The Primer Effects

Abstract

The discovery of primer effects in mice began in the 1950s. The findings at that time were all made with female mice. When female mice were housed in groups without the odor of males, their estrous cycles were extended (Lee–Boot effect) resulting in less frequently coming into estrus. Exposure to male odor reversed this, i.e., the females with longer estrous cycles returned to short estrous cycle length (Whitten effect). This influence of male odor was stronger when the females were kept in larger group sizes. Influences of male odors on females were also observed to accelerate the timing of vaginal opening in female pups (Vandenbergh effect), such that their vaginal opening occurred earlier following exposure to male odor. These influences of male odors on the estrous cycles of females and timing of puberty suggested that sex hormone secretion is stimulated by the odor of opposite sex. Another primer effect found in the 1950s was the disruption of the establishment of pregnancy by exposure to the odor of an unfamiliar male (Bruce effect). Similar observations in male mice were not to occur until much later. For example, the sperm density in males exposed to female-soiled bedding was higher and sperm motility of subordinate males was low. This suggested that the influence was mediated by the odor of the dominant male. Primer effects are found in both males and females affecting reproductive status.