402 G. O. Batzli et al. 



Food Consumption and Foraging Patterns 



The grazing regime imposed by these populations depends not only 

 on their biomass but also on their rate of food consumption and on the 

 composition of their diet. Rates of food consumption must allow at least 

 sufficient assimilation to supply energy requirements. Because of their 

 high energy requirements small mammals often put more grazing pres- 

 sure on tundra vegetation, even in the Prudhoe Bay region, than do cari- 

 bou. Although Dicrostonyx and Rangifer have similar assimilation effi- 

 ciencies (~0.55 to 0.65) Lemmus has much lower efficiency (0.33), which 

 further increases ingestion. 



The gut capacity of Lemmus for digestible nutrients is 60% that of 

 Rangifer in relation to body weight (Table 10-1 1), hence lemmings must 

 fill their stomachs more often. However, for their size lemmings eat 

 much faster, and the net result is that they need to spend only 20% as 

 much time eating as caribou when forage is easily available. Interest- 

 ingly, although the absolute turnover time of gut contents for lemmings 

 is much less than for caribou, when corrected for metabolic weight (pro- 

 portional to W'^^^ according to Kleiber 1961), the relative turnover times 

 are almost equal. 



The distribution of grazing differs in its timing as well as in its inten- 

 sity. Grazing by Lemmus becomes most intense during winter and at 

 snowmelt when the vegetation lies dormant. Caribou migrate, and most 

 of them leave the Prudhoe Bay region during winter, so the grazing pres- 

 sure from caribou is highest during summer when plants are growing. 



The species and parts of plants taken by the grazers also vary. Lem- 

 mus takes primarily graminoids with a supplement of mosses. During 



TABLE 10-11 Nutritional Characteristics of Mature Lemmus 

 and Rangifer, Assuming Body Weights of 80 g 

 and 100 kg. Respectively 



Note: Energetic calculations are for summer (15°C). 



