2. Endocrines and Populations 315 



fore changes in mean litter size apparently do not need to be corrected for 

 these variables. During each breeding season of the study there was an 

 appreciable decline in litter size as measured by embryos and uterine im- 

 plantation scars and this decrease was also paralleled by a decline in the 

 number of corpora lutea. There was also an increase in the general level of 

 the population for comparable months from 1952 to 1953. This general in- 

 crease coincided with an overall decline in mean litter size which, however, 

 was not significant. These changes were inversely related to the annual 

 increase in population density for the first two years. However, the popula- 

 tion declined sharply from spring to fall in the third year and yet the mean 

 litter size also declined. 



Prenatal mortality was directly related to population density in the first 

 breeding season, inversely in the third and bore no conspicuous relationship 

 in the second. Prenatal mortality was greatest after the population had 

 declined in September 1954. The mean prevalence of pregnancy also de- 

 clined with each successive year. 



The peak of the population was evidently in September of 1953, and the 

 decline followed shortly thereafter. Hoffmami noted that at this time the 

 proportion of fertile males began to decline and at the same time 24% of 

 the mature females weighed less than 33 gm. This figure increased to ap- 

 proximately 45% in June and July of the following season. The proportion 

 of mature males weighing less than 35 gm. also rose to reach a maximum of 

 33% in July of 1954. These morphologic functional changes resemble those 

 observed by Kalela and probably reflect suppression of growth at high 

 densities. The fact that there was a decline in male fertility as well as in the 

 incidence of pregnancy further supports such a conclusion. Hoffman called 

 attention to the fact that prenatal mortality was lowest at peak densities 

 and highest the following year. However, examination of his data 

 reveals that the proportion of multiparous females rose steadily throughout 

 the 1953 season to reach 100% in September, undoubtedly accounting for 

 for the low proportion of prenatal losses for two reasons: (1) it is likely 

 that the older parous females suffer less prenatal loss at any time; and (2) 

 more important, they probably represent the dominant animals and there- 

 fore those least affected by high densities in accordance with the results of 

 laboratory experiments (Retzlaff, 1938; Christian and LeMunyan, 1958). 

 The age of the animals could not be determined in these studies ; immature 

 animals were classed as such and were not further divided into those old 

 enough to have matured and young animals. Had it been possible to deter- 

 mine age, it is likely that these results would have been more decisive. 

 There was a low mortality of weanlings and juveniles during the period of 

 build-up in this population, but apparently there was a marked increase in 

 mortality in all age groups during the decline. 



