Sexual maturation and seasonal changes in plasma levels of sex steroids and fecundity of wild Norway rats selected for reduced aggressiveness toward humans.

Parameters of sexual maturation and reproductive function were compared in two groups of Norway rats, one selected for reduced aggressiveness towards man and the other for aggressiveness. It was found that selection for reduced aggressiveness produced an acceleration of sexual maturation as judged by earlier vaginal opening in the female and presence of mature spermatozoa in the testes of the male as well by earlier prepubertal increase in plasma testosterone in the male. In domesticated adults, seasonal variations in the parameters of the reproductive function narrowed; they showed neither the winter decrease in sexual activity nor the marked fall in plasma testosterone and estradiol exhibited by their aggressive counterparts.