Bidirectional selection for open-field activity in young chicks.

A selection program for open-field activity in 2-day-old chicks was performed over eight generations. All characters measured responded to selection, and for most of them the response was symmetric in the two strains. After eight generations of selection the distribution of latencies and of measures of activity overlapped only slightly. The two exceptions were the latency to move and the percentage of birds which defecated. From an evolutionary point of view the symmetry of the two lines, the relatively high heritabilities, and the absence of heterotic effects on these characters show that open-field behavior has been subjected to a stabilizing natural selection rather than a directional one. This conclusion agrees with observations on open-field behavior in this and other species.