Tonic immobility responses of white leghorn hens affected by induction techniques and genetic stock differences.

Tonic immobility (TI) was induced in about 70% of While Leghorn hens caught and immediately restrained in the dorsal position for a single 15 sec period. Temporary confinement of hens housed in floor pens in single-bird cages prior to restraint reduced initial induction of TI to 46%. Eye closure during TI occurred in less than or equal to 15% of hens and vocalization, jumping up, or both on termination was present about 80% of the time. Induction of TI on first restraint was greater when less support was provided for the hen's body. Latency until first gross leg and head movement after TI induction decreased with increased time required for catching of hens in floor pens, but righting time (until hens stool up) was unaffected. Correlations among the three latency measures revealed the following: between leg and head movement, r .41 to .71 (P less than .01); leg movement and righting, r .51 to .99 (P less than .01); head movement and righting, r -.62 (P less than .05) to .96 (P less than .01). Genetic differences were detected consistently by latency to first gross head movement and by righting time only. Strains selected for part-year egg mass showed more prompt head movement than did unselected controls. Random genetic drift between strains selected alike apparently was responsible for differences in righting time. Although strains within selection schemes differed, crosses between strains did not differ from the mean of the parental strains, providing evidence that genetic variance for righting time tends to be more additive than nonadditive.