Social rank affects reproductive development in male lambs

Under field conditions, dominant rams may prevent mating by other rams, thereby depressing flock fertility. However, little is known about the effects of social rank on prepuberal reproductive development in male lambs. The objective of this experiment was to determine if social rank of lambs is related to body weight, scrotal circumference, testosterone concentrations, semen production and courtship behavior until 38 weeks of age. An additional aim was to characterize male-male sexual behavior and to determine if this behavior was related to social rank. Body weight, scrotal circumference and testosterone serum concentrations were measured weekly in 13 male lambs. In addition, semen was obtained by electroejaculation, and sexual behavior towards non-estrous ewes was registered every 2 weeks. Male-male mounts were also recorded weekly. Individual social rank success indexes were determined using the food competition test when lambs were 3 and 6 months old. The five rams with greater (GR) and the five with lesser (LR) success indexes were used in the experiment. Body weight and scrotal circumference increases were greater in GR than in LR lambs. Testosterone concentrations increased with age, but were not related to social rank. Semen characteristics matured earlier in GR than in LR lambs. Most courtship and mating behaviors (lateral approaches, flehmen, ejaculations and ejaculations/total mounts) increased with age, but no effect of social rank was observed. The number of ano-genital sniffing and mount attempts increased with age and was greater in GR than LR. The total number of male-male mounts in which each lamb took part was positively related with the success index (r=0.48; P=0.05), and tended to be related with the number of times each lamb was mounted (r=0.42; P=0.06). No relation was observed between the number of times each lamb mounted and the social rank success index. We observed that GR lambs mature earlier than LR lambs, which is reflected by a greater increase of their body weight and scrotal circumference. Semen production and male-female sexual behavior also occurred earlier in high-ranked than in low-ranked lambs. It remains to be determined if low-ranked lambs can achieve similar development than high-ranked lambs over longer time intervals, or if their maximum developmental capacity is lower than the maximum obtained by high-ranked lambs.