REALIZED HERITABILITY OF TONIC IMMOBILITY IN WHITE LEGHORN HENS - A REPLICATED SINGLE GENERATION TEST

Fearfulness was measured as latency to recover from induced tonic immo bility using White Leghorn hens from a population originated by crossi ng three different strains selected for egg number and egg weight. Rea lized heritability was calculated in a single generation selection exp eriment with eight replicates, each consisting of 76 40-wk-old females from which 21% were selected for decreased tonic immobility duration. A randomly selected control population was maintained for each replic ate. Separately from this experiment, fear differences between ages an d sexes were investigated at 1, 2,4, 8, 12, 16, 20, 24, 28, and 32 wk. The realized heritability (.32 +/- .10) indicates that induced tonic immobility should respond to selection. It was numerically higher than the estimate calculated by haff-sib analysis of variance in the base population (.18 +/- .07); the logarithmic transformation did not yield higher heritability estimates. Crossing effects were evaluated in two different crossbreds, i.e., Leghorn x Castellana and Buff Prat x Whit e Prat, which suggested important negative heterotic effects for this fear-related trait (-31% and -54%, respectively). Significant (P < .05 ) quadratic and cubic variation between 1 and 32 wk of age was obtaine d in females and males, respectively; the involvement of these age eff ects make it difficult to generalize about realized heritability for t onic immobility. Tonic immobility increased from 32 to 72 wk. Differen ces between the sexes were apparent, males showing longer tonic immobi lity durations than females except in the postjuvenile period before s exual maturity, suggesting a relationship between this trait and hormo nal changes.