The Herbivore-Based Trophic System 349 



252 



210 



-, 168 

 ^ 126 



84 



42- 



^ \ADIVIR20 



Thermoregulation \ \ 

 80 \ \ 



W 



. I i \ i^i-J u 



■30 -20 -10 10 



Temperature , "C 



20 



60 



50 





u 



40 - 



o 



30 S. 



c 

 a> 



-20 



-10 



a. 



FIGURE 10-7. The relationship of energetic require- 

 ments of non-reproducing lemmings (solid lines) and 

 percentage of energy used for thermoregulation (dashed 

 lines) to ambient temperatures. Subscripts give the body 

 weight of lemmings in grams. 



which assimilated energy can be transformed into tissue. Growth rates of 

 lemmings have been summarized by BatzH (1975a) and allow for the de- 

 velopment of sexual dimorphism in size beginning as subadults, 30 to 60 

 days old. In order to calculate the total energy used for growth, 25% of 

 the energy stored in new tissue must be added, given an efficiency of tis- 

 sue growth of 0.80 (Blaxter 1967). Assuming that growth is independent 

 of ambient temperature, growth adds a constant energy increment to the 

 average daily metabolic rate. 



Energy requirements for maintenance and growth as a function of 

 ambient temperature differ markedly for juvenile and adult lemmings 

 (Figure 10-7). The average daily metabolic rate increases significantly 

 with declining ambient temperature; for the 20-g juvenile lemming it in- 

 creases nearly 3-fold over the temperature range -1- 17 ° to -25 °C, the an- 

 nual range of temperature usually encountered by a lemming. 



The average daily metabolic rate equation separates the energy cost 



