Changes in mother-offspring relationships with the increasing age of the lamb in hair sheep (Ovis aries)

Changes in the mother-offspring relationship with increasing age of the offspring were analyzed in 46 Saint Croix ewes and their single male lambs from 7 to 58 days after lambing. Physiological and behavioral variables were measured every second day during 10-minute separation periods (pen test) and after traveling 25 m toward their restrained offspring or mother (reunion test). The changes in the variables measured in both tests were analyzed using linear regression. No relationship was found between the respiration rate, heart rate, locomotor activity, number of bleats or serum cortisol concentration registered during pen tests and the age of the lambs. In contrast, although no relationship was found between heart rate, locomotor activity, and cortisol concentration, ewes displayed higher respiration rate (r = −0.45; P < 0.05) and vocalized more (r = −0.85; P < 0.05) in the pen tests during the first days postpartum. Bleats decreased from 22.00 ± 2.32 to 6.68 ± 0.05 bleats per minute from day 7 to day 58, respectively. In the reunion test, it was not until lambs were 16 days old that 100% of them could reach their mother. However, all ewes reached their lambs before 17 days postpartum but failed to complete the reunion test within the time limit (1 minute) or took more (P < 0.05) time to reach their lamb as the experiment progressed. These results support the hypothesis that, in Saint Croix sheep, the interest of the mother in her young is stronger immediately after parturition and declines with increasing age of the lamb, whereas the interest of the lambs shows an inverse relationship.