ADAPTATIONS TO TROPICS AND ALTITUDES 

 DISCUSSION 



HUDSON: Dr. Morrison, do any of your studies correlate these 

 tolerances of the oxygen tension within the aspect of the cardio- 

 vascular system? In the case of the house mouse you mentioned 

 the lung and the diaphragms, so I suppose that answers part of my 

 question. 



MORRISON: Our primary objective was an evaluation of over- 

 all performance, but weexaminedanumber of details. For example, 

 the hemoglobin level in native mice at high altitude appears no 

 greater than at sea level; but at both levels , hematocrits are higher 

 in the "vigorous" species as compared to "less vigorous" species. 

 We were not prepared to examine the factor which I suspect is the 

 most significant, namely the capillary dischargeof oxygen. Because 

 there were only modest changes in the other factors in the chain — 

 lung and heart size, heart and respiration rates, hemoglobin level, 

 etc. — we are forced to conclude that there is some specialization, 

 perhaps an increase in number, or a lengthening and contorting, 

 of the capillaries. This would really be an optimal adaptation with 

 minimal distortion of the normal pattern of the animal; and it would 

 seem that this normal pattern is rather important. The house mouse 

 does adapt, but I am sure that he is at a concomitant disadvantage 

 in some way because it has distorted the normal mammalian form 

 (i.e., lung fraction, heart fraction, hematocrit, etc.) which is a very 

 constant feature. I cannot say just why the normal proportions are 

 optimal, but I think it must be so. 



Of course, you human physiologists know the problems of get- 

 ting comparable material. I was impressed by this in Peru where 

 some studies compared miners from Ororococha to other subjects 

 from Lima. Some miners had more work capacity at 15,000 feet 

 than the urbanites had at sea level, but clearly the development 

 and conditioning of these subjects differed by much more than 

 altitude. 



HART: May I ask Dr. Morrison a question about the study of 

 mice at high altitude? Were they all small mammals in your high 

 altitude and low temperature comparison? 



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