Neonatal stress in lambs: Behavioral and physiological responses

For the 1st 5 weeks after birth, experimental lambs reared in small flocks under conditions that approximated the normal social environment for sheep were subjected 3 times a week to an experimental procedure that was designed to expose the lambs to neonatal stress. The experimental procedure consisted of maternal separation, handling, and exposure to a Pavlovian conditioning paradigm. Each lamb responded to this experimental procedure with elevated plasma corticosteroid levels and intense vocalization. In contrast to the experimental (or neonatal stressed) lambs, the control lambs remained undisturbed with their mothers during the time the experimental lambs were being stressed. When both groups of lambs were subsequently exposed to open‐field testing, the 2 groups did not differ in their behavioral responses (e.g., vocalization, latency to 1st movement, or locomotion) even though each of these behaviors has been found to be sensitive to the effects of neonatal social experience. Also no difference existed between the 2 groups in their initial adrenocortical response to open‐field testing; however, the plasma concentration of corticosteroids of the experimental lambs took longer to return to pretest levels. At 5 months of age, the lambs were subjected to brief restraint in a dark chamber, and the effect of this brief stress on heart rate and plasma corticosteroids was measured. No differences were found between the 2 groups in either the heart rate or adrenocortical response to this stress. Thus, we have no evidence from these studies that neonatal stress had any effect on the lambs' subsequent ability to respond to brief behavioral stress.