The birth weight of rabbits born after heterospermic insemination

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Abstract
1. Two rabbit sires were used for insemination of sixty-eight females. Insemination was either homospermic (one sire at a time) or heterospermic (mixed semen from the two sires). Each offspring could be traced to its sire by genetic marks. The sires differed in weight and were known to give offspring differing in mature weight and named Large and Small offspring. The object of study was the birth weight of these offspring in logarithmic transformation.2. After heterospermic insemination, there was no evidence of any real ‘heterospermic vigour’ in the average birth weight of the litter as a whole.3. After heterospermic insemination, the difference in birth weight between the two kinds of offspring was accentuated (enhancement effect). This confirms a previous experiment.4. The enhancement effect is ascribed to competition among embryos. It is not attributable to postulated changes in the gestation period. It appears to arise from an increase in the birth weight of Large offspring, together with a possible decrease in the birth weight of Small offspring. These changes in birth weight are attributable to changes in prenatal growth rate.5. The bearing of this work on Russian experiments with heterospermie insemination is discussed.