358 G. O. Batzli et al. 



winter. Continuous shifts in the diet mimicked the field situation. Nutri- 

 ent concentrations in graminoids were those reported for the Carex- 

 Oncophorus meadow for 1970 when nutrients were low and for 1973 

 when nutrients were high (Chapter 5; Chapin et al. 1975). 



Early runs of the model, using a combination of starting ages, dates 

 and nutrient concentrations, showed that only reproducing females 

 would experience serious deficiencies — depletion of normal body pool by 

 more than 25%. A series of runs were then conducted to compare the ef- 

 fects of different litter sizes, nutrient contents of forage, digestibility of 

 forage and food habits of reproducing females. In years when there were 

 low concentrations of nutrients in their forage, adult females (120 days 

 old) could barely support litters of seven during midsummer, four during 

 winter and two during late August. No offspring could be supported dur- 

 ing late June of such years. In winter, reproduction was limited by nitro- 

 gen, calcium and phosphorus, but in early and late summer only calcium 

 and phosphorus were limiting. During summers when there were high 

 levels of nutrients in the vegetation, 30-day-old subadults could raise lit- 

 ters of eight. As expected, smaller litters and slower growth in older ani- 

 mals improved the nutrient status of the lemmings. 



The simulation indicated that the condition of nutrient-deficient 

 lemmings could be improved during summer by an increase in the intake 

 of mosses or a reduction in digestive efficiency for energy and dry matter 

 (Figure 10-10). In winter these tactics also improved calcium and phos- 

 phorus deficiencies, but they caused worse deficiencies in nitrogen. Re- 

 duced digestibility allowed the extraction of minerals from more total 

 dry matter because the lemming had to eat more to meet energy require- 

 ments. These resuhs help to explain some unusual aspects of lemming bi- 

 ology. The low digestibility of forage by lemmings, compared to temper- 

 ate microtines, appears paradoxical since existence in an arctic environ- 

 ment increases energy requirements. However, the results of the model 

 suggest that lemmings confront a nutritional situation where calcium and 

 phosphorus availability are more critical than energy. Low digestive effi- 

 ciency for energy requires greater food intake and thus increases nutrient 

 availability. Adjusting the model so that energy digestibility was im- 

 proved caused reproducing lemmings to become calcium- and phos- 

 phorus-deficient, even when nutrient levels in the forage were high. Cal- 

 cium and phosphorus concentrations in arctic forage appear to be low 

 compared to those in temperate forage (Table 10-5). Low digestive effici- 

 ency for energy by lemmings may have evolved to assure an adequate in- 

 take of inorganic nutrients. The lower limit of digestibility must be deter- 

 mined by the actual ability of lemmings to find enough food and pass it 

 through the gut. The actual digestibilities are the resuh of several con- 

 flicting pressures. 



The model also helps to explain the presence of relatively high 



