Effects of an acute stressor on fear and on the social reinstatement responses of domestic chicks to cagemates and strangers

Genetic selection for appropriate levels of sociality (motivation to be wit h conspecifics) could benefit poultry welfare and performance. Runway tests that require chicks to traverse a corridor in order to reach other chicks in a goal box are commonly used to measure this behavioural trait. However, we need to determine if the chicks' responses in such tests are sensitive to certain experiential variables before we can recommend possible selectio n criteria for future breeding programmes. The present study focused on fea r and on the identity of the stimulus birds. Broiler chicks either remained undisturbed or were exposed to an acute stressor (mechanical restraint) be fore their tonic immobility fear responses were measured 1 h later in Exper iment 1. Exposure to the stressor significantly prolonged tonic immobility and, hence, presumably, underlying fear levels. In Experiment 2, the respon ses of stressed chicks and undisturbed controls were assessed when they wer e tested individually in a runway with a goal box containing either familia r or unfamiliar chicks of the same age. Our finding that stressed chicks em erged from the start box sooner and spent longer near the stimulus birds su ggests that exposure to a frightening event increased social reinstatement motivation. Furthermore, social affiliation was more pronounced when the go al box contained familiar cagemates rather than strange chicks, regardless of prior treatment. This finding demonstrates that broiler chicks that were housed in groups of twelve can discriminate between familiar and unfamilia r conspecifics encountered in novel surroundings. Thus, sociality was posit ively associated with fearfulness and broilers clearly showed social discri mination in runway tests. These findings highlight the dangers of disregard ing variables, such as fear and the capacity for social recognition in test s of social motivation. We strongly recommend that exposure to frightening events prior to test should be avoided and that the identity of the birds i n the runway goal box should be standardized, i.e. either familiar or unfam iliar, and noted. (C) 2001 Elsevier Science B.V. All rights reserved.