Effects of dense housing on the growth of reproductive organs, plasma testosterone levels and fertility of male mice.

Male mice were raised in populations of two different sizes from birth to adulthood: six animals (three of each sex) or 30 animals (15 of each sex) being maintained in cages of the same dimensions. Animals were killed every 10 days from weaning to 60 days and at 90 days. Increased population size induced decreased body and seminal vesicle weights; testicular, pituitary and adrenal weights were little affected. Plasma testosterone levels were lowered by increasing population size over the period from weaning to 50 days but they were increased at 60 and 90 days. The age at which first fertile matings occurred was not affected. These results indicate that the endocrine function of the testis, as determined by the measures used here, was specifically affected by this differential housing.