EVOLUTIONARY ASPECTS IN MAMMALS 



according to Krieg(1952), hibernate or have a similar torpid con- 

 dition. In Marmosa cinerea . which is considered old from a phylo- 

 genetic standpoint, Eisentraut (1955) recorded an average body tem- 

 perature of 34.7° G, and a range of from 29.3 C to 37.8 G, show- 

 ing great dependence upon conditions of activity. I would like to 

 give Eisentraut credit for his attempt at systematically arranging 

 body temperatures, not as fixed numbers, but as ranges of tem- 

 peratures. To indicate an animal's body temperature as a fixed 

 point, even when giving this as an average, is not as expressive 

 as listing the range in body temperature for the animal during 

 normal natural conditions. Such information requires undisturbed 

 recordings of body temperatures under all normal conditions rang- 

 ing from sleep to strenuous exercise. Information of this kind is, 

 unfortunately, available for only a very limited number of species. 

 Such information, however, will, in my mind, express more about 

 an animal's temperature regulating ability than do most of the para- 

 meters now in general use. Eisentraut (1956b) suggests "activity 

 temperature" as a term for such a range. This apparently excludes 

 conditions of sleep and rest, which ought to be included. I will, 

 therefore, propose to call it "body temperature range at normal 

 behavior." It is highly significant that the New World Didelphidae 

 (Didelphis, Marmosa) have lower body temperatures than the mar- 

 supials of the Old World. The taxonomists unanimously consider 

 the New World marsupials as the most primitive phylogenetically. 

 Morrison, in his work on two marsupials from Central America 

 (1956), states that the species studied, the brown opossum ( Meta - 

 chirus nudicaudatus ) and the Eten opossum ( Didelphis marsupialis ) , 

 showed a homeothermism in no way inferior to that of many higher 

 mammals. Sutherland's data (1897) show that even the very slug- 

 gish koala bear (Phascolarctos) maintained his body temperature 

 better than any of the placental mammals. A number of authors 

 comment on the well-developed ability of the common American 

 opossum ( Didelphis virginiana) to regulate its body temperature. 

 As we all know, this species has migrated extensively northward, 

 and it seems of the greatest significance that its temperature regu- 

 lation is decidedly superior to that seen in the closely related 

 tropical forms. 



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