2. Endocrines and Populations 313 



The rates of fecundity, pregnancy, and other measurements of reproductive 

 competence were unchanged from the preceding year. There was no appar- 

 ent change in htter size or intra-uterine mortaUty throughout the period of 

 the study of this population. There was no shortage of food at any time 

 during these studies, and especially not during periods of active reproduc- 

 tion. Therefore, the changes in reproductive function are not attributable 

 to ^'arying food supplies. Climatic factors, although variable, did not corre- 

 late with the functional changes and were probably noncontributory in 

 producing functional changes. 



This study shows, beyond reasonable doubt, that the shifts in reproduc- 

 tive function were reciprocally related to the density of the population in 

 1954 and 1955 and that increased density was the factor responsible for 

 the decline in reproductive function (with a total suppression of reproduc- 

 tion in the young of the year in one area) and attainment of maturity in 

 1955. It also shows that the primary effect of increased density was to in- 

 hibit maturation so that large numbers of immature animals accumulated 

 by late summer. At the same time, growth beyond a general level of 20-24 

 gm. was suppressed in both sexes. The data from the area of extreme 

 density, in which no animals matured are especially convincing. No direct 

 measures of endocrine function were made, but these occurrences in natural 

 populations of Clethrionomys rufocanus conform with the results from the 

 more highly controlled experiments with populations of mice or voles in 

 the laboratory. 



The apparently paradoxical failure in 1956 of reproduction to return to 

 the 1954, or higher, levels, together with the apparent stunting, also fits 

 the conclusions discussed earher. The effects of increased density on lacta- 

 tion and subsequent growth of the young and in turn their inability to 

 lactate adequately were discussed earlier, and attention was called to the 

 profound long-range effects of increased density on the animals experiencing 

 the increase as well as two generations of their progeny (Chitty, 1955; 

 Christian and LeMunyan, 1958) . The red-backed voles which overwintered 

 until the spring of 1956 had been subjected to increased density and had 

 experienced its effects. It was noted that these voles were unable to mature 

 in the year of their birth and were stunted. The following year their repro- 

 ductive performance and that of their offspring reflected a similar situation, 

 closely resembling the effects of increased density on laboratory popula- 

 tions. The population dechne was consequently due to the high mortahty 

 of those young born during the period of maximum density and probably 

 reflects decreased resistance to a variety of stimuli coincident with increased 

 activation of adaptive mechanisms. A very high rate of infant and juvenile 

 mortality was a major factor in the cessation of growth in laboratory popu- 

 lations, but in these there was no appreciable mortaUty of the adults. 



