The influence of ram mating preferences and social interaction between rams on the proportion of ewes mated at field joining

Abstract

This experiment investigated whether rams display mating preferences for particular oestrous ewes during field joining, and tested the hypothesis that such preferences are due to differences in the sexual “attractiveness’ of oestrous ewes. In addition, the influence of social interaction between rams on their mating preferences was determined. Each of 10 rams was presented with oestrous ewes in a 0.4-ha mating plot for up to 11 days. No observations were made in a preliminary 5-day period of joining in single-sire mating groups with at least two, and usually six, oestrous ewes per ram per day. For the next 6 days the rams were joined singly and then in groups of two and three at the rate of six oestrous ewes (that were 0–4 h into oestrus) and 30 non-oestrous ewes per ram. The rams were allocated to groups of two and three on the basis of their dominance status, which had been established earlier from competition by the rams for a bucket of feed. The behaviour of all animals was observed continuously until one of the ewes was no longer in oestrus. All rams displayed clear mating preferences, and this resulted in an uneven distribution of mating among the available oestrous ewes, with 24.8% ( 58 234 ) of the ewes not being mounted. Rams were actively discriminating between ewes, since they investigated 87% ( 80 92 ) of the ewes that were not mated and were able to locate the oestrous ewes in a group that also contained non-oestrous ewes. Also, the correlation between the number of ejaculations received by ewes and the number of times they “solicited” attention from the rams was only 0.28. This suggests that the “soliciting” behaviour of the ewes had less influence on whether or not they were mated than did the rams mating preferences. The Spearman's rank correlations between the ejaculations and the observable components of ram behaviour received by the ewes were highly significant ( P<0.0001) `and all about 0.6. These data indicate that rams display mating preferences for particular oestrous ewes because ewes differ in their individual sexual “attractiveness”. When the rams were joined in groups, dominance-subordinance relationships were clear but these did not influence the number of times individual rams mated per observation period. Social interaction between rams modified their mating preferences, but the extent of this seemed to depend on the amount of competition between rams. In groups of two, subordinate rams mated about the same number of ewes (46.4%) as dominant rams (50%), indicating that they had become less selective and had mated with different, and less “preferred”, ewes than the dominant rams. In groups of three, preferences were not modified and the most dominant ram in each case mated with the most ewes. These results emphasis the complexity of social interaction between rams.