EVOLUTIONARY ASPECTS IN MAMMALS 



increases their independence from the environment. I think, fur- 

 thermore, that this is an important point in the phylogeny of homeo- 

 thermy and more than anything that it should be subjected to further 

 study. 



My data on the tolerance to cold in a long-time experiment 

 differs markedly from apparent results of earlier investigations 

 on armadillos. The conspicuous increase in rectal temperatures 

 has not been reported previously and it may have escaped notice 

 because the rectal temperatures during cold exposure were not 

 continuously recorded. Wislocki (1933) observed the following rectal 

 temperature patterns in armadillos transferred from 28 C to 

 C air temperature: at time, 35 C; at 1-hour exposure, 34 C; 

 at 2- hour exposure, 31.5 C; at 3- hour exposure, 30 C. Recently, 

 Enger (1957) reported that the armadillo, opposum, and three- 

 toed sloth are poor thermal regulators and lose body heat to a 

 considerable extent during a cold stress, that is, 4 to 6 hours at 

 10° C air temperature. These results differ basically from those 

 of the present study. It would be highly desirable to determine 

 whether this difference stems from the fact that Wislocki 's and 

 Enger's measurements were made on animals from a strictly 

 tropical habitat, Barro Colorado, in Panama, whereas the subjects 

 of this study came from Texas. This would be a most notable 

 demonstration of adaptation to cold. 



The increase in oxygen consumption upon lowering of the air 

 temperature for a representative experiment is shown in Figure 

 10, and for all experiments in Figure 13. There was more than 

 a five-fold increase in oxygen uptake when the air temperature 

 was reduced from 30 C to -10 C. The relatively great spread 

 in the data at the lower temperatures is probably related to the 

 differences in the rectal temperatures of different animals. No 

 attempt was made to test responses to temperatures below -10 C 

 because some animals try to escape exposure to this temperature. 

 These activities of course involved the abandonment of the protec- 

 tive posture and large losses of heat occurred. Ordinarily the 

 animals showed an amazing ability to remain crouched in their 



ball- like posture for hours when the temperature was as low as 



o 

 C. 



Ill 



