EVOLUTIONARY ASPECTS IN MAMMALS 



reasoning I feel that such a strict sequential play-off of the com- 

 pensatory mechanisms will require a rather precise central ner- 

 vous thermostatic control which in many forms may not have 

 reached the perfection needed. 



For the metabolic rate of the armadillo, my studies showed a 

 low resting value of about 250 cc/(kilo x hr) at an air tempera- 

 ture of 30 C. These measurements are in accord with those 

 reported by Scholander et al. (1943). They found values averag- 

 ing 180 cc/(kilo xhr) and varying between 150 and 280. These values 

 are roughly half the size of those reported by de Almeida and 

 de A. Fialho (1924). However, Scholander etal. felt that the experi- 

 mental approach may have been a factor in the recording of the 

 high values by these authors. The resting metabolism of the arma- 

 dillo, then, is obviously less than that common in mammals of the 

 same size. It is slightly more than half that simultaneously measured 

 in rabbits of comparable size. The measured metabolic rate in 

 rabbits corresponded with values given by Benedict in 1938. The 

 resting metabolism of the armadillo, although somewhat higher, 

 approaches that found in sloths by Irving etal. (1942). Martin (1903) 

 reported an air temperature of 30 C coinciding with the lowest 

 metabolic value for the monotremes, Echidna and Omithorhynchus , 

 and for several marsupials. Throughout the whole temperature range 

 studied, Martin found the metabolism to be lower in monotremes and 

 marsupials than in higher mammals like cats and rats. The ques- 

 tion arises: Is this low level of resting metabolism related to the 

 low resting body temperatures found in these species ? The results 

 of my study on the armadillo suggest that this may be partly true, 

 but that differences in the thermostatic mechanisms are also impor- 

 tant factors in the dissimilarity between the armadillo and more 

 advanced homeotherms. For the same reason critical temperatures 

 and critical gradients are more or less meaningless when applied 

 to these species. As mentioned, the armadillo will occasionally 

 start shivering and increase its body temperature at an air tempera- 

 ture of 30 G. The word "critical," as in critical temperature and 

 critical gradient, is obviously not pertinent to this situation. Martin 

 argued similarly that the low metabolism in monotremes and mar- 

 supials can result from the following factors: a greatly diminished 

 heat loss, a lower body temperature level, and failure to maintain a 

 constant body temperature. 



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