ADAPTATIONS TO DESERTS 



temperature as far down as we record it, and this goes to a 

 little below C. We found we cannot get any leveling off as 

 high as we have measured it, which is about 32 C. You do not 

 find a leveling off or thermo-neutral zone type of thing, as Dr. 

 Hudson found in the Poor-will. We found also that at the very low 

 temperature, the shivering was so intense that it obscured the heart 

 rate except at very short intervals, when the heart rate would come 

 through. I wondered how you recorded your heart rate. 



HUDSON: Of course, we started off by measuring it in the 

 thermal neutral zone and the Poor-will is probably a particularly 

 good bird for this sort of thing, since it is quite tractable. As 

 we get below the lower critical temperature of course the shiver- 

 ing begins to appear on the EKG record, but does not make it 

 impossible to pick out the QRS complex until we begin to get down 

 to ambient temperatures around 20 C. Now, at those tempera- 

 tures, we have found that by giving the bird, and it appears to 

 have an extremely rapid and sensitive response to this, a quick 

 burst of heat, not enough to seriously interfere with its meta- 

 bolism or its body temperature in any way, that it will imme- 

 diately cut out shivering and then we can pick up a clearer EKG 

 record. Then in a matter of minutes, of course, it starts to shiver 

 again. 1 wouM like to counter by asking you a question, and that 

 is; do you find any sort of a correspondence between the lower 

 critical temperature of the heart rate and metabolism, or are all 

 your measurements of heart rate made in the thermo-neutral 

 zone? 



WEST: I never find a thermo-neutral zone for the small birds 

 I have studied. 1 never go to high enough temperatures. Unfor- 

 tunately, we are so concerned with cold, we do not go much over 

 30° C. 



HUDSON: Most of the small birds have a thermo-neutral 

 zone or point that would be around or above 30 C. 



WEST: But we get perfect linear correlation of temperature 

 on metabolism and on heart rate, as far as we can go up and down. 



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