MORRISON 



o o 



procedure involved a cold stress (at 5 C to 10 C) to raise the 



metabolic level. The oxygen tension was then lowered in successive 

 steps until a reduction in oxygen consumption was observed. This 

 was always followed by more or less severe hypothermia depend- 

 ing on the duration of the experiment. 



As an index of performance, we choose the pO at which the 

 metabolism was reduced to twice the basal level. Of our "low" 

 species, the least effective was the Chilean degu ( Octodon degu) , 

 a rat- sized, histricomorph rodent. The "critical" pressure for 

 the degu was sometimes reached atanpO of 110-120 mm, a reduc- 

 tion of only 1/4 from that at sea level. The other extreme was seen 

 in one of the species of the high-altitude genus of Akodon, a small 

 cricetid rodent, which could still be effective at a pO of 50-60 mm, 

 or about a third that at sea level. These were the extremes, and 

 although animals from high altitude were on the whole much more 

 effective than animals from sea level, a spectrum of "critical" 

 pressures was seen. Thus, the best "low" species ( Oryzomys 1. 

 lon gicaudatus ) was more effective than several of the "high" species. 

 The differental performance of different species from the same 

 environment appeared to relate to general "fitness" or "athletic 

 development." Thus, the Oryzomys was markedly the most vigorous 

 of the low species, and it is quite reasonable that their greater meta- 

 bolic potential will also be effective under the handicap of hypoxia. 

 In a similar manner, wild guinea pigs showed significantly greater 

 performance than their more sedentary domestic relatives taken 

 from the same altitude. 



In sunrvmary, Andean rodents from high altitude do show superior 

 thermoregulation to cold stress when measured at low oxygen pres- 

 sures. This facility appears to be unrelated to the moderate increase 

 in cold stress on the altiplano, and relates rather to the general 

 improvement in transport capacity by which the species adapt to 

 the requirements of their hypoxic environment. 



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