Ewes prefer subordinate rather than dominant rams as sexual partners

Abstract

When rams and ewes interact freely, dominant rams perform the greatest number of copulations. However, the sexual preference of the female might influence the final distribution of ejaculations, and preference for subordinate rams could be evolutionary advantageous. To determine the sexual preference of the ewe for dominant (DOM) or subordinate (SUB) rams when rams’ activity is restricted, 28 ewes and 8 rams were used. Rams were classified into DOM and SUB by competition in pairs for an estrous ewe and were kept in dyads until the end of the experiment. Ewes were induced into estrus using progestagen-impregnated intravaginal sponges, and 24 h after sponge withdrawal, the sexual preference of the ewe for DOM or SUM rams was determined individually. Both rams were tethered with a 4 m long lasso in opposite corners of a 4m × 12m pen, and all the interactions were video-recorded. The test was repeated with seven ewes for each dyad of rams. The evaluation period started when the ewe allowed her first mount and ended with the last. At the end of each test, each ewe was removed from the pen and the location of the rams were exchanged. The time the ewe received the first to the last mating averaged 16:55 ± 1:25 h:min. Overall, ewes entered more times and remained longer in the zone of the SUB (entrances: 68.9 vs 29.1; pooled SEM = 9.0; P = 0.001; time into the zone: 3.66 ± 0.56 h vs 1.04 ± 0.51 h; P=<0.0001, for SUB and DOM rams, respectively). In 25 % of the tests, only the SUB ram mated the ewes. The number mounts and mates were significantly greater in SUB than DOM rams (mounts: 22.35 ± 3.07 vs 10.86 ± 2.84; P = 0.0005; mates: 10.50 ± 0.94 vs 3.79 ± 1.07; P < 0.0001 for SUB and DOM rams respectively). It was concluded that estrous ewes prefer to interact and be mounted and mated by subordinate rather than by dominant rams when rams’ activity is restricted.