Tonic Immobility and Open Field Responses in Nagoya, White Leghorn, and White Plymouth Rock Chicks.

Innate fear responses to tonic immobility (TI) and open field (OF) were evaluated in newly hatched chicks of three breeds with distinct breed origin and genetic relationships. The breeds studied were Nagoya (NAG), a native Japanese breed; White Leghorn (WL), a representative of layers; and White Plymouth Rock (WPR), a parental breed of common broilers. The TI test revealed that WL was the most sensitive to extensive fear evoked by the TI test among the three breeds, followed in order by WPR, and NAG. In contrast, the OF test revealed that NAG was the most sensitive to mild fear evoked by the OF test, followed in order by WPR, and WL. The different fear responses between NAG and WL were supported by minimal phenotypic correlations between TI and OF traits in each breed. These results demonstrated that NAG and WL breeds exhibit extreme and opposite responses to TI and OF fears.