2. Endocrines and Populations 311 



These results indicate that reproductive performance is inversely related 

 to population density in Norway rats, at least for increasing and stationary 

 populations. The status of decreasing populations is equivocal and may 

 represent a partial recovery of reproductive function in declining popula- 

 tions, although decreasing populations are presumably at maximum den- 

 sity. However, we already have noted that adrenocortical hypertrophy is 

 greatest in populations spontaneously decreasing from asymptotic levels. 

 These results represent a large number of populations and carry consider- 

 able weight; nevertheless in another experiment in which three urban 

 populations of Norway rats were artificially reduced about 50% there was 

 no corresponding increase in the rates of pregnancy (Davis and Christian, 

 1958). Two factors may have obscured such an occurrence. One was that 

 the populations were increasing and therefore had a fairly high pregnancy 

 rate at the time of the reduction (30.2%) , and the other was that the main 

 breeding season was declining at the time the next samples were collected 

 (April, May, and June) from which a mean pregnancy rate of 32.2% was 

 obtained. 



Experiments designed to analyze reproductive function in relation to 

 density of populations of rats suffer from the inability to determine the 

 age of the animals, therefore attainment of maturity and other reproduc- 

 tive end points are based on the sizes of the animals. There is evidence to 

 indicate that rats in increasing populations grow faster, and so are larger 

 at a given age, than rats from stationary populations (Davis, 1951b). 

 Therefore inhibition of growth in populations of high density may tend to 

 obscure evidence of partial inhibition of reproductive function, such as 

 delayed puberty, since it is likely that reproductive function and growth 

 are equally inhibited. Thus, these results are evidence to support the earlier 

 comments regarding the desirability of determining age in mammals when 

 evaluating reproductive function. 



One of the more conclusive studies to date on the relationship between 

 density of population and reproduction was conducted by Kalela (1957) 

 on red-backed voles {Clethrionomijs rufocanus) in Finland. A major factor 

 contributing to the value of this study was the ability to determine and 

 approximate age of the animals by the root development of the first molars. 

 These molars are rootless at the end of September of the year of birth, but 

 the roots are visible by the following spring and continue to develop further 

 (Zimmermann, 1937; Kalela, 1957). The question might be raised whether 

 the growth and rooting of the molars would be suppressed along with sup- 

 pression of growth, but molars were rooted in animals in which growth and 

 maturity were definitely suppressed (Kalela, 1957) ; so that any suppression 

 of molar growi^h and rooting would make the results even more dramatic. 

 []This and other means of estimating age in voles has been more fully re- 



