MORRISON 



But the summer animals did not maintain their temperature in 

 the daytime. So this si^gests that there is some kind of adaptation 

 of thermoregulatory control not in the metabolic capacity nor in 

 the insulation, but in the ability to respond to a stimulus which it 

 may not encounter. 



Figure 7 describes the koala. This is a familiar sluggish mar- 

 supial (Phalangeridae) which has poor temperature regulation to 

 cold as can be seen from the very substantial slope to the T curve. 

 In this regard the koala seems quite inadequate, but since the T 

 curve continues almost in a straight line across the isothermal 

 line, it actually has quite effective regulation to heat; much better, 

 indeed, than many of our higher mammals, such as the rodents, 

 which cannot maintain the T below the T . There is again a con- 

 siderable scatter in these points, but much of this could be elim- 

 inated by proper definitions of the conditions. This is a particular 

 problem with a sluggish animal which adjusts only slowly to new 

 circumstances. As one might expect, the afternoon T is warmer 

 than the morning T . But this is a diurnal animal and so part of 



this slope of the T curve reflects the daytime activity. 

 B 



The examples given thus far might tempt us to characterize 

 the marsupials as animals with very large diurnal cycles, and 

 even to represent a measure of thermal instability, although this 

 is a matter of some argument. But Figure 8 shows, for contrast, 

 a small wallaby ( Macropodidae ) with no daily cycle at all. Simi- 

 larly, Figure 9 presents one of the larger macropods in which the 

 diurnal cycle is again absent. But in part this effect is spurious 

 as a representation of the animal in nature because an animal as 

 large as the kangaroo is not able to engage in his normal activity 

 when maintained in close captivity. If it is normally occupied with 

 moving and feeding, higher nocturnal values are obtained. How- 

 ever, Figure 9 is principally of interest in illustrating or suggest- 

 ing another phenomenon. The checkered circles averaged by the 

 upper dotted curve represent T values that were taken during the 

 week following a critical heat test in which the animal was exposed 

 for 6 hours at 40 . During this exposure the T was not markedly 

 elevated (only to 35.4 G), since kangaroos are excellent regulators 

 to heat. Nevertheless, following this heat exposure, an elevated 



396 



