JOHANSEN 



The increase in heat production when air temperature declines 

 has several noteworthy features. The steepness of the slope of 

 metabolism versus air temperature is fairly great and of about 

 the same magnitude as for the two-toed sloth. The fact that the 

 naked armadillo and the hairy sloth with about the same resting 

 metabolism showed similar regressions in this respect supports 

 the idea that the armadillo has a potent vasoconstrictor ability. 

 The fact that the armadillo showed a step- wise increase in insula- 

 tion may, however, somewhat invalidate comparison of it to the 

 sloth. The peak metabolic values in the armadillo vary from five 

 to six times larger than the resting values. Such high metabolic 

 rates were recorded for as long as 6 hours, which was the longest 

 time oxygen consumption was measured at one fixed low tempera- 

 ture. According to Scholander et al. (1950) the maximum increase 

 in heat production is time-dependent and seldom more than four 

 times the resting value. They state that this relation is valid for 

 long-time experiments, but they do not define "long time." In the 

 armadillo the increased heat production results from a progressive 

 augumentation of shivering. The shivering pattern is closely related 

 to the crouched posture and the armadillo rarely utilizes moving 

 about as a means for increasing heat production in the cold. 

 Occasionally when they abandoned the immobile posture, the ani- 

 mals experienced great heat losses. To demonstrate more expli- 

 citly the difference in metabolism and response of body temperature 

 to cold exposure, I did some experiments with rabbits. In Figure 

 14, we can see that deep rectal temperatures and oxygen consump- 

 tion were followed at various ambient temperature levels ranging 

 o o o CO 



from +30 Gto-6 G.AtSO C air temperature, the rectal tempera- 

 tures of the armadillos are more than 4 C lower than for the rab- 

 bits. Upon a gradual step-wise decrease in air temperature, the 

 armadillos show an increasing body temperature, accompanied by 

 a steep increase in oxygen consumption, whereas the rabbits' con- 

 dition is unchanged. The procedure lasted 4 to 5 hours. These studies 

 demonstrate a conspicuous over-compensation to the cold stress 

 in the armadillo. Inother words, the body temperature is drastically 

 raised at the expense of an increased metabolism. The fact that this 

 over-compensation takes place so rapidly and to such a large extent 



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