Effect of social rank in hair rams on the number of lambs sired and their postnatal development

Abstract

The impact of social rank among hair rams on reproductive efficiency has been extensively studied, particularly regarding its influence on ewes and rams under various lambing scenarios, both within and outside the breeding season. However, limited information exists on the specific effects of social rank on lamb paternity. The present study aimed to evaluate the influence of social rank during a 35 d mating period on paternity outcomes and the postnatal development of lambs during the breeding season. A total of 108 adult ewes were divided into six groups, with 18 ewes per group. Each group was paired with two rams: one dominant ram (DRam) and one subordinate ram (SRam). Lamb development was monitored from birth through weaning and up to 150 d of age. Paternity was determined for 107 lambs using a panel of 116 single-nucleotide polymorphism (SNP) markers to assign sires from among the 12 rams included in the study. Social rank showed significant differences in lambing outcomes depending on the type of birth ( P <0.05). DRams sired 67 % of lambs, while SRams sired 33 %. DRams exhibited a significantly higher proportion of twin births (52.9 %) compared to SRams (32.4 %; P <0.01). However, no significant differences were observed between the ram groups for other lambing types. Lambing type also significantly influenced lamb weight gain, with single-born lambs achieving greater weight gain than twins and triplets ( P <0.05). Additionally, lamb sex ratios differed significantly between ram groups, with DRams producing a higher proportion of male lambs (50.7 %) compared to SRams (41.2 %; P <0.05). This study demonstrates that social rank significantly affects reproductive outcomes, including lamb paternity, lambing type, birth weight and body weight at 3 months of age. These findings highlight the importance of considering social hierarchy when managing breeding programs to optimize reproductive efficiency in hair sheep production systems.