the changes in plasma sodium and potassium levels being similar to those observed
in the quokka (REID and McDoNALD, 1968). These possums could be kept alive
by giving them injections of cortisol, aldosterone being ineffective, though the dose
of cortisol could be reduced if both of these steroids were given simultaneously.
The adrenal cortex of Australian marsupials thus seems to playa vital role in their
ability to regulate sodium and potassium levels. Injections of aldosterone decrease
renal sodium excretion in the American opossum but have no consistent effect on
urinary potassium (BECK, BROWNELL, and BESCH, 1969). As we shall see, other
studies on marsupials have identified and implicated corticosteroids in their regulatory mechanisms.
The water metabolism of two large species of kangaroo has been studied in far
hotter and drier areas than those occupied by Setonix. TIM EALEY led a series of
ecological and physiological investigations on a wild population of the hill kangaroo, or euro, Macropus robustus, living in a hot dry desert region of north west
Australia (Fig. 3.2). ALAN NE~'SOME carried out a similar investigation on the red
kangaroo, Megaleia (or Macropus) rufus, living in an arid central region of the continent near Alice Springs. The average annual rainfall in both the areas is about
25 em, most of it falling during the summer months. The rain, however, is sporadic
and unreliable, and both areas may undergo periods of drought when little rain
falls for periods of three or four years. The summer temperature is high, the average
daily maximum often being greater than 40 °C for many weeks and may rise to
49 o C. The relative humidity usually is low so that the potential evaporation is
high. Despite these osmotically adverse conditions the kangaroos in these areas
continue to survive and multiply. A man lost in these regions in the height of summer can expect to survive for less than one day (ADOLPH, 1947) so the adaptability
of the kangaroos is impressive. Both the euro and the red kangaroo may form a
highly concentrated urine, which can be 2700 m-osmole/l in the red kangaroo (A.
NEWSOME, quoted by SCHMIDT-NIELSEN, 1964a) and 2200 in the euro (EALEY,
BENTLEY, and MAIN, 1965). Such an ability must aid water conservation by th ese
animals but is not a predominant factor in their survival. EALEY and his collaborators observed that euros can withstand dehydration equivalent to 30% of their body
weight, and that they reduce evaporation by sheltering in rocky caves during the
heat of the day . Euros drink periodically and even during hot dry periods only
come in to obtain water on an average of once every three days. During the intervening period they incur a water deficit equivalent to about 12% of their body
weight.
NEWSOME (1965 a and b) found that the red kangaroos also only drank sporadically, apparently gaining most of their water from their diet, which consists preferentially of succulent green shoots of the herbage. The animals limit evaporation
during the day by seeking the shelter of bushy woodlands and avoiding the open
plains . The red kangaroo, like the quokka, stores arginine-vasopressin and oxytocin
in its neurohypophysis, and the ratio of these conforms to that usually found in
marsupials of 4 or 5 to 1 (FERGUSON and HELLER, 1965). The estimated concentration of vasopressin in the neurohypophysis is 1O9M/kg
body weight, which is low
compared to other species, but this could result from the fact that the animals had
been hunted and shot before the pituitary was obtained. It can be reasonably surmised that vasopressin has a similar role, in regulating the urine volume and concen102
in the quokka (REID and McDoNALD, 1968). These possums could be kept alive
by giving them injections of cortisol, aldosterone being ineffective, though the dose
of cortisol could be reduced if both of these steroids were given simultaneously.
The adrenal cortex of Australian marsupials thus seems to playa vital role in their
ability to regulate sodium and potassium levels. Injections of aldosterone decrease
renal sodium excretion in the American opossum but have no consistent effect on
urinary potassium (BECK, BROWNELL, and BESCH, 1969). As we shall see, other
studies on marsupials have identified and implicated corticosteroids in their regulatory mechanisms.
The water metabolism of two large species of kangaroo has been studied in far
hotter and drier areas than those occupied by Setonix. TIM EALEY led a series of
ecological and physiological investigations on a wild population of the hill kangaroo, or euro, Macropus robustus, living in a hot dry desert region of north west
Australia (Fig. 3.2). ALAN NE~'SOME carried out a similar investigation on the red
kangaroo, Megaleia (or Macropus) rufus, living in an arid central region of the continent near Alice Springs. The average annual rainfall in both the areas is about
25 em, most of it falling during the summer months. The rain, however, is sporadic
and unreliable, and both areas may undergo periods of drought when little rain
falls for periods of three or four years. The summer temperature is high, the average
daily maximum often being greater than 40 °C for many weeks and may rise to
49 o C. The relative humidity usually is low so that the potential evaporation is
high. Despite these osmotically adverse conditions the kangaroos in these areas
continue to survive and multiply. A man lost in these regions in the height of summer can expect to survive for less than one day (ADOLPH, 1947) so the adaptability
of the kangaroos is impressive. Both the euro and the red kangaroo may form a
highly concentrated urine, which can be 2700 m-osmole/l in the red kangaroo (A.
NEWSOME, quoted by SCHMIDT-NIELSEN, 1964a) and 2200 in the euro (EALEY,
BENTLEY, and MAIN, 1965). Such an ability must aid water conservation by th ese
animals but is not a predominant factor in their survival. EALEY and his collaborators observed that euros can withstand dehydration equivalent to 30% of their body
weight, and that they reduce evaporation by sheltering in rocky caves during the
heat of the day . Euros drink periodically and even during hot dry periods only
come in to obtain water on an average of once every three days. During the intervening period they incur a water deficit equivalent to about 12% of their body
weight.
NEWSOME (1965 a and b) found that the red kangaroos also only drank sporadically, apparently gaining most of their water from their diet, which consists preferentially of succulent green shoots of the herbage. The animals limit evaporation
during the day by seeking the shelter of bushy woodlands and avoiding the open
plains . The red kangaroo, like the quokka, stores arginine-vasopressin and oxytocin
in its neurohypophysis, and the ratio of these conforms to that usually found in
marsupials of 4 or 5 to 1 (FERGUSON and HELLER, 1965). The estimated concentration of vasopressin in the neurohypophysis is 1O9M/kg
body weight, which is low
compared to other species, but this could result from the fact that the animals had
been hunted and shot before the pituitary was obtained. It can be reasonably surmised that vasopressin has a similar role, in regulating the urine volume and concen102