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HETEROSPERMIC INSEMINATION AND ITS EFFECT ON THE OFFSPRING RATIO IN RATS

Department of Dairy Science, University of Illinois, Urbana, Illinois 60801, U.S.A.

(Received 28th April 1975)

The consecutive insemination of an oestrous female by two different males has often been reported to yield disproportionate numbers of offspring. This disparity in the number of offspring has been suggested as being independent of the number of spermatozoa from each sire (Allen & Champion, 1955; Edwards, 1965; Roche et al., 1968; O'Reilly et al., 1972). The heterospermic impregnation technique has also been used to determine the effect of semen storage and semen diluents and to evaluate the relative fertility of males (Roche et al., 1968; Beatty et al., 1969). In the mouse (Lewis & Wright, 1935) and rat (Blandau & Money, 1944) sperm transport is almost instantaneous and it might be expected that the spermatozoa deposited by the first sire would be at an advantage in reaching the site of fertilization before the spermatozoa