Investigation of the Bruce effect in the Mongolian gerbil (Meriones unguiculatus).

Strange males and females blocked implantation in recently mated female Mongolian gerbils. Even the presence of the partner male failed to annul the influence. The females were, however, affected only when not lactating (e.g. in first pregnancies or when a litter had died). Pregnancies could also be ended by alterations of the caging conditions, but there were quantitative differences between social and non-social influences, both in numbers of females reacting and in their subsequent fertility. These results with gerbils do not support the suggestion that the biological advantage of pregnancy blocking lies in enhanced male reproductive success.