382 G. O. Batzli et al. 



protein, calcium and phosphorus levels in graminoids well above those in 

 nonfertilized areas. Fertilization apparently increased winter reproduc- 

 tion in 1968; there were about 75 winter nests ha"' in the fertilized area 

 and none in the control areas. The effect continued through 1971, al- 

 though by then it was less dramatic. Melchior (pers. comm.) reported 14 

 nests ha"' in the fertilized areas and 2 ha'" on the control plots. These re- 

 sults suggest that, at least during some winters, reproductive perform- 

 ance of lemmings can be stimulated by improving vegetation quality. 



In summary, lemming populations often increase up to a Hmit im- 

 posed by their food supply and begin to decline when there is not enough 

 food to meet energy demands. However, in summer primary production 

 exceeds the lemmings' requirements, and lack of food cannot explain the 

 continued population decline. Several lines of evidence suggest that lem- 

 mings can survive on low quality forage because of nutritional adapta- 

 tions, but the lack of nutrients may still reduce winter reproduction. 



Intrinsic Factors 



All the factors influencing lemming populations discussed so far are 

 extrinsic, residing outside of the population itself. Several investigators 

 have suggested that intrinsic factors such as behavior, physiology and 

 genetics may be equally important and that social interactions and ag- 

 gression increase with increasing population density. Christian and Davis 

 (1965) proposed a hormonal imbalance to account for a population's de- 

 cline. Chitty (1967) and Krebs et al. (1973) argued that some types of 

 lemmings emigrate more or die sooner than others, thus changing the 

 genetic composition of the remainder of the population. Changes in gen- 

 otypic frequencies within the population are held to be responsible for 

 changing reproductive and survival rates. 



Some work has been done on physiological changes in lemmings at 

 Barrow. Mullen (1965) looked at blood glucose and formed elements of 

 blood during four summers. There was no evidence of physiological 

 changes associated with population density. Krebs and Myers (1974) 

 found no evidence that physiological stress played a role in the produc- 

 tion of microtine cycles. Andrews et al. (1975) reported changes in adre- 

 nal activity and kidney disease associated with population density and 

 climatic factors, but often these do not appear to be consistent or statis- 

 tically significant. Thus the consequences of these endocrine adjustments 

 for population dynamics of lemmings are unclear. Using a process of 

 elimination Krebs (1964) concluded that genetic changes influenced lem- 

 ming populations at Baker Lake, Canada, but gave no direct evidence. 

 No studies of emigration or genetics have been done on lemmings at 

 Barrow. 



