urine in the adult manner. This inability
primarily appears to be due to an insufficiently developed kidney, probably inadequate differentiation of the loops of HENLE. There is also a deficiency of neurohypophysial peptides and these do not appear in reasonable levels in the gland until
the young are about 135 days old. In the protected environment of the mother's
pouch, the young Setonix is not normally subjected to stresses like dehydration,
so that such mechanisms are not essential at that time.
Setonix lives in coastal areas where dietary supplies of sodium are adequate,
and it has even been suggested that it may drink sea-water during the summer
drought period and so possibly acquire a considerable excess of salt . These animals
can concentrate urinary electrolytes to about 800 m-equiv/l, which is higher than
the level of 560 m-equiv/l in sea-water. Some quokkas, in extreme circumstances
when no other fluid is available, drink small amounts of sea-water and make a net
gain of osmotically-free water (BENTLEY, 1955). However, it seems unlikely that
this species normally drinks sea-water though a similar small wallaby, the Tammar,
Macropus eugenii, apparently can do so (KINNEAR et al., 1968). Quokkas sweat
in hot environments, especially from areas on their fore and hind paws . The sodium
loss by this route is unknown, but LORNA GREEN (1961) has suggested that the
reason that they lick their paws at such times is to retrieve salt secreted by the sweat
glands. The first studies to be carried out on the endocrine regulation of sodium
and potassium metabolism were those of BUTTLE et al. (1952). Earlier work on
the American opossum, Didelphis virginiana, suggested that the adrenal cortex may
not be as important to marsupials as to placental mammals, since this species could,
in certain circumstances, survive adrenalectomy for prolonged periods, with little
or no change in electrolyte level. JENNY BUTTLE and her collaborators found that
adrenalectomy in thequokka was usually fatal within 2 days . Death was accompanied by elevated plasma potassium and decreased plasma-sodium concentrations.
The mean survival time could be increased two or three times by either giving the
quokkas 1% sodium-chloride solutions to drink, or by injecting them with a corticosteroid, deoxycorticosterone acetate. Cortisol and corticosterone have recently
been identified in the blood of Setonix (ILETT, 1969). Australian brush-tailed possums, Trichosurus vulpecula,also die within a few days following adrenalectomy,
101
the kidney. The injection of vasopressin also increases the total sodium and chloride
excretion in the urine of Setonix (P.]. BENTLEY; P. WOOLLEY, quoted by WARING
et al., 1966), but the physiological significance of these observations is not clear .
These actions of neurohypophysial peptides in Setonix are similar to those observed in the American marsupial, Didelphis virginiana. The posterior pituitary
gland of this opossum contains arginine-vasopressin and oxytocin (SAWYER et al.,
1960). Posterior pituitary extracts have an antidiuretic action and increase excretion
of urinary chloride in this species (SILVETTE and BRITTON, 1938).
The young Setonix at birth weighs about 0.25 g, about one-fifteen thousandth
of the maternal weight, and is deficient in many morphological and physiological
characters when compared with the parent, or with new-born placental mammals
(see SHIELD, 1961). Such deficiencies include an inability to form a hyperosmotic
urine or respond to the injection of vasopressin (BENTLEY and SHIELD, 1962). It
is not until an age of more than 120 days that they respond to dehydration, or injected vasopressin, and form a hypertonic urine in the adult manner. This inability
primarily appears to be due to an insufficiently developed kidney, probably inadequate differentiation of the loops of HENLE. There is also a deficiency of neurohypophysial peptides and these do not appear in reasonable levels in the gland until
the young are about 135 days old. In the protected environment of the mother's
pouch, the young Setonix is not normally subjected to stresses like dehydration,
so that such mechanisms are not essential at that time.
Setonix lives in coastal areas where dietary supplies of sodium are adequate,
and it has even been suggested that it may drink sea-water during the summer
drought period and so possibly acquire a considerable excess of salt . These animals
can concentrate urinary electrolytes to about 800 m-equiv/l, which is higher than
the level of 560 m-equiv/l in sea-water. Some quokkas, in extreme circumstances
when no other fluid is available, drink small amounts of sea-water and make a net
gain of osmotically-free water (BENTLEY, 1955). However, it seems unlikely that
this species normally drinks sea-water though a similar small wallaby, the Tammar,
Macropus eugenii, apparently can do so (KINNEAR et al., 1968). Quokkas sweat
in hot environments, especially from areas on their fore and hind paws . The sodium
loss by this route is unknown, but LORNA GREEN (1961) has suggested that the
reason that they lick their paws at such times is to retrieve salt secreted by the sweat
glands. The first studies to be carried out on the endocrine regulation of sodium
and potassium metabolism were those of BUTTLE et al. (1952). Earlier work on
the American opossum, Didelphis virginiana, suggested that the adrenal cortex may
not be as important to marsupials as to placental mammals, since this species could,
in certain circumstances, survive adrenalectomy for prolonged periods, with little
or no change in electrolyte level. JENNY BUTTLE and her collaborators found that
adrenalectomy in thequokka was usually fatal within 2 days . Death was accompanied by elevated plasma potassium and decreased plasma-sodium concentrations.
The mean survival time could be increased two or three times by either giving the
quokkas 1% sodium-chloride solutions to drink, or by injecting them with a corticosteroid, deoxycorticosterone acetate. Cortisol and corticosterone have recently
been identified in the blood of Setonix (ILETT, 1969). Australian brush-tailed possums, Trichosurus vulpecula,also die within a few days following adrenalectomy,
101