EFFECT OF GROUP-SIZE ON TONIC IMMOBILITY IN LAYING HENS

It has been shown that duration of tonic immobility (TT) reflects pred ation risk and levels of fearfulness. Since the chances of escaping pr edation are higher in larger groups, birds from these groups should ha ve shorter TI duration. On the other hand, increased competition betwe en birds in larger groups might lead to greater fearfulness and longer TI in larger groups. The aim of this study was to test these two hypo theses. Laying hens raised in floor pens in group sizes of 15, 30, 60 and 120, each with four replicates, were used. Tonic immobility tests were performed on adult birds, either directly 'in' their home pen or 'out' in a separate room. In the test 'in', duration of TI increased w ith group size, with a significant difference between group sizes 15 a nd 120 (P = 0.012). In the test performed 'out', there was a trend for TI to be longer in larger groups. The duration of TI in groups tested 'in', was significantly shorter than in groups tested 'out' (P < 0.00 1). The results of this study suggest that although the ultimate funct ion of TI is to reduce predation, the social environment is a proximat e factor affecting TI duration in domestic hens. (C) 1998 Elsevier Sci ence B.V. All rights reserved.