Competition in utero between twin lambs

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Abstract
1. The weights at birth of 822 pairs of twin lambs have been studied with respect to the differences between males and females. Data were obtained during five lambing seasons from various breeds and crosses in five flocks.2. The sex distribution of twin pairs came close to the ratio .3. Analyses made within season and within the major breed categories showed that sex differences were small relative to other sources of variation, but that there was a significant effect of sex of co-twin on birth weight. The following average differences were found (the sex of co-twin being indicated in brackets): These differences had an average S.E. of 0·078 lb. (Table 3). The sex difference in litters of mixed sex was twice as large as that obtained from like-sexed litters.4. The data for particular flocks and seasons suggest that in addition to these average effects, there may be some interaction between sex and breed or season which can cause deviations from the averages.5. These results are interpreted to mean that there is competition between twins for some element of growth in limited supply; and that in twins of mixed sex, the result tends to favour the male.