Observations on the ability of the stud male to block pregnancy in the mouse.

The block to pregnancy in newly mated female mice caused by the proximity of a strange male (`Bruce effect') occurs in a greater number of females when the male belongs to a different strain from the female (alien male) than when he belongs to the same strain (strange male) (Bruce, 1960; Parkes & Bruce, 1961). The difference between strains implies that the uniformity of odour or `spectrum of odours' of a strain is more powerful than individual differences between males. The proportion of blocked pregnancies was not influenced between males of the same strain when the `strange males' were litter-mates of the female and/or of the stud male (Bloch & Wyss, 1972). Some observations, however, suggest a certain ability of the female to discriminate between individual males. According to Bruce (1960, 1968, 1969), the recently mated female can be removed from her stud partner within a few hours of coitus and returned 24 hr later without disturbance to the pregnancy. This presupposes at least partial recognition of the stud male as an individual. The reaction is governed by the ability of the female to discriminate between the stud male 'made familiar by coitus' and the unfamiliar male to which she is exposed (Bruce, 1969). We assumed that the female recognizes the stud male after being separated from him for 24 hr, not because he was the coital partner, but because usually he had been with the female for a certain time before mating so that she had become accustomed to his odour. Experiments were therefore designed to ascertain the difference between stud males which had mated with females after being in the same cage for only 24 hr and males which had been with the females for at least 3 weeks but had not achieved coitus. All male and female mice belonged to the same random-bred strain (NMRI). Three groups of experiments were performed.