Effect of Social Hierarchy Upon Reproductive Traits of Dorper Rams from Northern Mexico.

Social hierarchies among animal species are established on the dominance-aggressiveness level of the animal aligned with the subordination of their mates. This study aimed to evaluate the association among social rank hierarchy, scrotal circumference (SC), semen quality (SQ), and sexual drive (SD). Adult Dorper rams (n=24) with similar live weight (LW), body condition score (BCS), and age (AG) were subjected to a behavioral test to define the success index (SI) based on male-to-male interactions, determining three social ranks: low hierarchy rams (LHR; SI = 0-0.33; n=9), medium hierarchy rams (MHR; SI=0.34-0.66; n=10) and high hierarchy rams (HHR; SI= 0.67-1; n=5). Afterward, the SC, SQ, and SD were evaluated. The GLM procedure of SAS (SAS Institute, Cary, NC) was used to analyze LW, AG, BCS, SC and SQ variables such as ejaculate volume, and sperm concentration. Microscopic characteristics of sperm cells were subjected to an ANOVA using the MIXED procedure of SAS. The GENMOD procedure was used to assess the effect of social rank (three levels) on the SD variables. Differences were considered as significant at P < 0.05. SC was greatest (P< 0.05) for HHR rams (40.0±3.5) and least for rams with medium social rank (MHR; 29.3±1.20). SQ was greatest (P< 0.05) for HHR rams (3,848±187) and least for LHR (2,660±463). The rejection percentage to mount an estrous ewe or SD was 77.7 for LHR and 20% for HHR and MHR rams (P < 0.05). To conclude, the HHR rams presented the highest successful copulation, the greatest scrotal circumference, and the greatest sperm cell concentration compared with rams with lesser social rank scores. [ABSTRACT FROM AUTHOR]