the secretion of milk. Oxytocin ma y thus function as a hormone controlling 'milk
let down' as in other mammals, but the action of vasopressin on urine flow has
not been tested. Large doses of oxytocin and vasop ressin respectively decrease and
increase the blood pre ssure of the plat ypus (FEAKES et al., 1952), but this is generally
considered to be a pharmacological response which is, incidentall y, more similar
to that of reptiles than other mammals.
The adrenal gland s in monotremes do not show the distinct delineation into
a cortex of interrenal tissue and a medulla of chromaffin cells. Indeed the echidna
adrenal is somewhat akin in its morphological arrangement to reptiles, while the
platypus has the potentialities of form seen in marsupials and placentals (WRIGHT,
CHESTER JONES, and PHILLIPS, 1957).
WEISS and McDONALD (1965) have identified corticosterone in the adrenal
venous and peripheral blood of the echidna and this is present at about 5 times
the concentration of cortisol. This type of ratio, in the rates of secretion of these
two corticosteroids, is more similar to reptiles than to that which is usual in mammals. There are, nevertheless, exceptions; a placental mammal like the rat secretes
corticosterone and no cortisol. The rates of secretion of corticosteroids in the echidna are much lower than in other mammals.
After adrenalectomy echidnas can survive for extended periods of time with
no persistent changes in the plasma or urinary electrolyte levels (McDONALD and
AUGEE, 1968). However, when the environmental temperature is reduced (to 5 °)
the onset of torpor (hibernation?) is more rapid in the absence of the adrenals
(AUGEE and McDONALD , 1969). This torpor, which is a characteristic response
in the echidna, is accompanied by reduced oxygen consumption and low ered
plasma levels of glucose and fatt y acids . The injection of cortisol (or glucos e) into
the adrenalectomized animals prevents it. Thus in the echidna, adrenocorticosteroids appear to exhibit a 'glucocorticoid' role but their relationship to electrolyte
metabolism is not yet clear .
4. The Marsupials (Pouched Mammals)
Two families of marsupials live in the Western Hemisphere: the Didelphidae (opossums) and the Caenolestidae (shrew-like animals living in South America).
Most marsupials, however, live in the Australian region, where the contemporary
families occupy the variety of ecological niches on the continent, in a fashion that
parallels the manner in which placentals occupy similar places elsewhere. Various
species have adopted insectivorous, herbivorous and carnivorous diets . They ma y
be arboreal like the possums, fossoriallike marsupial moles or they may live in a
more conventional way on the surface. Large areas of Australia are desert with little
and irregular rainfall , combined with a high environmental temperature and a low
humidity. Other regions on the continent are covered with tropical rain forest ,
while there are also many temperate areas with more equitable climates. The marsupial fauna has occupied all these regions with success, often to the chagrin of the
sheep farmers whose flocks may fare poorly compared to the indigenous species.
In many of the more barren regions sheep and cattle, in contrast to marsupials, require supplement s of trace elements, protein and water inorderto survive and breed.
98
let down' as in other mammals, but the action of vasopressin on urine flow has
not been tested. Large doses of oxytocin and vasop ressin respectively decrease and
increase the blood pre ssure of the plat ypus (FEAKES et al., 1952), but this is generally
considered to be a pharmacological response which is, incidentall y, more similar
to that of reptiles than other mammals.
The adrenal gland s in monotremes do not show the distinct delineation into
a cortex of interrenal tissue and a medulla of chromaffin cells. Indeed the echidna
adrenal is somewhat akin in its morphological arrangement to reptiles, while the
platypus has the potentialities of form seen in marsupials and placentals (WRIGHT,
CHESTER JONES, and PHILLIPS, 1957).
WEISS and McDONALD (1965) have identified corticosterone in the adrenal
venous and peripheral blood of the echidna and this is present at about 5 times
the concentration of cortisol. This type of ratio, in the rates of secretion of these
two corticosteroids, is more similar to reptiles than to that which is usual in mammals. There are, nevertheless, exceptions; a placental mammal like the rat secretes
corticosterone and no cortisol. The rates of secretion of corticosteroids in the echidna are much lower than in other mammals.
After adrenalectomy echidnas can survive for extended periods of time with
no persistent changes in the plasma or urinary electrolyte levels (McDONALD and
AUGEE, 1968). However, when the environmental temperature is reduced (to 5 °)
the onset of torpor (hibernation?) is more rapid in the absence of the adrenals
(AUGEE and McDONALD , 1969). This torpor, which is a characteristic response
in the echidna, is accompanied by reduced oxygen consumption and low ered
plasma levels of glucose and fatt y acids . The injection of cortisol (or glucos e) into
the adrenalectomized animals prevents it. Thus in the echidna, adrenocorticosteroids appear to exhibit a 'glucocorticoid' role but their relationship to electrolyte
metabolism is not yet clear .
4. The Marsupials (Pouched Mammals)
Two families of marsupials live in the Western Hemisphere: the Didelphidae (opossums) and the Caenolestidae (shrew-like animals living in South America).
Most marsupials, however, live in the Australian region, where the contemporary
families occupy the variety of ecological niches on the continent, in a fashion that
parallels the manner in which placentals occupy similar places elsewhere. Various
species have adopted insectivorous, herbivorous and carnivorous diets . They ma y
be arboreal like the possums, fossoriallike marsupial moles or they may live in a
more conventional way on the surface. Large areas of Australia are desert with little
and irregular rainfall , combined with a high environmental temperature and a low
humidity. Other regions on the continent are covered with tropical rain forest ,
while there are also many temperate areas with more equitable climates. The marsupial fauna has occupied all these regions with success, often to the chagrin of the
sheep farmers whose flocks may fare poorly compared to the indigenous species.
In many of the more barren regions sheep and cattle, in contrast to marsupials, require supplement s of trace elements, protein and water inorderto survive and breed.
98