354 G. O. Batzli et al. 



low compared to the 45% to 55% digestibility of temperate grasses by 

 other microtine rodents (Batzli and Cole 1979), tundra graminoids are 

 higher in total nonstructural carbohydrate and lipids than are temperate 

 graminoids (Chapter 5). Thus, the higher digestibilities achieved by tem- 

 perate microtines must result from the breakdown of a significant por- 

 tion of the structural tissue. For lemmings, however, the rate of energy 

 and nutrient assimilation is maximized at the expense of efficiency. Food 

 is passed rapidly through the gut and only the most easily digested frac- 

 tion assimilated. Melchior (pers. comm.), in laboratory feeding trials us- 

 ing graminoids as food, showed that hunger reached maximal levels after 

 two hours of food deprivation, and we found that guts were virtually 

 empty after three hours. Following a change of diet, fecal pellets at- 

 tributable to the new diet appeared in 35 minutes. 



The rapid passage of food through the gut and the high daily energy 

 demand require that a significant amount of each day be spent foraging. 

 During summer adult lemmings spend about 60 to 70% of their time out 

 of the burrow (Banks et al. 1975, Peterson et al. 1976). Melchior (pers. 

 comm.) estimated the stomach capacity of adult male lemmings by feed- 

 ing animals to satiation; food consumed was approximately 10% of body 

 weight. This gives a stomach capacity of about 125 usable joules per 

 gram of lemming: 



Stomach capacity = 0.1 H^x0.20x 18,900x0.33 = 125 J g"' 



where Wis the body weight of the lemming in grams, 0.20 is the propor- 

 tion dry weight in forage, 18,900 represents the average number of joules 

 per gram dry weight of forage and 0.33 is the proportion of joules assimi- 

 lated. Given this stomach capacity, and assuming that the value derived 

 for adult males holds for all age classes, we may calculate the number of 

 times the gut must be filled each day to satisfy the energy requirements of 

 average daily metabolic rate plus growth. Since both stomach capacity 

 and body weight are proportional to the volume of the animal, we 

 assume that stomach capacity increases linearly with weight, whereas 

 average daily metabolic rate increases with ^P ". Gut capacity increases 

 more rapidly with size than does metabolism, and the number of gut fill- 

 ings needed to satisfy average daily metabolic rate falls. Growth rate also 

 decreases with size, further reducing the required number of fillings per 

 day. Small lemmings (20 g) require 22 to 52 fillings per day while large 

 lemmings (80 g) require only 14 to 22 as temperature varies from -i- 15 to 

 -25 °C. The large number of fillings does not represent the number of 

 foraging bouts, however, as lemmings may spend an hour or more out of 

 the burrow at a time, at least during summer, and probably refill the 

 stomach before it is empty (Peterson et al. 1976). 



Perhaps the most revealing expression of energy demand for the 



