Behavioral, cardiac and cortisol responses to brief peer separation and reunion in cattle.

Behavioral, cardiac, and adrenal responses of heifers to short-term isolation and to subsequent reunion with familiar or nonfamiliar conspecifics were measured. Two groups of heifers were studied according to their different social reactivity: Aubrac heifers (n = 12) reared under suckler conditions and Friesian heifers (n = 12) reared under dairy management. Because these two groups could also react differently to human beings, testing social isolation was realized by removing pen mates without handling the subject. Moreover, heifers were tested in confinement to avoid an alteration of the cardiac response to isolation by an excessive motor activity. Although physical restraint can influence the reactions, this effect is assumed to be weak because heifers had been exposed to the experimental procedures, including confinement for 3 days before isolation test, in addition to brief periods of physical restraint occurring regularly according to rearing practices. Results show that social separation induced struggling and large increases in vocalization, heart rate, and plasma cortisol concentrations in all heifers. Except for vocalization, these effects were more severe in Aubrac than in Friesian heifers. For all heifers, isolation-induced distress was positively correlated with the duration of social contacts they engaged with the pen mates prior to separation. Behavioral responses, i.e., struggling and vocalization, decreased when conspecifics were brought back, independently of their familiarity to the subject. In contrast, the heart rate decline induced by the entrance of conspecifics was more pronounced in response to reintroduction of pen mates. These findings indicate that social isolation is a severe psychological stress in cattle and that the mere sight of conspecifics reduces behavioral distress regardless of peer identity. The isolation-induced distress depends on the genetic and rearing backgrounds of the heifers without allowing to differentiate their respective effects.