286 /. /. Christian 



tive purposes. However, in female voles adrenal weight increased with in- 

 crease in body size; therefore for females it was necessary to use relative 

 adrenal weights for purposes of comparison. 



3. Reproduction 



a. Male. Reproductive function was depressed in proportion to increases 

 in population size in both male and female house mice from populations of 

 fixed size. The results of experiments with freely growing confined popula- 

 tions of house mice and voles also show a progressive decrease in reproduc- 

 tive function with increasing density. We have seen that there was a delay 

 in the onset of puberty in male house mice from freely growing populations 

 of high density, as demonstrated by the delayed involution of the adrenal 

 X-zone and the development of the testes, seminal vesicles, and preputial 

 glands. The size of the gonads and sex accessories was less than their con- 

 trols from segregated mice of all but the heaviest body weight. Rapid de- 

 velopment of the seminal vesicles and preputial glands did not begin until a 

 body weight of 16 gm. was reached, which coincides exactly with the be- 

 ginning of X-zone involution in animals. These results strengthen the con- 

 clusion that androgen secretion was delayed. In the segregated controls, the 

 sex accessories had begun rapid growth by the time the animals had reached 

 a weight of 13 gm. These results were confirmed by histologic examination 

 of the testes. The size of the testes primarily reflected the advancement of 

 spermatogenesis and the development of the seminiferous tubules. Sperma- 

 togenesis was inhibited to the same degree as the sex accessories (Christian, 

 1956) . 



Male mice from freely growing populations of intermediate size exhibited 

 an inhibition of the reproductive organs which was approximately inter- 

 mediate between that exhibited by the males from the high populations and 

 the segregated controls (Christian, 1956). 



It is of additional interest that testes and sex accessories of male mice 

 weighing 28 gm. or more, the heaviest weight group in the experimental 

 populations, were about the same size as those of the isolated controls. 

 Furthermore, there was no inhibition of spermatogenesis in these animals. 

 In general, this weight group contained the dominant animals in the four 

 high-density populations examined and was represented by a total of 21 

 animals. These mice represented the initially introduced males and those 

 males from the first litter or two that had established their dominance early 

 in the histories of the populations. These observations are confirmed by 

 Crowcroft and Rowe (1957), who observed that the productive females in 

 in confined populations of house mice were the introduced animals or those 

 from their first few litters. Detailed observations relating the weights of 



