The Herbivore-Based Trophic System 351 



210 



TO 



c 

 a) 

 a. 



X 



UJ 



0) 



c 

 UJ 



2 4 6 



Number of Pups 



FIGURE 10-8. The simu- 

 lated daily energy expend- 

 iture by a reproducing 

 female lemming with a 

 winter nest in relation to 

 litter size, weight of pups 

 and ambient temperature 

 T. (MacLean and Thom- 

 sen, pers. comm.) 



the radius of the nest. Under winter conditions heat flow from the nest is 

 considerably less than the average daily metabolic requirement without a 

 nest. For a 60-g lemming at an air temperature of -30 °C the average 

 daily metabolic rate maintained over the entire day gives 199 kJ of energy 

 used. The time that can be spent in a nest depends upon energy reserves, 

 which can be estimated from the stomach capacity equation discussed 

 below. Assuming 30-minute bouts of foraging with metaboHsm at the 

 average daily rate, interspersed between bouts of nest use, total energy 

 use by a 60-g lemming with a nest of 12-cm radius is 129 kJ day"'. 



A large nest is crucial for winter reproduction. It reduces the energy 

 cost to the female and prevents rapid cooling and death of the sucklings 

 prior to the development of homeothermy at 10 to 12 days of age. The 

 physiological processes involved in reproduction are energy-demanding, 

 especially lactation. However, much of the heat from respiration by the 

 female goes into the nest, and thus contributes to homeothermy. Before 

 the sucklings develop homeothermy the major costs of reproduction 

 come from 1) the increased radius of the nest contents, the female plus 

 young, which leads to greater heat flow from the nest; 2) the need to re- 

 warm cooled sucklings following a period of absence; and 3) the growth 

 of the sucklings. The estimated energy expenditure of a lemming sup- 

 porting a maximum reproductive load, six 11-g heterothermic sucklings 

 at an ambient temperature of -30 °C, is 186 kJ (Figure 10-8), or 144% of 



