ADAPTATIONS TO TROPICS AND ALTITUDES 



for the other two primates are also compared in Figure 21. Most 

 ol the observed differences relate to the differences in size. The 

 values for the two marmosetts lie just above the mean curve relat- 

 ing conductance to body weight in some temperate small mammals, 

 C = W (Morrison and Ryser, 1951). Aotus, by virtue of its more 

 effective insulation, has a conductance appreciably below the mean 

 curve. 



In summary, this limited survey of thermoregulation in the 

 primates has again shovm some regularities in the daily cycles, 

 but also some variety in this and in the response to cold. Also, to 

 consider again the general question as to common thermoregula- 

 tory features shared by .tropical mammals, there appears to be 

 none. Certainly there is great variation in the maintained levels 

 both diurnal and nocturnal. The response to cold and as well, to 

 heat, appear equally variable since either or both may be present 

 or absent. Even the criterion of inferior insulation cited by 

 Scholander et al., (1950) does not hold for many of the smaller 

 tropical species. Indeed, perhaps we can only characterize the 

 tropical mammals by the complete heterogeneity of the thermo- 

 regulatory responses. 



Altitude and Thermoregulation 



The relation between thermoregulation and altitude appears 



even more tenuous than that of the tropics. It is true that if oxygen 



is sufficiently withheld from a mammal in the cold, its T will 



B 

 fall (Nielsen et al., 1941). But other functions and activities will 



be similarly impaired. Of course, to the extent that thermoregula- 

 tion may require a considerably greater energy output than other 

 functions, it will be preferentially affected — and also, as a regula- 

 tion that is, perhaps, less critical than some others, it might be 

 preferentially dispensed with as in the camel (Schmidt- Nielsen 

 et al., 1957). 



We have recently investigated the altitudinal responses of a 

 number of Andean rodents, and the matter of their transport capa- 

 city for at varying altitudes bears on the present point since it 

 represents a limit for energy output. Indeed, our experimental 



411 



