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Summary. In wild female house mice a skewed distribution of vaginal plugs on the days following pairing indicates an oestrous synchronization (Whitten effect) correlated with the presence of the male. Inseminated wild house mice were exposed to strange males or subjected to various types of cage changes and handling during the pre-implantation period. The strange males produced a significant reduction in pregnancy rates (Bruce effect). Changes in the physical environment produced compar- able reductions in the absence of a strange male. Several socio-dependent reproductive phenomena are known to occur in some strains of laboratory mice. Two of these are the Whitten or oestrous synchroniz¬ ing effect and the Bruce or pregnancy blocking effect (Parkes & Bruce, 1961). The odour of a male is capable of stimulating the oestrous cycle, hence syn¬ chronizing the females so that a large percentage becomes inseminated on the 3rd night following pairing. Similarly the odour of a strange male will block implantation, following which the female will return to oestrus. The object of the present study was to determine the occurrence of these two phenomena in wild house mice maintained under laboratory conditions. The mice used in this study were produced in an outbred laboratory colony derived from local stock. Wild-trapped animals were periodically added. Mice were fed a standard laboratory ration and housed in large-mouthed, 1-gal glass jars placed horizontally. A 4 in. diameter opening was cut in each jar top and fitted with a $ in. screen. This closure provided ventilation and access for the water bottle drinking tube. The wild house mice seem to adjust well to this 'cage'. Mice were weaned at 21 days of age and kept three to five per cage. Eight- to 12-week-old virgin females were placed with a stud male of proven fertility and examined each morning for vaginal plugs, which were considered evidence of insemination. The number of days between pairing and insemination was recorded and analysed for evidence of oestrous synchronization. Following insemination the male was removed and the female isolated for 24 hr (Day 0). On Day 1 the females were randomly assigned to one of the eight treatments described in Table 2. Pregnancy was determined by autopsy and examination

OESTROUS SYNCHRONIZATION AND PREGNANCY BLOCKING IN WILD HOUSE MICE (MUS MUSCULUS)