350 G. O. Batzli et al. 



of thermoregulation, 1.87 kJ °C"', from the cost of maintenance. The 

 cost of thermoregulation expressed as percent of total energy require- 

 ments, average daily metabolic rate plus growth, is zero at +17°C, 

 which is the lower limit of thermoneutrahty, compared with 36"7o of the 

 total for the 80-g lemming at -25 °C and 60% for the 20-g lemming at 

 -25 °C. 



Energy required for reproduction includes that invested in growth of 

 fetuses and production of milk for sucklings. Fetuses grow from to 3.3 

 g over the 21 -day gestation period, an average growth rate of 0.25 g 

 day"'. Given an average summer litter of seven (Figure 10-4), the repro- 

 ductive female must support an average fetus growth of 1.75 g day"'. Ac- 

 tually, the growth is concentrated in the latter phases of gestation. After 

 a tissue growth efficiency of 0.80 is applied, the cost of pregnancy is about 

 75% greater than that for growth of the 20-g lemming (Figure 10-7). 



The cost of suckling growth must also be supported by the breeding 

 female. Applying a growth rate of 0.8 g day"' to a litter of seven gives a 

 value of 5.6 g of suckHng growth per day. Since 1 g suckling live weight 

 has an energy content of 4. 19 kJ, this is equal to 23.5 kJ. This must be di- 

 vided by 0.3, the value for efficiency of conversion of milk to suckHng 

 tissue, and by 0.7, the value for efficiency of milk production by the 

 mother (Brody 1945, Hashizume et al. 1965). The energy requirement is 

 therefore 1 13 kJ day"', equal to the average daily metabolic rate of a 40-g 

 lemming at 5 °C. Thus, lactation plays an immense role in the energetics 

 of lemmings. Securing this additional energy requires more activity by 

 reproducing females, which further increases energy demand. Lemmings 

 usually are able to satisfy this demand during summer, but during 

 winter, when the cost of thermoregulation is high and forage is sparse, 

 litter size declines to three (Figure 10-4). 



Reproduction would not be possible at all during winter without the 

 construction of nests. The value of the nest to the lemming was explored 

 by MacLean and Thomsen (pers. comm.) using a heat flow model. The 

 model regards the lemming as a homeothermic body of temperature 71 

 and radius b, proportional to the cube root of body mass. The lemming 

 must produce heat at a rate q that is equal to the heat flow from the 

 warm lemming to the cold surroundings. The nest forms an insulating 

 layer of inner radius b and outer radius a around the lemming. 



The model shows that an equiUbrium heat distribution is rapidly es- 

 tablished in the inner layer of the nest around the lemming. At this time 



g = 4nkiT,-T^)[ab/(a-b)] 



where k is the thermal conductivity of the nest material in J s"' cm"' °C"' 

 and Ta is the temperature of the snow around the nest. Heat flow is 

 determined by the temperature gradient, the radius of the lemming and 



