Social rank during pre-pubertal development and reproductive performance of adult rams

Abstract

Reproductive performance of rams when they were 1.5 and 2.5 years old during two breeding seasons was investigated. The objective was to compare the reproductive performance of the rams on the basis of their social ranks, which had been scored for each subject during the lamb development stage. The subjects were aged matched Corriedale × Milchschaf rams ( n = 12: all singles born during the same week from multiparous ewes) were used. Data were collected from the five rams with greater (GR) and lesser (LR) social rank scores when lambs. These data included body weight, scrotal circumference, sexual behavior toward estrual ewes (e.g. incidence of ano-genital sniffing, lateral approaches, Flehmen, mount attempts, actual mounts, and mounts with ejaculations, and ejaculation/total mount ratio) during a 30-min period, and semen variables (semen volume, mass motility, individual sperm motility and total number of spermatozoa in the ejaculate). During the first breeding season, GR rams were heavier (62.9 ± 2.7 kg) than LR rams (69.4 ± 3.0 kg; P < 0.0001). There were no differences in scrotal circumference or testosterone concentration measures. Semen volume was greater in GR rams (0.5 ± 0.1 mL compared to 0.7 ± 0.1 mL for GR and LR rams; P < 0.004), but it tended to have a greater percentage of abnormal spermatozoa. Although some courtship behaviors were more frequent in LR rams (lateral approaches and Flehmen) and ejaculation/total mount ratio of LR rams also tended to be greater, mounting activity (mount attempts and mounts) was more frequent in GR rams. During the second year, GR rams still tended to be heavier (83.3 ± 2.0 kg compared to 85.9 ± 2.4 kg; P = 0.09) but had smaller scrotal circumference than LR rams (37.5 ± 0.2 cm compared to 36.5 ± 0.6 cm; P < 0.045). Semen characteristics were not different for the rams, but mass motility tended to be greater for LR rams. Generally, LR rams displayed more lateral approaches, showed Flehmen more often, mounted with ejaculation more often, and tended to have a greater mount with ejaculation/total mounts ratio. Conclusions were courtship of rams may differ in line with the social status they had as lambs although there are not clear effects on other sexual behaviors or semen characteristics.