356 G. O. Batzli et al. 



rate of intake may increase with size, but an 80-g lemming would have to 

 eat 66% faster than a 20-g lemming at low temperatures and 156% faster 

 at high temperatures to spend similar amounts of time feeding. No rela- 

 tionship between body size and maximum feeding rate could be found in 

 Melchior's data. 



Actual foraging times would include search for and selection of 

 food and would be much greater than minimum feeding times. Further- 

 more, during winter lemmings must leave the nest to forage under lower 

 subnivean temperatures. While they are away from the nest, nest temper- 

 ature falls. More time spent foraging increases energy demand, which in- 

 creases the necessary foraging time. A positive feedback relationship ex- 

 ists and accentuates winter energy requirements. 



Nutrient Relationships 



One of the most intriguing aspects of lemming nutrition is its rela- 

 tionship to plant nutrient concentration and nutrient flux. Nutrient con- 

 tents of plants vary widely with plant species, plant part, site, season and 

 year (Chapter 5). Changes in forage quality have serious implications for 

 lemming nutrition. For instance, the preference for Dupontia fisheri in 

 midsummer may be related to the fact that it tends to grow in nutrient- 

 rich areas. Indeed, the general propensity of lemmings to use vegetation 

 more in troughs and wet meadows than in drier areas may be related to 

 the higher nutrient status of graminoids in those areas. 



Interest in the role of nutrients in lemming population dynamics led 

 to examination of the nutrient dynamics of lemmings in relation to food 

 habits and nutrient content of the forage. Of the energy ingested by the 

 population as a whole, 1.5% is retained as production (Table 10-3): 



P/I - A/IxP/A - 0.33x0.045. 



The balance is lost as urine and feces, or through respiration. Since the 

 energy and nutrients come from the same food, we may calculate the P/I 

 value for nutrients as 



P//,^, = 1.5(L/F[^,xl.2-') = 1.3L//^,A,, 



where P/I[n] is the percentage of a nutrient retained by the lemming 

 population, 1 .5 is the percentage of energy retained, L/Fi^vi is the ratio of 

 nutrient concentration in lemmings to forage, and 1.2 is the ratio of 

 energy concentration in lemmings to forage. 



Calculated values for forage (Table 10-4) represent typical values 

 and ignore wide variation for some nutrients, so only major trends 



