tration in such large kangaroos, as it does in placental mammals. Considering the
environmental osmotic problems this role could be substantial, and could necessitate prolonged periods of secretion by the neurohypophysis.
The few marsupials and the single monotreme that have been examined exhibit
an elevated ratio of vasopressin/oxytocin (V/O) concentration in their neurohypophyses; it is 6.2 in the Australian possum, 3.8 in the quokka, 4.8 in the red kangaroo (FERGUSON and HELLER, 1965), 2.8 in the American opossum and 3 in the
echidna (SAWYER et al., 1960). The V/ 0 ratio in most placentals is 1 to 2 with the
exception of the camel and llama (Tylopoda) in which it is 3 to 4. It is unknown
whether such variations reflect differences in the function of these peptides, but
they nevertheless are of phyletic interest. The relatively higher storage levels of
vasopressin, compared to oxytocin, could reflect genetic differences in the rates
of synthesis or release of these pep tides, or the y could result from the formation
of an unidentified, structurally different, hormone. The oxytocin activity in the
neurohypophyses of the species w ith a high V/ 0 ratio has not been exhaustively
characterized, and it is~sible that this peptide is not identical in molecular structure with oxytocin. Thus a peptide with a lower specific oxytocin activity could
result in an elevated V/O ratio. Neurohypophysial peptide hormones similar, but
not identical, to oxytocin have been identified in a number of vertebrates including
the reptiles, amphibians and fish, and these were initially also thought to be oxytocin.
The grasses of the central Australian regions are deficient in sodium compared
to those in coastal areas (DENTON, 1965) so that conservation of this solute could
be of particular importance to many kangaroos. However, there is no definitive
information about this for the red kangaroos living in the NEWSOME study area.
The urine (NEWSOM E, quoted by SCHMIDT-NIELSEN, 1964a) of these animals has
a Na/K ratio of 0.13 , while the euros in the EALEY study area exhibit a ratio of
0.33. This suggests that relatively more sodium may be available to th e euro than
the red kangaroo. Two populations of the grey kangaroo, Macropus giganteus, living on grasses with contrasting contents of sodium, have been studied by COGHLAN
and SCOGGINS (1967). One group of these kangaroos lives in the coastal areas of
south east Australia, where the grasses contain more than 150 m-equiv sodium/kg
dry weight, and the other occupies the Snowy Mountain plains, where the herbage
contains less than 10 m-equiv sodium/kg dry weight. Aldosterone, cortisol and
corticosterone have been identified in the blood of the red kangaroo (WEISS and
McDONALD, 1967) as well as the grey kangaroo. The red kangaroos were studied
in the laboratory, and whil e aldosterone was secreted at rates comparable with
those seen in placenrals, the cortisol levels were somewhat lower. Rates of corticosteroid secretion were measured in grey kangaroos caught in the field and it was
found that animals living in the sodium-deficient Snowy Mountains area have blood
aldosterone levels 8 times as great as those from the sodium-replete coastal regions.
There was little difference in the blood levels of cortisol or corticosterone. The
zona glomerulosa in the adrenal cortices of the animals from the Snowy Mountains
plains was larger than that in the other group.
Corticosteroids have been detected, and measured, in the adrenal venous and
peripheral blood of several Australian marsupials. In addition to the kangaroos,
aldosterone has also been detected in the blood of the wombat, Vombatus hirsutus
103
environmental osmotic problems this role could be substantial, and could necessitate prolonged periods of secretion by the neurohypophysis.
The few marsupials and the single monotreme that have been examined exhibit
an elevated ratio of vasopressin/oxytocin (V/O) concentration in their neurohypophyses; it is 6.2 in the Australian possum, 3.8 in the quokka, 4.8 in the red kangaroo (FERGUSON and HELLER, 1965), 2.8 in the American opossum and 3 in the
echidna (SAWYER et al., 1960). The V/ 0 ratio in most placentals is 1 to 2 with the
exception of the camel and llama (Tylopoda) in which it is 3 to 4. It is unknown
whether such variations reflect differences in the function of these peptides, but
they nevertheless are of phyletic interest. The relatively higher storage levels of
vasopressin, compared to oxytocin, could reflect genetic differences in the rates
of synthesis or release of these pep tides, or the y could result from the formation
of an unidentified, structurally different, hormone. The oxytocin activity in the
neurohypophyses of the species w ith a high V/ 0 ratio has not been exhaustively
characterized, and it is~sible that this peptide is not identical in molecular structure with oxytocin. Thus a peptide with a lower specific oxytocin activity could
result in an elevated V/O ratio. Neurohypophysial peptide hormones similar, but
not identical, to oxytocin have been identified in a number of vertebrates including
the reptiles, amphibians and fish, and these were initially also thought to be oxytocin.
The grasses of the central Australian regions are deficient in sodium compared
to those in coastal areas (DENTON, 1965) so that conservation of this solute could
be of particular importance to many kangaroos. However, there is no definitive
information about this for the red kangaroos living in the NEWSOME study area.
The urine (NEWSOM E, quoted by SCHMIDT-NIELSEN, 1964a) of these animals has
a Na/K ratio of 0.13 , while the euros in the EALEY study area exhibit a ratio of
0.33. This suggests that relatively more sodium may be available to th e euro than
the red kangaroo. Two populations of the grey kangaroo, Macropus giganteus, living on grasses with contrasting contents of sodium, have been studied by COGHLAN
and SCOGGINS (1967). One group of these kangaroos lives in the coastal areas of
south east Australia, where the grasses contain more than 150 m-equiv sodium/kg
dry weight, and the other occupies the Snowy Mountain plains, where the herbage
contains less than 10 m-equiv sodium/kg dry weight. Aldosterone, cortisol and
corticosterone have been identified in the blood of the red kangaroo (WEISS and
McDONALD, 1967) as well as the grey kangaroo. The red kangaroos were studied
in the laboratory, and whil e aldosterone was secreted at rates comparable with
those seen in placenrals, the cortisol levels were somewhat lower. Rates of corticosteroid secretion were measured in grey kangaroos caught in the field and it was
found that animals living in the sodium-deficient Snowy Mountains area have blood
aldosterone levels 8 times as great as those from the sodium-replete coastal regions.
There was little difference in the blood levels of cortisol or corticosterone. The
zona glomerulosa in the adrenal cortices of the animals from the Snowy Mountains
plains was larger than that in the other group.
Corticosteroids have been detected, and measured, in the adrenal venous and
peripheral blood of several Australian marsupials. In addition to the kangaroos,
aldosterone has also been detected in the blood of the wombat, Vombatus hirsutus
103