MORRISON 



Marsupialia 



The marsupials have often been stigmatized as indifferent 

 homeotherms usually because of the low reported level of the body 

 temperature (T^. But this conclusion suffers on three counts; 

 first, a somewhat lower T is a poor criterion of homeothermism, 

 we do not consider birds to be more homeothermic than mammals 

 simply because they maintain a higher T ; second, a fairly limited 

 assortment of marsupials has been studied; and third, since most 

 marsupials are nocturnal, their study by diurnal physiologists has 

 resulted in a falsely low estimate of their T level. 



The first figure shows the relation between body temperature 

 and ambient temperature (T ) in a small American (brown) opos- 

 sum ( Didelphidae ) in day and night. The diurnal values are quite 

 distinct from the nocturnal ones, but both are accurately regulated 

 (over a range) in response to cold. This relation (T vs T ) des- 

 cribes the sum of regulatory activities with a horizontal curve rep- 

 resenting complete regulation and one with a 45 slope (reference 

 line), representing the absence of regulation seen in a poikilotherm. 

 In addition to the slope, we must also consider the relation of the 

 curve to the reference line since an animal may have a fairly labile 

 T and yet maintain it well above the T . A third criterion of regu- 

 lation is the variability of the T around the mean curve, but this 

 may sometime provide a spurius mdex, since the scatter may only 

 reflect a variation in the circumstances under which the measure- 

 ments were made. 



Figure 2 compares a rat-sized Australian representative (Das- 

 yuridae) with an even more striking diurnal depression, near noon 

 its T is about 34 C, but at night it is 38 C. Since the latter 

 level IS equivalent to that in the dog, Chaetocercus can hardly 

 be considered defective or primitive in its level of regulation. But 

 this is a form which can show a daily torpor. Another smaller rela- 

 tive (Antechinus) shows an even higher level during periods of 

 activity (to 40 ) although these periods do not follow a 24-hour 

 cycle. Accordingly, it is necessary to identify any daily (or other) 

 cycle and choose either or both, the maximum and minimum peri- 

 ods—the active and resting phases--to characterize the species. 



390 



