Stress response and sexual performance of acutely grouped dominant and subordinate rams of different ages

It was previously reported that dominant and subordinate rams modify their behavior similarly after grouping. However, the question of whether these differences are due only to social stress or an influ-ence of age has yet to be clarified. Therefore, the aim of the present experiment was to compare the stress response and the individual sexual behavior toward an estrous ewe of rams that were grouped in unequal dyads according to age and body weight, resulting in a dominant or subordinate ram after grouping. Eighteen Saint Croix sexually experienced rams were housed in individual pens (2 m X 2 m) from Day-30 to Day 0. On Day 0, one adult and one yearling ram were grouped together and kept in a different pen until the end of the study. Body weight and scrotal circumference were recorded on Days-6,-2, 2, 5, 8, 11, and 21, while the social position was determined based on food competition tests from Day 1 to Day 10. Blood samples for cortisol determination were collected by jugular venipuncture every 30 min for 7 h, beginning 1 h before grouping, and every hour for 5 more hours, and on daily basis between 9.00 and 9.30 h from Day-1 to Day 17 for testosterone measures. The sexual behavior of each ram was tested on Days-1, 3, 6, 9, 12, and 22. Scrotal circumference was greater in dominant than sub-ordinate rams ( P = 0.04), and varied with time, without interactions between social position and time. Cortisol concentration did not differ according to the social position but varied with time ( P < 0.0 0 01). Testosterone concentration was greater in subordinate than dominant rams ( P = 0.05) but did vary with time or an interaction between social position and time. The frequency of lateral approaches was greater in subordinate than in dominant rams ( P = 0.02), while the frequency of mates did not vary according to the social position but varied with time ( P < 0.0 0 01). However, dominant rams had a shorter reaction time and mated more frequently than the subordinate rams. It was concluded that grouping unknown adult and yearling rams was stressful and rams modified their sexual strategy, with slight differences according to their social position. In general, after grouping, all differences in sexual behavior favored dominant rams.(c) 2022 Elsevier Inc. All rights reserved.