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Mating has been shown to have effects on ovulation in a number of species which ovulate spontaneously, such as rats (Aron, Asch & Asch, 1961; Aron, Asch & Ross, 1966; Zarrow & Clark, 1968) and sheep (Parsons, Hunter & Rayner, 1967; Van der Westhuysen, Van Niekerk & Hunter, 1970). In the sow, the results obtained by Pitkjanen (1955) and Lebedev (1957) suggest that mating hastens ovulations and shortens the time during which ova are liberated. Direct inspection of the ovaries by coelioscopy (Locatelli, 1971) allows rapid and accurate determination of the time ofovulation and causes minimal trauma. By this technique, the earlier results have been confirmed in a preliminary experiment (Signoret, 1970). The present experiment was designed to show by how many hours mating advances ovulation and shortens its total duration. Fifty-nine Large White gilts were mated at the time of the second oestrus following withdrawal of methallibure (I.C.I.), which had been given for 21 days (100 mg/day in the feed) in order to control ovulation and to facilitate detection of the time of ovulation. The gilts were mated at the first sign of oestrus, and again 6 hr later, by boars of known fertility. They were then randomly assigned to one of four experimental groups in which coelioscopy took place 30, 35, 40 and 45 hr, respectively, after the onset of oestrus. The time ofovulation in this group (A) was compared with that in a group of thirty-seven unmated gilts (B) given a single injection of pmsg 24 hr after the last treatment with methallibure, and in a second control group (C) of seventy-four unmated gilts on their second oestrus following withdrawal of methallibure. The findings in these last two groups have already been reported (du Mesnil du Buisson, Mauléon, Locatelli & Mariana, 1970). Oestrus was detected by teaser boars put with the gilts for a few minutes every 6 hr (Signoret, 1967). The results are summarized in Table 1. By probit analysis (Finney, 1962), the median time between the beginning of oestrus and the start of ovulation was found to be 34-1 hr in the mated animals (A) and 38-0 hr in the controls (C). In the two groups, ovulations finished 35-0 hr and 41-8 hr, respectively, after the start of oestrus. Thus after two copulations, the median time for the onset of ovulation was 3-9 hr earlier than in the unmated animals and the duration of ovulation was reduced by 2-8 hr (Text-fig. 1). It has been suggested that the effects of stimuli associated with mating could be mediated by modifications in the patterns of discharge of pituitary gonado-

EFFECT OF MATING ON THE ONSET AND DURATION OF OVULATION IN THE SOW