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

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... These results with gerbils do not support the suggestion that the biological advantage of pregnancy blocking lies in enhanced male reproductive success. Materials and General Methods Test animals were Mongolian gerbils which have been bred at our Institute since 1974. The Makrolon housing, breeding and test boxes measured 50 35 20 cm (Type A) or 30 15 x 13 cm (Type B). Before the introduction of each pair, cages were washed with a 10% Sagrotan solution and strewn with a 3 cm layer of fresh sawdust, removed after completion of every test and controls. Half a flowerpot served as shelter. Room temperature was kept at 20-22°C, with a 12 h light: 12 h dark cycle. The animals were fed daily ad libitum with a dry food mixture of