Sex and strain comparisons of tonic immobility ("Righting time") in the domestic fowl and the effects of various methods of induction.

Tonic immobility was induced in adult laying hens using different methods and substrates, factors which have shown great variety of response in previous work. The fact that topic immobility was induced in every situation confirms the robust nature of this phenomenon. Placing hens on their backs and restraining them in a U-shaped wooden cradle covered with cloth was the most reliable and consistent method. Not only was the duration of response greatest when this method was used, but less handling, with its possible stressful effects, was required to induce it. There were no significant differences in the latency to the first head movement or in the duration of immobility between adolescent 'T' (Rhode Island Red × Light Sussex) and 'S' (White Leghorn) lines but both these characteristics were considerably shorter in the 'J' line birds (Brown Leghorn). There were no significant sex differences in the duration of tonic immobility but male 'T' and 'S' line birds showed greater latencies to the first head movement and fewer subsequent alert head movements than females. These results are discussed in terms of sex and strain differences in fearfulness. The varied nomenclature used to describe the immobility reaction is discussed and an alternative term "righting time" is proposed. It is further suggested that the immobility response consists of an inhibitory and an alert stage.