[Studies on the pheromonal pregnancy block in the mouse. II. Discriminatory and/or memorial mechanisms in female mice responding to the stud male].

The present study was carried out to examine whether the stud male strain have a blocking effect on the pregnancy in stud female. Four inbred mouse strains, C3H, C57BL/6, BALB/c and CBA, were used in these experiments. Virgin female mice were caged together with a male of a different strain at 10-20 weeks of age for copulation. The mated females were removed from the male, and housed with a male strange or alien from the stud male. The "Bruce effect" was evident in C3H, BALB/c and CBA strains, but not in C57BL/6. It was shown in the strains having the "Bruce effect" that pregnancy block occurred if the strange male was of a strain different from the stud male. Further experiments were designed to examine whether the "Bruce effect" might depend on imprinting or memory in a stud female by preproximity to a strange or alien male before stud mating. In this experiment, the "Bruce effect" did not occur in BALB/c or CBA strain by "imprinting" from preproximity to the strange or alien male, except for BALB/c males. From these findings, it was suggested that the discrimination or memory must play a leading role in the "Bruce effect" in that the female was able to recognize her own stud male.