Sexual performance and stress response of previously unknown rams after grouping them in dyads.

The establishment and maintenance of individual social status is a stressor that affects negatively the reproductive function. Therefore, the aims of this study were to determine if after grouping two unknown rams there is a stress response, and their sexual behavior is affected, and if those responses differ between the rams that resulted dominants and those that resulted subordinates. Twelve adult rams were maintained in individual pens for one month, and on Day 0, hour 0 two rams of similar body weight were grouped in a neutral pen. Social rank was determined using the food competition test, in which each pair of rams was allowed to compete for feed in a bucket only large enough for one of them; the ram that was able to eat from the bucket for more than 1 min, was considered to be the dominant individual. Cortisol concentrations were measured on Day 0 during the first 12h after grouping. Testosterone concentrations were measured from Day -9 to Day 16. Sexual behavior of rams toward estrus ewes was determined before and after grouping in individual pen tests. There was no difference between dominant and subordinated rams in none of the studied variables. Cortisol concentration had a peak 30 min after grouping the rams (P<0.0001). There was a significant decrease in testosterone concentration on Day 5 and Days 8-14 compared to concentration before grouping (P=0.02). The frequency of ano-genital sniffing and flehmen of estrus ewes decreased after grouping, with no differences between dominant and subordinated rams (P=0.009 and 0.03 ano-genital sniffing and flehmen, respectively). However, the frequency of lateral approaches (P=0.005), and all mating behaviors towards ewes increased after grouping the rams (mount attempts: P=0.02; mounts: P=0.008; mounts with ejaculation: P=0.05; total mounts: P=0.003; ejaculation/total mounts: P=0.0004). It was concluded that grouping unknown rams modified their sexual strategy resulting in an increase in the number of mating behaviors when rams were tested alone with estrus ewes. However, grouping two unfamiliar rams was a stressful event that negatively affected testosterone concentrations but the stress and behavior responses did not differ according to rams' social rank.