316 /• /• Christian 



These data, like those from the preceding study, indicate that the effects 

 of high densities on reproduction and growth persist for some time after 

 the peak has been reached and passed. It should be remembered that all 

 animals beginning the breeding season after a peak are sur^-ivors from the 

 peak densities. Although the highest numerical size of Hoffmann's popula- 

 tion was in September of 1953, the actual peak could just as easily have 

 been the relatively high spring population of 1954. The data suggest this 

 to be the case, as there was a brief period at the beginning of the 1954 

 (June) breeding season in which prenatal and infant mortality were lower 

 than usual and then climbed sharply. Hoffmann concluded that ovulation 

 rate and Utter size tend to vary inversely with population density and thus 

 to run counter to the cychc trend of the population, except during the 

 crash decUne period in Microtus montanus, when lowered Utter size may 

 contribute to the drop in density. He also concluded that speed of attain- 

 ment of reproductive maturity remained rather constant despite changes 

 in density, which is quite the reverse of Kalela's results and conclusions. 

 This conclusion is, however, based on the assumption that there is no 

 inhibition of growth or that it is totally independent of changes in reproduc- 

 tive function. Since the age of these voles could not be determined, age 

 was assumed to be reflected in body weight. 



In view of the likelihood that inhibition of growth and reproduction are 

 not independent, but rather are two manifestations of one reaction, as we 

 have seen for Clethrionomys as weU as for Mus in experimental populations, 

 the conclusion is not justified that the speed of attainment of maturity 

 remains constant in the presence of changes in density. Furthermore, 

 Hoffman's figures indicate that a high proportion of mature animals were 

 much smaller in peak and declining populations than usually found at 

 lower densities. Of course these data could be interpreted to indicate an 

 early attainment of maturity with respect to size, but this interpretation is 

 most unUkely in view of the other data on reproduction and the results of 

 other experiments. This problem could be settled by determining the age 

 of these animals; a problem ah-eady discussed at some length. 



Chitty (1952) also studied a population of voles {Microtus agrestis) and 

 obtained results similar to those of Kalela. During the peak breeding season 

 of 1937 he found that young males did not increase in weight above 22.2 

 gm., whereas normally adult males seldom weigh less than 22.3 gm. after 

 May. Young females weighed more than 22.2 gm. only while pregnant; 

 otherwise the heavier females in this population were old adults with a few 

 exceptions in September and October. The prevalence of pregnancy in the 

 peak year was apparently less than in other years and appeared to be due 

 mainly to a failure of maturation in the young of the year, as older animals 

 exhibited a uniformly high pregnancy rate. In one area all age groups ceased 



