Suppression of oestrus in grouped mice: the effects of various densities and the possible nature of the stimulus.

2 experiments designed to investigate the effect of grouping of mice on the suppressio nof estrus and the possibel nature of the stimulus is reported. Virgin C57BL/6 and BALB/Ci female mice were used. 120 female subjects from each strain (68-104 days) were placed in equal numbers into 5 housing conditions: 1 2 4 6 or 8 females per cage. They were put into previously unused clean cages where they remained undisturbed for 15 days. Vaginal smears were taken daily between 1300-1800 hours from each female for 28 days following this. Housing conditions had a significant effect on the number of complete estrous cycles during the 28-day period. There was also a significant effect of strain and a significant interaction of housing conditions and strain. C57BL mice decreased with increasing cage density to the 4 cagelevel after which significant differences were absent. BALB females showed a similar decrease to the 6 females/cage level. To examine the nature of the stimulus 12 females of each strain were divided into 2 groups of equal size. The experimental females were placed individually into cages in which adult females of the same strain had been housed for 4 days. The results indicate that the lengthening of the diestrous phase of the cycle seen in the 1st experiment was also present when animals were caged in soiled cages. It is suggested that a pheromene may be the stimulus for suppression.