Realized heritability of tonic immobility in White Leghorn hens: a replicated single generation test

Fearfulness was measured as latency to recover from induced tonic immobility using White Leghorn hens from a population originated by crossing three different strains selected for egg number and egg weight. Realized heritability was calculated in a single generation selection experiment 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 replicate. Separately from this experiment, fear differences between ages and 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 half-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 White 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 obtained in females and males, respectively; the involvement of these age effects make it difficult to generalize about realized heritability for tonic immobility. Tonic immobility increased from 32 to 72 wk. Differences between the sexes were apparent, males showing longer tonic immobility durations than females except in the postjuvenile period before sexual maturity, suggesting a relationship between this trait and hormonal changes.