310 J. J. Christian 



So it appears to be with population density. Only those female mice which 

 were moderately affected by density produced young which were then 

 permanently affected by reduction in lactation, whereas those mice more 

 seriously affected failed to produce at all and exhibited 100% resorption of 

 embryos (Christian and LeMunyan, 1958). Therefore, when one considers 

 the enormous number of variables affecting a population, it is not sur- 

 prising that there is so much variation among individuals and among 

 populations wdth respect to the effects on reproduction. Reproduction 

 should include the entire process of producing young from maturation of the 

 parents to weaning when discussing or measuring the effects of physiologic 

 adaptive mechanisms and the effects on the endocrine organs. 



In spite of these problems, a number of studies on natural populations of 

 small mammals have been sufficientlj'' detailed and critical to permit exami- 

 nation of the relationships between population density and reproduction. 

 The most conclusive of these have been carried out on Norway rats (Rattus 

 norvegicus) and voles {Microtus sp. and Clethrionoimjs) , but there have been 

 others on muskrats (Ondatra), cotton rats {Sigtnodon) , woodchucks {Mar- 

 mota) , and hares (Lepus americanus) . Most of these suffer to one extent 

 or another from a lack of dependable criteria for determining age, as dis- 

 cussed above. 



The reproducti^'e performance of Norway rats from increasing, de- 

 creasing, and stationary populations was investigated (Davis, 1951a). 

 These rats have a major reproductive season in late winter and early 

 spring and a secondary minor peak in reproductive activity in the fall. 

 Increasing populations have a higher prevalence of pregnancy than either 

 decreasing or stationary populations. These differences are especially 

 marked in the spring breeding season when the prevalence of pregnancies 

 was 41.6% in increasing, 25.3% in decreasing, and 14.4% in stationary 

 populations. The incidences of pregnancy were, repsectively, 6.1, 6.3, and 

 3.8 per year. However, there were no apparent differences in litter size and 

 the prevalence of lactation Avas essentially the same in all three categories 

 although the data on lactation could not be analyzed separately for the 

 two breeding seasons. However, an analysis of lactation by seasons could 

 not account for the similarity of the overall figure and one must conclude 

 paradoxically that, although the pregnancy rate was highest in increasing 

 populations, there was also a greater parturitional mortality and at the 

 same time a better survival of those young which were not lost soon after 

 birth. 



Similar results were obtained when reproductive performances of Norway 

 rats from a rural population were compared at two different relative popula- 

 tion densities. The pregnancy rate was 48.3% a month after artificially 

 reducing the population a third, compared to 14.4% before the reduction. 



