(WEISS and McDoNALD, 1966a; COGHLAN and SCOGGINS, 1967) but has not been
found in the Australian possum, Tricho suru s vulpecula (CHESTER JONES et al.,
1964a; WEISS and McDoNALD, 1966b), though it is pres ent in the American opossum, Didelphis virginiana (DAVIS et al., 1967). Cortisol is the predominant corticosteroid in all of the Australian marsupials so far examined, the ratio of cortisol/corticosterone (compounds F/B) usually ranges from 3 to 12. This contrasts
with the echidna (Monotremata) in which the F/B ratio is less than 0.5 (W EISS and
McDoNALD, 1965). Placental mammals, however exhibit widely varying FIB ratios; it is, for instance, less than 0.05 in the laboratory rat and 20 in a monkey (BUSH,
1953). The possible phyletic and functional significance of thes e differences is not
known.
The control of secretion of corticosteroids in marsupials appears to conform to
the placental pattern, but the information is incomplete. DAVIS et al. (1967) found
that the injection of placental corticotrophin into the American opossum, Didelphis virginiana, produced a ' striking' increase in secretion of aldosterone, cortisol and corticosterone. Renin extracted from opossum kidneys had similar effects
when injected into these animals, and if hypophysectomized opossums were used ,
the action of the renin was more selective with respect to aldosterone. Sodium deple- .
tion of the opossum resulted in increased granulation of the renal juxtaglomerular
cells, suggesting increased production of renin. The kidneys and blood of the Australian brushtailed possum, Trichosurus uulpecula, have also been shown to contain
renin (REID and McDONALD , 1969). The circulating levels of this hormone increased in possums which were sodium-deficient and there was also a granulation
of the renal juxtaglomerular cells in such animals. Thus in both the American opossum and the Australian possum the control of corticosteroid secretion appears to
conform to the placental pattern involving the corticotrophin and renin-angiotensin axes. WEISS and McDONALD injected ovine and porcine corticotrophin int o
Australian wombats, possums and kangaroos, and found increases in the rate of
secretion of cortisol and corticosterone. The activity of marsupial corticotrophin
has not yet been described.
5. The Placental Mammals
Generically and numerically the placentals are the predominant group of mammals.
All of the marine and most aquatic mammals belong to this group. They are present
in all of the world 's deserts. The placentals have thus adapted to a wide ran ge of
differing osmotic circumstances.
The Rodentia make up about one-third of all placental genera. This order has
a very cosmopolitan distribution, occupying diverse habitats including the deserts
of Africa, Asia, America and Australia, where, with the possible exception of the
latter area, they are the principal mammals present. Osmoregulatory problems in
such dry hot areas result in the exclusion of most mammals, except certain rodents,
equ ipped with behavioural and physiological adaptability. KNUT and BODIL
ScHMIDT-NIELSEN investigated the water metabolism of the banner-tailed kangaroo
rat, Dipodomys spectab ilis, and revealed the manner in which th is little rodent lives
104
found in the Australian possum, Tricho suru s vulpecula (CHESTER JONES et al.,
1964a; WEISS and McDoNALD, 1966b), though it is pres ent in the American opossum, Didelphis virginiana (DAVIS et al., 1967). Cortisol is the predominant corticosteroid in all of the Australian marsupials so far examined, the ratio of cortisol/corticosterone (compounds F/B) usually ranges from 3 to 12. This contrasts
with the echidna (Monotremata) in which the F/B ratio is less than 0.5 (W EISS and
McDoNALD, 1965). Placental mammals, however exhibit widely varying FIB ratios; it is, for instance, less than 0.05 in the laboratory rat and 20 in a monkey (BUSH,
1953). The possible phyletic and functional significance of thes e differences is not
known.
The control of secretion of corticosteroids in marsupials appears to conform to
the placental pattern, but the information is incomplete. DAVIS et al. (1967) found
that the injection of placental corticotrophin into the American opossum, Didelphis virginiana, produced a ' striking' increase in secretion of aldosterone, cortisol and corticosterone. Renin extracted from opossum kidneys had similar effects
when injected into these animals, and if hypophysectomized opossums were used ,
the action of the renin was more selective with respect to aldosterone. Sodium deple- .
tion of the opossum resulted in increased granulation of the renal juxtaglomerular
cells, suggesting increased production of renin. The kidneys and blood of the Australian brushtailed possum, Trichosurus uulpecula, have also been shown to contain
renin (REID and McDONALD , 1969). The circulating levels of this hormone increased in possums which were sodium-deficient and there was also a granulation
of the renal juxtaglomerular cells in such animals. Thus in both the American opossum and the Australian possum the control of corticosteroid secretion appears to
conform to the placental pattern involving the corticotrophin and renin-angiotensin axes. WEISS and McDONALD injected ovine and porcine corticotrophin int o
Australian wombats, possums and kangaroos, and found increases in the rate of
secretion of cortisol and corticosterone. The activity of marsupial corticotrophin
has not yet been described.
5. The Placental Mammals
Generically and numerically the placentals are the predominant group of mammals.
All of the marine and most aquatic mammals belong to this group. They are present
in all of the world 's deserts. The placentals have thus adapted to a wide ran ge of
differing osmotic circumstances.
The Rodentia make up about one-third of all placental genera. This order has
a very cosmopolitan distribution, occupying diverse habitats including the deserts
of Africa, Asia, America and Australia, where, with the possible exception of the
latter area, they are the principal mammals present. Osmoregulatory problems in
such dry hot areas result in the exclusion of most mammals, except certain rodents,
equ ipped with behavioural and physiological adaptability. KNUT and BODIL
ScHMIDT-NIELSEN investigated the water metabolism of the banner-tailed kangaroo
rat, Dipodomys spectab ilis, and revealed the manner in which th is little rodent lives
104