Influence of Pre- and Post-Natal Stress on the Social Motivation and Fear Response in Lambs

The impact of maternal separation on the capacity of lambs to respond to social challenges and stressful situations might be modulated by pre-natal stressors such as environment conditions experienced by gestating females. In this study, we explored the impact of two experimental group sizes (GS) experienced by gestating ewes (pre-natal stress) and of early maternal separation (post-natal stress) on the social motivation and fear responses in lambs. Fifty-four pregnant Latxa ewes (Basque Country, Spain) were randomly assigned to two experimental pre-natal GS treatments of 6 or 12 ewes/pen; GS6, GS12). After birth, lambs from each treatment were either left with mothers (MR) or early separated (ES) 24 h after birth. Thirty two, 2 to 8-day-old lambs of similar ages were split into groups of 3, coming from the same pre- and post-natal treatment. Each lamb was sequentially subjected to a novel arena test, a novel object test, and a social motivation test. Lamb behavior, movement patterns, and vocalizations were collected. The effects of pre- and post-natal treatments, their interaction, and male/female differences were determined using the GLIMMIX procedure in SAS. Pen during gestation nested within group size, and individual ewe nested within pen during gestation and group size, were included as random effects. No significant interactions were detected between pre- and post-natal treatments (P>0.05). Latency to move during the social motivation test was shorter for MR than for ES lambs (3.5 ± 1.6 vs. 7.3 ± 1.7 s, for MR and ES, respectively; P=0.0327), who also had longer inactive periods (38.8 ± 3.6 vs. 15.0 ± 3.0 %; P=0.0278) and explored less (22.1 ± 3.4 vs. 40.9 ± 5.5 %; P=0.0332). GS6 lambs remained closer to the other test lambs than GS12 in the social motivation tests (93.9 ± 1.1 vs 90.08 ± 0.9 % of total time; P=0.0044). Females walked longer net distances than males in the novel arena tests (254.8 ± 7.6 vs. 221.9 ± 8.7 cm, respectively; P=0.0257). Females also had shorter latencies to move (2.1 ± 1.6 vs. 8.7 ± 2.0 s; P=0.0488) and walked longer net distances during the social motivation tests (249.6 ± 10.5 vs. 217.5 ± 9.8 cm respectively; P=0.0227). No evidences of impact on fear responses were obtained. These results appear to suggest that MR lambs, and to a lower extent those born from ewes housed at GS6 during pregnancy, had increased social motivation. Females appear to show a higher social motivation.