Effects of auditory and physical enrichment on 3 measurements of fear and stress (tonic immobility duration, heterophil to lymphocyte ratio, and fluctuating asymmetry) in several breeds of layer chicks.

The purpose of this study was to analyze the effect of auditory enrichment (by means of classical music) or physical enrichment (by means of hanging colored string bunches and barley grains on the floor) on tonic immobility duration, heterophil to lymphocyte ratio, and fluctuating asymmetry (FA) in chicks of several layer breeds. In experiment 1, 192 chicks from 8 Spanish breeds and 1 White Leghorn population were reared in cages with or without music auditory enrichment until 8 wk of age. The effect of music auditory enrichment was significant for heterophil to lymphocyte ratio (P < 0.05). The ratios were higher in chicks reared without music than in those reared with music, suggesting that auditory enrichment from classical music reduces stress in chicks. There were significant differences in morphological trait measurements (relative asymmetry of wing length, leg width, and combined asymmetry; P < 0.05), being greater in chicks reared without music. This result suggests that FA is a good indicator for stress level in chicks, given that it follows the same trend as that found for heterophil to lymphocyte ratio. There was a significant treatment by breed interaction (P < 0.05) for tonic immobility duration, indicating no consistent effect by auditory enrichment on tonic immobility duration across breeds. In experiment 2, 180 chicks from 3 Spanish breeds were housed in pens with or without physical enrichment (colored plastic string bunches and barley grains on the floor) until 6 wk of age. The effect of physical enrichment on tonic immobility duration, heterophil to lymphocyte ratio, and FA was not significant, indicating no effect on fear and stress in layer chicks. In conclusion, auditory enrichment by means of classical music is a reliable method for reducing stress levels in several breeds of layer chicks. However, music auditory enrichment was not effective in reducing fearfulness in any of the layer breeds. Physical enrichment by means of colored plastic string bunches and floor barley grains does not appear to be an effective method for reducing stress and fear in layer chicks.