The ability of Merino ewes and lambs to reunite after separation, as affected by divergent selection for ewe multiple rearing capacity

Lambs and ewes from 2 Merino lines that had been selected divergently from the same base population for (High line) or against (Low line) multiple rearing success, were assessed for their ability to reunite after temporary separation. Selection in the lines began in 1986, and data for this study were obtained from 1998 to 2002. Postnatal lamb mortality for lambs born during 1998-2002 were generally lower (P < 0.05) in high line lambs than in low line contemporaries. When tethered 20 m away from their dams at 1 day of age, lambs in the High line tended (P < 0.10) to be more likely to bleat and tug on the tethering rope than Low line contemporaries. Dams of lambs in the high line were quicker (P < 0.05) to reunite with a tethered lamb than low line ewes. They were also quicker (P < 0.05) to establish contact with the full litter after separation of multiples. In an experiment conducted in pens at an age of 3 days, no line difference was found for the ability of ewes to reunite with their lambs or litters when confronted by a choice of 3. When lambs were assessed in the same way, 89% was able to reunite with their dams within 5 min, with no evident line difference (P > 0.05). When released from the pens, the dams of 95% of single lambs returned for their lambs, the proportion tending (P < 0.10) to be higher in the High line than in the Low line. The dams of multiple lambs returned for 73% of the lambs tested, with no clear line difference. Lambs in the High line were more (P < 0.01) likely to follow next to the front legs or midside of their dams than Low line lambs. Line differences in some behavioural patterns in favour of the High line are considered to be conducive to lamb survival when compared with Low line performance.