The Herbivore-Based Trophic System 369 



TABLE 10-7 Sex Ratios of Lemming Popula- 

 tions in Spring 



it consumed an average of 7.5 60-g lemmings day"'. After its weight sta- 

 bilized, food consumption fell to 5.4 lemmings day"', a value 35% 

 greater than Gessaman found for summer. 



Maher (1970) estimated the food requirement of pomarine jaegers 

 by direct observation of adults, which were unfettered, and chicks, which 

 were penned in the field. Chicks near the age of fledging consumed about 

 3.3 60-g lemmings day"', about one lemming less than adults. 



Because of the very high insulating value of the winter pelage rela- 

 tive to the summer coat, the maintenance energy requirement of arctic 

 foxes does not change appreciably through the year (Underwood 1971). 

 Deposition of fat during summer, use of fat during winter and changing 

 activity patterns modify the annual energy budget greatly. Total assimi- 

 lated energy requirements for a single fox varied more than five-fold over 

 the year. Requirements were greatest in^ August, about 4609 kJ day"', 

 equivalent to 14.7 60-g lemmings, and least in April, about 838 kJ day"' 

 or 2.7 60-g lemmings. This remarkable difference may help to explain the 

 greater density of foxes during winter than summer, even though food 

 appears to be more available in summer. 



The energy requirements of arctic weasel populations have not been 

 measured. But extrapolating from Brown and Lasiewski's (1972) equa- 

 tions for Mustela frenata, MacLean et al. (1974) estimated that a 65-g 

 least weasel living at an ambient temperature of -20 °C would have a rest- 

 ing metabolic rate of 409 kJ day"', which would require consumption of 

 1.3 60-g lemmings day"'. However, weasels use lemming nests in winter, 

 and maintaining a microclimate at 0°C reduces the resting metabolic rate 

 to 281 kJ day"', requiring only 0.9 60-g lemming day"'. 



Estimates of winter predation by weasels can be based upon remains 



