Chronic stressors, tonic immobility and leucocytic responses in the domestic fowl

Abstract

Tonic immobility (TI) fear reactions and heterophil/lymphocyte (H/L) ratios were measured in individually caged Brown Leghorn pullets before and after their exposure to one of three procedures lasting approximately 72 hr. These were: ad lib feeding (controls), fasting (removal of food) or frustration of feeding (fasting plus denial of access to visible food). H/L ratios were elevated after fasting or frustration, illustrating the stressful nature of these procedures, but remained unaltered in the controls. Pretreatment TI responses were similar in all groups. Susceptibility and response duration were reduced upon retesting presumably through habituation, but this effect was relatively homogeneous across groups. Any direct effect of fasting/frustration upon fearfulness seemed unlikely because the mean TI responses of control and stressed birds were similar. However, whereas a rank order based on TI durations was repeatable upon retesting in the controls, it was destabilised in the stressed groups. Positive intraindividual correlations were found between pretreatment TI durations and subsequent H/L responses to fasting/frustration. This suggests that leucocytic responses to chronic stressors may be greater in fearful pullets than in less fearful birds.